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Lakhmir Singh Class X Chemistry Chapter 5 – Periodic Classification of Elements


Exercise : Solution of Questions on page Number : 281


Answer 1:
(a) Mendeleev arranged the elements in his periodic table on the basis of atomic masses.
(b) In the modern periodic table, the elements are arranged on the basis of atomic numbers.


Answer 2:
(a) False
Newlands divided the elements into horizontal rows of seven elements each.
(b) False
According to Mendeleev’s periodic law, the properties of elements are a periodic function of their atomic masses.
(c) False
The elements in a period have consecutive atomic numbers.


Exercise : Solution of Questions on page Number : 282


Answer 3:
Dmitri Mendeleev was a Russian chemist who said that the properties of elements are a periodic function of their atomic masses.


Answer 4:
(a) Periods have elements with consecutive atomic numbers.
(b) The statement is correct.
(c) The statement is correct.


Answer 5:
(a) John Newlands is the scientist who gave the Law of Octaves in the early classification of elements.
(b) Johann Dobereiner, a German chemist gave the Law of Triads in the early classification of elements.


Answer 6:
A, B and C are the elements of a Dobereiner’s triad. If the atomic masses of A and C are 7 and 39 respectively, then according to Dobereiner’s triad law, the atomic mass of B is the average of the atomic masses of A and C.
So, the average of the atomic masses of A and C =7+392=462=23
Therefore, the atomic mass of B is 23.


Answer 7:
There are six elements between X and Y.
Since X and Y are the two elements having similar properties and obey the Newland’s law of octaves, the number of elements between X and Y, including both, must total eight.


Answer 8:
The atomic mass was Mendeleev’s basis for the classification of elements.


Answer 9:
In the classification of the then known elements, Mendeleev was guided by two factors, which are:
1. Increasing atomic masses
2.Grouping together of elements having the same properties


Answer 10:
Gallium (Ga) and scandium (Sc) are the two elements whose properties were pExercise : Solution of Questions on page Number : 281redicted on the basis of their position in Mendeleev’s periodic table.


Answer 11:
(i) Eka-boron was named scandium (Sc).
(ii) Eka-aluminium was named gallium (Ga).
(iii) Eka- silicon was named germanium (Ge).


Answer 12:
Germanium (Ge) and Scandium (Sc) are the two elements whose properties were predicted on the basis of their position in Mendeleev’s periodic table.


Answer 13:
Lithium (Li), sodium (Na) and potassium (K) with atomic masses 7, 23, 39 constitute a Dobereiner’s triad. The atomic mass of sodium (middle element) is half-way between those of the other two. This can be seen in the illustration below:
Average of the atomic masses of lithium and potassium = Atomic mass of lithiumAtomic mass of potassium =7+392 =23


Answer 14:
Noble gases are the group of elements which could be placed in Mendeleev’s periodic table later on, without disturbing the order. This is because they are inert (unreactive) elements, as their valence shells are completely filled with electrons. Therefore, Mendeleev decided to include these elements in a separate group.


Answer 15:
(a) The basis for the modern periodic table is the atomic number.
(b) The horizontal rows  in a periodic table are called periods.
(c) Group 1 elements are called alkali metals.
(d) Group 17 elements are known as halogens.
(e) Group 18 elements are called noble gases.
(f) According to Newland’s classification of elements, the properties of sulphur are similar to those of oxygen because sulphur is the eighth element, starting from oxygen.


Answer 16:
(a) (i) A group is a vertical column of elements in a periodic table.
(ii) A period is a horizontal row of elements in a periodic table.
(b) There are seven periods and eighteen groups in the long form of the periodic table.
(c) (i) Lithium (Li) and sodium (Na) are two elements of group 1.
(ii) Chlorine (Cl) and bromine (Br) are two elements of group 17.
(iii) Neon (Ne) and argon (Ar) are two elements of group 18.


Answer 17:
(a) Lithium and beryllium are the metals among the first ten elements in the modern periodic table.
(b) The significance of atomic numbers in the modern periodic table is that it helps in arranging the elements according to their electronic configuration. Since, the number of valence electrons is the same, elements placed in the same group show similar chemical properties.
For example, if we take the element lithium (Li), its atomic number is 3. Its electronic configuration is (2,1). As it has one valence electron, it is placed in the first group of the periodic table. Similarly, all the elements having one valence electron will be placed in group 1, and they show similar chemical properties. In this manner, classification of elements is made according to the electronic configuration of the elements.


Answer 18:
(a) The isotopes of elements are not given a separate place in the periodic table. This is because isotopes have the same atomic number and a different atomic mass, and the modern periodic table is based on the atomic number of elements. Hence, to give it a separate place, the elements should be arranged according to their atomic mass.
(b) As per Mendeleev’s classification, elements are arranged in an increasing order of atomic masses, but cobalt, with a higher atomic mass was placed before nickel. Cobalt has an atomic mass of 58.9 and nickel has an atomic mass of 58.7, which is slightly lower than that of cobalt. Mendeleev could not explain this point. This problem was resolved in the modern periodic table. According to the modern periodic law, elements are arranged in the increasing order of their atomic numbers. The atomic numbers of cobalt and nickel are 27 and 28 respectively. So, cobalt with a lower atomic number should come before nickel.
(c) Generally, hydrogen is treated as a special element and placed alone at the head of the periodic table. The position of hydrogen in the periodic table is unclear because it resembles both the alkali metals and halogens in some of its properties. However, it is placed in the first group, above all the alkali metals in the modern periodic table because its electronic configuration is similar to that of alkali metals. But due to the small size of hydrogen, it exhibits properties different from that of the alkali metals.


Answer 19:
(a) On the left side of the periodic table, we find metals.
(b) On the right side of the periodic table, we find non-metals.
(c) Metalloids are the names of those elements which divide metals and non-metals in the periodic table.


Exercise : Solution of Questions on page Number : 283


Answer 20:
(a) Lithium (Li), sodium (Na) and potassium (K) are three elements (metals of group 1) that have only a single electron in their outermost shell.
(b) Magnesium (Mg) and calcium (Ca) are two elements (metals of group 2) that have two electrons in their outermost shell.
(c) Helium (He), neon (Ne) and argon (Ar) are three elements (inert gases of group 18) that have a completely filled outermost shell.


Answer 21:
According to Dobereiner’s law of triads, when elements are arranged in increasing order of their atomic masses, a group of three elements with similar chemical properties is obtained. This group is called a triad. The atomic mass of the middle element is equal to the arithmetic mean of the atomic masses of the other two elements.
For example, the set of elements: calcium (Ca), strontium (Sr) and barium (Ba), with atomic masses 40, 88 and 137 respectively form a triad because they have similar chemical properties, and the atomic mass of strontium is approximately equal to the average of the atomic masses of calcium and barium.
Arithmetic mean of atomic masses of calcium and barium = 40+1372=88.5


Answer 22:
According to Newlands’ law of octaves, when elements are arranged in increasing order of their atomic masses, the properties of the eighth element are a repetition of the properties of the first element. Newlands divided the elements into horizontal rows. Each horizontal row had 7 elements.
For example, let us take a row from Newlands’ classification of elements. If we take the elements Li, Be, B, C, N,O, F and Na, lithium (Li) is the first element, and sodium the eighth. It has been found that the properties of sodium are a repetition of the properties of lithium. In short, both lithium and sodium have similar chemical properties.


Answer 23:
(a) Yes, Dobereiner’s triads also existed in the columns of Newlands’ law of octaves. This can be explained by taking the second column of the Newlands’ classification of elements as an example. The second column has the elements lithium (Li), sodium (Na) and potassium (K), which form a Dobereiner’s triad.
(b) Limitations of Dobereiner’s classification of elements:
It could identify only three triads from the elements which were known at that time. It failed to arrange all the elements which were known then in the form of triads of elements with the same chemical properties.
(c) Limitations of Newlands’ law of octaves:
1. Newlands’ law of octaves was applicable only up to the element calcium and not beyond that, i.e, it only works for lighter elements.
2. According to Newlands, only 56 elements existed in nature and there would be no elements discovered in the future. However, this assumption was proved wrong with the discovery of several elements later on which did not fit into Newlands’ law of octaves.
3. A unique slot was not given to all the elements, as Newlands put two elements which have different properties together in a single slot.
4. The element iron which resembles cobalt and nickel in its properties was given a position far away from them.


Answer 24:
(a) According to Mendeleev’s periodic law, the properties of elements are a periodic function of their atomic masses. This means that when elements are arranged in an increasing order of their atomic masses, the elements with similar properties repeat at regular periods.
However, Henry Moseley, a scientist, showed that the atomic number of an element is a more fundamental property and a better basis to classify elements than the atomic mass. The atomic number increases consistently by 1, from one element to the next, whereas the atomic mass does not change regularly. Also, the atomic number of an element does not change and remains fixed. This led to a change in Mendeleev’s periodic law.
(b) Noble gases are placed in a separate group of the periodic table because they are inert elements. They are unreactive as their valence shells are completely filled with electrons. Their properties are different when compared to all the other elements.


Answer 25:
(a) The merits of Mendeleev’s classification of elements:
1. Mendeleev’s periodic law predicted the existence of certain elements such as gallium, scandium and germanium, which were not discovered at that time.
2. Mendeleev’s periodic table could also predict the properties of various elements based on their position in the periodic table.
3. Mendeleev’s periodic table could place group 18 elements, called Noble gases in the periodic table, when they were discovered.
(b) Anomalies of Mendeleev’s periodic classification of elements:
1. Mendeleev’s periodic classification of elements could not explain the position of isotopes in the periodic table: Since, isotopes of the same metal have similar chemical properties but different in their atomic masses. So, the isotopes of the same element should be given different place in periodic table as Mendeleev’s periodic table was arranged in increasing order of their atomic masses. However, isotopes were not given separate place in his periodic table.
2.Mendeleev’s periodic classification of elements could not assign a proper place for the element,hydrogen. Since, hydrogen resembled both alkali metals and halogens in some of the properties.


Answer 26:
(a) The properties of eka-alumiunium, predicted by Mendeleev, which was undiscovered then, were almost similar to the actual properties of the element discovered later on. The comparison table below shows the similarities in the properties of the undiscovered element then and the actual element (Gallium).

Property Eka-Aluminium Gallium
Atomic Mass 68 69.7
Density 5.9g/cm3 5.94g/cm3
Melting point low 30.2oC (low)
Formula of oxide Ea2O3 Ga2O3
Formula of chloride EaCl3 GaCl3

(b) (i) Scandium (Sc) was known as eka-boron.
(ii) Gallium(Ga) as eka-aluminium.
(iii) Germanium (Ge) as eka-silicon.


Answer 27:
(a) Elements are classified into certain groups in such a manner that elements belonging to the same group exhibit similar properties. This eases the study of elements, as we can reduce the study to a few groups of elements rather than studying the properties of all the 115 elements known at present, which is very difficult. This is the reason for classification of elements.
(b) The two criteria used by Mendeleev to classify the elements in his periodic table are:
(i) Increasing atomic masses
(ii) Grouping of elements which exhibited similar properties, placed under the same vertical column (group).
(c) Mendeleev left some gaps in his periodic table for the elements which were not known at that time, to ascertain that elements having the same properties fell in the same vertical column or group, without disturbing the arrangement of previous elements.
(d) There was no mention of noble gases like helium, neon and argon in Mendeleev’s periodic table as those gases were not known at that time.
(e) The isotopes of chlorine Cl-35 and Cl-37 should be placed in the same slot because their chemical properties are the same. The two isotopes of chlorine have the same atomic number and chemical properties. We know that the arrangement of elements is made on the basis of atomic number in the periodic table. And chemical properties of the element depend on the atomic number of the atom.


Answer 28:
(a) Mendeleev’s periodic law: According to Mendeleev’s periodic law, the properties of elements are a periodic function of their atomic masses. This means, when the elements are arranged in an increasing order of their atomic masses, the elements with similar properties repeat at a regular interval of the periods.
(b) Mendeleev  used the formula of the oxides and hydrides produced by the elements as the basic chemical properties of elements, in creating his periodic table.
(c) Limitations of Mendeleev’s classification of elements are:
i) Mendeleev’s periodic law could not explain the position of isotopes in the periodic table.
ii) Mendeleev’s periodic law could not assign a proper position to the element hydrogen in the periodic table.
iii) Mendeleev’s periodic law failed to explain the wrong order of atomic masses of some elements.
(d) Germanium (Ge) and scandium (Sc) are the two other elements besides gallium, which have since been discovered, for which Mendeleev had left gaps in his periodic table.
(e) Group 18 consisting of noble gases was missing from Mendeleev’s original periodic table as the noble gases were not known at that time.


Answer 29:
(a) Modern periodic law: According to the modern periodic law, the properties of elements are a periodic function of their atomic numbers. This means, if the elements are arranged in an increasing order of their atomic numbers, the elements exhibiting similar properties will repeat after regular intervals of periods.
(b) The elements are arranged in an increasing order of atomic number, and it helps in arranging the elements according to their electronic configuration. Since elements having the same number of valence electrons show similar chemical properties, they are placed under the same group.
(c) As the modern periodic law is based on the atomic numbers of elements, all the anomalies of Mendeleev’s periodic table are removed. For example:
1. The position of isotopes could be explained, which the Mendeleev’s periodic table failed to explain. As all the isotopes of an element have the same atomic number, they can be placed under the same group in a periodic table.
For example: Cl-35 and Cl-36, which are the isotopes of Chlorine with the same atomic number 17, they can be placed in the same group of the periodic table.
2. Mendeleev could not explain why cobalt was placed before nickel. This problem was solved by the modern periodic law. The atomic numbers of cobalt and nickel are 27 and 28 respectively. Hence, cobalt with a lower atomic number was placed before nickel in the modern periodic table.
(d) It is not possible to place an element with an atomic number of 1.5 between hydrogen and helium. This is because elements are placed in the increasing order of their atomic numbers. Atomic numbers are always whole numbers and cannot be in decimals, as half an electron or half a proton cannot exist .
(e) Niels Bohr is the scientist who prepared the modern periodic table.


Answer 30:
(b) Beryllium.
In Mendeleev’s periodic table, a gap was left for gallium, scandium and germanium.


Answer 31:
(b) Calcium.
The Newlands’ law of octaves for the classification of elements was applicable only up to the element calcium.


Answer 32:
(d) Increasing atomic masses.
According to Mendeleev’s periodic law, the elements in the periodic table were arranged in the order of increasing atomic masses.


Answer 33:
(c) All metalloids
The three elements having the chemical symbols Si, B and Ge are metalloids.


Exercise : Solution of Questions on page Number : 284


Answer 34:
(c) Ge.
Germanium (eka- silicon) is the element which found a vacant place in the periodic table later on.


Answer 35:
(c) 78
The atomic mass of the element Z is 78.
According to Dobereiner’s law of triads, atomic mass of Y=X+Z2
Substituting the atomic masses we get:
46=14 + Z2
92=14+Z
Therefore, Z=78.


Answer 36:
(c) R.
R (Si) is a metalloid since its atomic number is 14 and it has 4 valence electrons.


Answer 37:
(d) In the modern periodic table, the isotopes of chlorine, having different atomic masses are kept in the same group.
The isotopes of chlorine, having different atomic masses but same atomic numbers are kept in the same group in the modern periodic table.


Answer 38:
(c) It has 18 vertical columns known as groups.
In the modern periodic table, there are 18 vertical columns known as groups.


Answer 39:
(b) Group III.

The element which forms the oxide X2O3 is placed in group 3 of the periodic table.


Answer 40:
(c) Bohr.
The modern periodic table of elements was prepared by Neils Bohr.


Answer 41:
(b) Proton.
Proton is an atomic particle whose number in the atoms of an element remains the same always, and forms the real basis for the modern classification of elements.


Answer 42:
(a) The atomic masses of three elements X, Y and Z are 7, 23 and 39 respectively. The average atomic mass of X and Z is nothing but the arithmetic mean of the atomic masses of X and Z.
Therefore, the average atomic mass of  X and Z = 7+392=23.
(b) From the above calculation, we observe that the average atomic mass of the elements X and Z is equal to the atomic mass of the element Y.
(c) Dobereiner’s law of triads is illustrated by the above example.
According to Dobereiner’s law, when elements are arranged in an increasing order of atomic masses, a three element group (triads), having the same chemical properties is formed. The atomic mass of the middle element is equal to the average of the atomic masses of the other two elements.
(d) The elements X,Y and Z , having atomic masses 7, 23, 39 are lithium (Li), sodium (Na) and potassium (K). They belong to the alkali metal group (group 1), with the same valency of 1 and exhibit similar chemical properties.
(e) The elements calcium (Ca), strontium (Sr) and barium (Ba) are a set of alkali earth metals (group 2 elements), with atomic masses 40,88 and 137 respectively, which are classified according to Dobereiner’s law of triads.


Answer 43:
In the given set of elements, i.e., calcium, magnesium, sodium and beryllium, sodium does not belong to the set. This is because sodium is a group 1 element and the remaining three elements​ belong to group 2. Elements of the same group show similar chemical properties.


Answer 44:
In the given set of elements, i.e., oxygen, nitrogen, carbon, chlorine and fluorine, chlorine does not belong to the set. The element chlorine belongs to the third period while the remaining elements belong to the second period.


Answer 45:
(a) The atomic masses of Na, Si and Cl are 23, 28 and 35.5 respectively.
According to Dobereiner’s law of triads, when elements are arranged in the increasing order of their atomic masses, a three element group with similar chemical properties is obtained. The atomic mass of the middle element is equal to the arithmetic mean of the atomic masses of the other two.
The above set of elements does not form a triad because arithmetic mean of the atomic masses of Na and Cl = 23+35.52=29.25
This value is approximately equal to the atomic mass of silicon, but they do not form a triad because the elements do not exhibit similar chemical properties.
(b) The set of elements Be, Mg and Ca with atomic masses 9, 24 and 40 respectively, form a triad because they have similar chemical properties and the atomic mass of magnesium (Mg) is approximately equal to the average of the atomic masses of beryllium (Be) and calcium (Ca).
The arithmetic mean of the atomic masses of Be and Ca =9+402=24.5 which is approximately equal to 24.


Answer 46:
(a) Sodium (Na), magnesium (Mg) and aluminium (Al) belong to the same(third) period of the periodic table.
(b) Lithium (Li), sodium (Na) and potassium (K) belong to the same group(group 1) of the periodic table.


Answer 47:
(a) Neon with atomic number 10.
Electronic configuration:- Shells: K L
Electrons arranged: 2 8
(b) Magnesium is the element with the electronic configuration of 2,8,2 as its atomic number is 12, since the atomic number is the sum of all the electrons present in the electronic configuration.
Electronic configuration:- Shells: K L M
Electrons arranged: 2 8 2
(c) Silicon is an element with a total of three shells and four electrons in its valence shell. Its atomic number is 14 and the electronic configuration is 2,8,4.
Electronic configuration:- Shells: K L M
Electrons arranged: 2 8 4
(d) Boron is an element with a total of two shells, with three electrons in its valence shell. Its atomic number is 5 and electronic configuration is 2,3.
Electronic configuration:- Shells: K L
Electrons arranged: 2 3
(e) Carbon is the element with twice as many electrons in its second shell as its first shell. The atomic number of carbon is 6 and its electronic configuration is 2,4.
Electronic configuration:- Shells: K L
Electrons arranged: 2 4


Exercise : Solution of Questions on page Number : 285


Answer 48:
(i) Lithium (Li), sodium (Na) and potassium (K) are three metals of group 1 having a valency of 1.
(ii) Calcium (Ca), strontium (Sr) and barium (Ba) are three metals of group 2 having a valency of 2.
(iii) Chlorine (Cl), bromine(Br) and iodine (I) are three non-metals of group 17 which are known as halogens, and have seven valence electrons.


Answer 49:
(a) (i) Gallium is the element which has taken the place of eka-aluminium.
(ii) Germanium is the element which has taken the place of eka-silicon.
(b) Gallium and Germanium are placed in the 4th period of the periodic table.
(c) Gallium belongs to group 13 and Germanium belongs to group 14 in the periodic table.
(d) Gallium is a metal and Germanium, a metalloid.
(e) Gallium, which belongs to group 13, has 3 valence electrons in its atom, and germanium which belongs to group 14 has 4.


Answer 50:
(a) Newlands’ law of octaves is illustrated by the above arrangement of elements.
(b) John Newlands is the scientist who proposed this classification of elements.
(c) This classification is compared with a characteristic of the musical scale because the repetition of the properties of elements is just like the repetition of the eighth note in an octave of music.
(d) Limitation: Newlands’ law of octaves could only be applied up to the element calcium.


Exercise : Solution of Questions on page Number : 302


Answer 1:
(i) The metallic property of elements decreases as we move horizontally from left to right.
(ii) The atomic size of elements decreases as we move horizontally from left to right.


Answer 2:
The tendency to gain electrons increases on moving from left to right in a period of the periodic table, because the metallic character decreases from left to right, and the non-metallic character increases. Also, a non-metal accepts electrons.


Answer 3:
The tendency to lose electrons decreases as we go from left to right across the periodic table.
This happens because the atomic size decreases from left to right, and the force of the nuclear attraction on the outermost electron increases. So, it requires more energy to lose electrons.


Exercise : Solution of Questions on page Number : 303


Answer 4:
(a) The chemical reactivity of alkali metals increases on going down in group 1 of the periodic table.
(b) The chemical reactivity of halogens(non-metals) decreases on going down in group 17 of the periodic table.


Answer 5:
All elements in the same column of the periodic table as boron have the same valency of 3.


Answer 6:
All elements in the same group of the periodic table as fluorine are halogens. Halogens are placed in group 17 which have seven valence electrons. Hence, their valency is 1.


Answer 7:
(a) The number of valence electrons in the atoms of first elements in a period is 1.
(b) The usual number of valence electrons in the atoms of the last element (inert gas) in a period is 8.


Answer 8:
False
On going down in a group of the periodic table, the number of valence electrons remains the same.


Answer 9:
The first element in every period of the periodic table have 1 valence electron. The elements are known as alkali metals.


Answer 10:
As we go from left to right in a period of the periodic table, the atomic radii decrease.


Answer 11:
The metallic character of elements increases as we go down in a group of the periodic table.


Answer 12:
i) In the first period of the periodic table, the number of valence electrons increases from 1 to 2, on moving from left to right.
ii) In the second period of the periodic table, the number of valence electrons increases from 1 to 8 on moving from left to right.


Answer 13:
The valency of the elements increases from 1(Na) to 4 (Si) and then decreases to 0 (Ar), on moving from left to right in the third period of the periodic table.


Answer 14:
The valency of elements remains the same on going down a group of the periodic table.


Answer 15:
(a) Sodium (Na) is the element which is in the first group and the third period.
(b) Fluorine (F) is the element which is in the seventeenth group and the second period.


Answer 16:

Element in the second period Li (3) Be (4) B (5) C (6)  N (7) O (8) F (9)  Ne(10)
Electronic configuration 2,1  2,2 2,3  2,4  2,5  2,6  2,7  2,8

Answer 17:
The elements in the increasing order of their atomic radii are as follows:
F<N<Be<Li.


Answer 18:
The elements in the increasing order of their metallic character are as follows:
Ga < Mg < Ca < K


Answer 19:
(i) Elements in the same group have equal valency.
(ii) The metallic character of the elements in a period decreases gradually on moving from left to right.


Answer 20:
(a) The horizontal rows in the periodic table are called periods.
(b) On moving across a period (right to left) in the periodic table, the atomic size of an atom increases.
(c) On moving from right to left in the second period, the number of valence electrons decreases.
(d) On moving down in a group of the periodic table, the metallic character of elements increases.
(e) The tendency to gain an electron decreases on moving down in a group of the periodic table.


Answer 21:
Electronic configuration of nitrogen (atomic number 7) :
Shells: K L
Electrons: 2 5
Electronic configuration of phosphorus (atomic number 15) :
Shells: K L M
Electrons: 2 8 5
The element nitrogen will be more electronegative because of the smaller size of its atom compared to phosphorus. Since, nitrogen has a smaller atomic radius than phosphorus, the attraction of its nucleus towards the incoming electron is more than phosphorus. Therefore, nitrogen accepts electrons more easily.


Answer 22:
(a) The number of valence electrons in element X is 2 because this element belongs to group 2.
(b) The valency of the group 2 element X is 2.
(c) Valence electrons of group 15:
Group number = Valence electrons + 10
15 = Valence electrons + 10
Valence electrons = 15 – 10 = 5
(d) Valency of the group 15 element Y is 3, that is, (8 – 5).


Answer 23:
(a) A period is a horizontal row of elements in a periodic table. As the atomic number increases from left to right in a periodic table, the size of the atom decreases.
(b) (i) On moving from left to right in a period of the periodic table, the chemical reactivity of the elements first decreases and then increases.
For example, in the third period elements comprising of Na, Mg, Al, Si, P, S and Cl, sodium (Na) is very reactive as it has one valence electron, and can therefore, lose its electron easily. The chemical reactivity gradually decreases as we go to aluminium (Al) and silicon (Si). This is because the number of valence electrons increases, making it difficult to lose electrons. Moving further right in the period towards non-metals, the chemical reactivity again gradually increases. Phosphorus (P) has 5 valence electrons and needs 3 electrons to complete its octet. Sulphur (S) has 6 valence electrons and needs 2 more electrons to complete its octet. Chlorine (Cl) has 7 valence electrons and needs only one more electron to complete its octet. As chlorine can easily accept an electron as compared to phosphorus and sulphur, the chemical reactivity increases from phosphorus to chlorine.
(ii) On moving from left to right in a period of the periodic table, the basic nature of oxides decreases and their acidic nature increases. For example, taking the third period elements again, the oxides of sodium are highly basic whereas those of chlorine are highly acidic.


Answer 24:
(a) The size of the atoms decreases on moving from left to right in a periodic table. This happens because the number of electrons and protons also increases and the nucleus of the atom becomes more positively charged. The nucleus exerts greater force of attraction on the electrons and pulls them tightly. As the nucleus pulls the outermost electrons towards it, the size of the atom (or the atomic radius) decreases.
(b) The metallic (electropositive) character of the elements decreases as we move from left to right and the non-metallic (electronegative ) character increases in a period of the periodic table.


Exercise : Solution of Questions on page Number : 304


Answer 25:
(a)
(i) All the elements of a group have similar chemical properties because they have the same number of electrons in their outer most shell.
ii) All the elements of a period have different chemical properties because the number of valence electrons for each varies.
(b) The elements in the increasing order of atomic numbers in the period is X<Z<Y. On moving from left to right in a period, the atomic number increases. Hence, the size of the atomic radius decreases.


Answer 26:
(a) The electropositive character of the elements increases on going down in a group of the periodic table.
(b) (i) In a group, all the elements will have the same valency because all the electrons have the same number of electrons in their outermost shell.
(ii) In a period, different elements have different valencies. On moving from left to right in a period, the valency of the elements increases from 1 to 4 and then decreases to 0.


Answer 27:
(a)

Group I Electronic configuration Group II Electronic configuration
Li (Lithium) 2, 1 Be (Beryllium) 2,2
Na (Sodium) 2, 8, 1 Mg (Magnesium) 2,8,2
K (Potasium) 2, 8, 8, 1 Ca (Calcium) 2,8,8,2

(b) (i) Elements having the same number of valence electrons in their atoms will be chemically similar because the chemical properties of an element depend on the number of electrons present in the outermost shell (valence electrons) of an atom of the element.
(ii) The first element of each period has only one outermost electron. For example, if an element has an electronic configuration of (2,1), we understand that the element belongs to group 1.


Answer 28:
(a) The usual number of valence electrons of group 18 elements is 8. Therefore, the valency of the group 18 elements is: 8-8=0.
(b) On moving down in a group of the periodic table, the number of valence electrons in the atoms of elements remains the same.


Answer 29:
(a) The last elements in the periods of the periodic table have their valence shells completely filled with electrons. They are known as noble or inert gases. Their valency is zero.
(b) The number of elements in (a) 1st period is 2 and (b) 3rd period is 8 in the modern periodic table.


Answer 30:
(a) The atomic size increases on moving from top to bottom in a group of the periodic table. This is because, on moving down in a group, new shells of electrons are added.
(b) Lithium, sodium and potassium are all metals which belong to group 1, and have 1 valence electron in their atoms. They all have the same electronic configuration. Therefore, each element shows similar chemical properties when reacting with water and liberates hydrogen gas.


Answer 31:
(a) In group 1 of the periodic table, the tendency to lose electrons increases on moving from top to bottom. This happens because an electron shell gets added at every stage and, the atomic radius increases. As the size of the atom increases, the distance between the valence electrons and the nucleus increases. This reduces the nuclear attraction force, causing the atom to lose its valence electrons easily, and form cations.
(b) In group 17 of the periodic table, the tendency to gain electrons decreases on moving from top to bottom. On moving down in a group, an electron shell gets added at every stage. As a result, the atomic radius increases. Since the size of the atom goes on increasing, it becomes difficult for the nucleus to attract the incoming electron, due to which the atom cannot form anions easily.


Answer 32:
(a) The size of atoms decreases progressively, on moving from sodium (Na) to chlorine (Cl) in the third period because the atomic number of the elements increases. Likewise, the number of electrons and protons in the atoms also increases. As the positive charges on the nucleus increase, the outermost electrons are held more tightly by the nucleus. Therefore, the size of the atom decreases.
(b) The atoms of helium and neon have their valence shells completely filled with electrons. The number of valence electrons is 8. So, they do not show any reactivity.


Answer 33:
(a) In the modern periodic table, elements are arranged according to their increasing atomic number. The atomic numbers of cobalt (Co) and nickel (Ni) are 27 and 28, respectively. Therefore, although cobalt (Co) has higher atomic mass, it appears before nickel (Ni) in the periodic table.
(b) Mendeleev’s periodic law could not assign a fixed position to hydrogen in the periodic table because hydrogen resembled both alkali metals (Group 1) and halogens (Group 17) in some of its properties. Hydrogen reacts with metals to form ionic compounds called hydrides and also with non-metals to form covalent compounds.


Answer 34:
(a) Periods are the horizontal rows of elements and Groups, the vertical columns of elements in the periodic table.
Characteristics of groups:
1. Elements of the same group have the same number of valence electrons.
2. Elements of the same group exhibit similar chemical properties as they have the same valency.
Characteristics of periods:
1. Elements of the same period have the same number of shells.
2. The valency of elements in the same period changes, and therefore, the elements in a period exhibit different chemical properties.
(b) The size of the atom increases on moving from top to bottom in the same group because at every step, new shells of electrons are added.
In a period, the size of the atom decreases on moving from left to right. This is because the atomic number of the elements increases on moving from left to right. Thereby, the number of electrons and protons also increases. Due to the increment in the number of protons, the nuclear charge also increases and the electrons get held more tightly to the nucleus.
(c) When we move vertically down from Li to Fr:
(i) The size of the atom increases.
(ii) The metallic character of the elements increases.
(d) On moving from top to bottom in a group , the size of the atoms and the metallic character of the elements increases.
(e) Periods have elements with consecutive atomic numbers.


Exercise : Solution of Questions on page Number : 305


Answer 35:
(a) The first period has only two elements because the first shell (K shell) can take a maximum of only two electrons. The second period has eight elements because the second shell (L shell) can accommodate a maximum of 8 electrons. The number of elements in a period depends upon the number of shells.
(b) The properties of an element depends on the number of valence electrons present in an atom of the element. Elements of a particular group have similar electronic configurations and the same number of valence electrons in their atoms. Hence, all the elements belonging to a certain group show similar properties. However, the electronic configurations of elements change on moving from left to right in a periodic table. Different groups have different valencies. Therefore, elements in different groups show different properties.
(c)

 Elements Least atomic radius Chemically least reactive
(i) F, Cl, Br F Br
(ii)Li, Na, K Li Li

(d) Fluorine and chlorine have a valency of 1 and show similar chemical properties. So, they are placed in the same group.
(e) The merits of the Modern Periodic Table of Elements are as follows:
1. It is based on the atomic number of elements.
2. The modern periodic table clearly explains the reason behind elements showing similar properties in a group and dissimilar properties in different groups.
3. This also explains the reason for the periodicity in the properties of elements.
4. The modern periodic table explains the reason for a repetition in the properties of elements after 2,8,18 and 32 elements.
5. In the modern periodic table, there are no anomalies in the arrangement of elements.


Answer 36:
(a) A group is a vertical column of elements in the periodic table.
In a group:
(i) The greatest metallic character is expected by elements in the lowest part of the group, as the metallic (electropositive) character increases on going down in a group.
(ii) The largest atomic size is found at the bottom of the group, as the atomic size increases on moving down in a group of the periodic table.
(b) Let us explain the properties by taking the example of sodium (Na) in group 1 and chlorine (Cl) in group 17:

Properties Group 1 Group 17
i) Atomic number Na, atomic number is 11 Cl, atomic number is 17
ii) Size of the atom Na has a smaller size than Cl Cl has a larger atomic size comparatively
iii) Metallic character Na is a metal, shows more metallic character Cl is non-metallic
 iv) Valence electron Number of valence electrons is 1 for Na metal Number of valence electrons is 7 for Cl
v)Electronegativity/ Electropositivity Na is electropositive in nature Cl is electronegative in nature
vi) Chemical reactivity Na is basic in nature Cl is acidic in nature

(c) The number of valence electrons in atoms determines which element will be the first and which the last in a period of the periodic table.
(d) The electronic configurations are repeated because 2, 8, 18 and 32 are the maximum number of electrons that can be placed in the four atomic shells, namely, K, L, M and N. The properties of the elements are repeated after 2, 8, 18 and 32 because the electronic configurations of elements are repeated after 2, 8, 18 and 32.
(e) Advantages of the periodic table:
(i) The study of elements and its properties is made easy.
(ii) By knowing the position of the element in the periodic table, prediction of the type of compounds that the elements form becomes easy.
(iii) The study of chemistry has been made easy in schools and colleges by using the periodic table chart as a teaching-aid.


Answer 37:
(c) The atoms lose their electrons more easily.
On moving from left to right in the periodic table, the tendency of an atom to lose electrons decreases.


Answer 38:
(c) 14th group
Group number = 10 + 4
= 14
Hence, the element X is placed in the 14th group.


Answer 39:
(b) 4th period.
Electronic configuration : K L M N
2 8 8 2
Hence, from the electronic configuration, it is clear that the outermost electron goes into the 4th shell. So, the element would be placed in the 4th period.


Answer 40:
(b) B, C, D:
B, C and D, with atomic numbers 3,7 and 10 respectively, belong to the same period of the periodic table. This is because the elements B, C and D have the same valence shell (L shell).


Answer 41:
(c) A and C
Electronic configuration of A (9) : K L
2 7
Electronic configuration of C (17): K L M
2 8 7
Group number of A and B = valence electron + 10
= 7+10 = 17.


Answer 42:
(c) K
K would lose an electron easily as it is a group 1 metal whose atomic number is greater than that of Na, which also belongs to group 1. Mg and Ca are group 2 metals and the tendency to lose electrons decreases on moving from left to right in a period of periodic table.


Answer 43:
(b) F
F is the element that does not lose an electron easily, as it is a non-metal (halogen) belonging to group 17 of the periodic table.


Answer 44:
(c) Group 18.
The element with an electronic configuration of (2, 8) belongs to group 18 (noble or inert gases) of the periodic table, as the elements in this group have their valence shell completely filled with electrons.
Group number = valence shell + 10
= 8+10 = 18.


Answer 45:
(b) Group 14.
Carbon is the essential constituent of all organic compounds and belongs to Group 14 of the modern periodic table.
Electronic configuration of carbon (6): K L
2 4
Group number = valence shell+10
= 4+10 = 14.


Answer 46:
(c) L shell.
L is the valence shell for elements of the second period of the modern periodic table, since the period number is decided by the number of valence shells.


Answer 47:
(b) P.
Phosphorus (P) has the maximum number of valence electrons, equal to 5.
Electronic configuration of P (15): K L M
2  8  5
Sodium (Na) has 1, Silicon has 4 and Aluminium (Al) has 3.


Answer 48:
(d) F, O, N
The increasing order of the atomic radii of oxygen, fluorine and nitrogen is fluorine, oxygen and nitrogen. This is because the atomic radius decreases from left to right in a period.


Exercise : Solution of Questions on page Number : 306


Answer 49:
(c) K.
Potassium (K) with an atomic number of 19 has the largest atomic radius. This is because sodium and potassium are elements of group 1. On moving from top to bottom in a group, the atomic radius increases. Magnesium and calcium are elements of group 2. The size of the atomic radius decreases on moving from left to right in a period of the periodic table


Answer 50:
(d) (ii) and (iv).
Isotopes of an element have the same atomic number and the same chemical properties.


Answer 51:
(c) Ea2 O3
The correct formula for the oxide of eka-aluminium, as predicted by Mendeleev, is Ea2O3.


Answer 52:
(b) 16.
The element which can form an acidic oxide should be the one with atomic number 16. Sulphur has an atomic number of 16 and sulphur oxide is acidic.
Note: Option (a) 6: carbon also forms an acidic oxide but the acidic strength of the oxides of sulphur (16) is greater than that of carbon (6).


Answer 53:
(d) 19.
Potassium (K) forms a basic oxide and has an atomic number of 19. The atomic numbers of 18,17 and 14 correspond to acidic oxides.


Answer 54:
(c) Valence electrons
The number of valence electrons does not increase while moving down the group in the periodic table.


Answer 55:
(b) Decreases.
The size of the atom decreases on moving from left to right in a period of the periodic table. With an increment in the atomic number, both the proton number and the nuclear positive charge increase. As the nucleus attracts the outermost electron, the size of the atom decreases.


Answer 56:
(d) Be, Mg, Ca.
Be, Mg and Ca are elements of group 2, written in order of their increasing metallic character. On moving down in the group, the metallic character increases.


Answer 57:
(i) In order to determine the group of the elements, let us first write down the electronic configuration of the elements from their atomic numbers.
Element Atomic Number Electronic Configuration
Shells: K L
X 2 2
Y 6 2 4
Z 10 2 8
In this case, X and Z are in the same group. Generally, elements which contain the same number of valence electrons belong to the same group of the periodic table. However, here, the element X (2) fills its outermost shell with two electrons and Z (10), with 8 electrons. Both belong to group 18 (noble gas).
(ii) The elements which have the same valence shell belong to the same period in the periodic table. We can see from the electronic configurations of Y and Z , that they have the same valence shell (L). Therefore, elements Y and Z belong to the same period in the periodic table.


Answer 58:
(a) The atomic number of the atom is 9 (electronic configuration 2, 7), since it is the sum total of the electrons present in an atom.
(b) The atom would be chemically similar to 17Cl (electronic configuration 2,8,7).
(c) They are similar because both have the same number of valence electrons (7) in their atoms.


Answer 59:
(i) 18Ar and 2He are very stable elements, as their valance shells are completely filled. They belong to the group of inert or noble gases.
(ii) 20Ca and 4Be are the two elements that belong to group 2 of the periodic table. This is because their valency is 2 and they are electropositive.
(iii) 8O (2,6) and 16S (2,8,6) are the two elements that belong to group 16 of the periodic table.
Group number = Valence shell + 10
= 6 + 10 = 16


Answer 60:
(a) Mg has a bigger size than Cl because the atomic size decreases on moving from left to right in a period of the periodic table. Mg and Cl belong to group 3 of the periodic table.
(b) K has a bigger size than Na because the atomic size increases on going down in a group of the periodic table. Na and K  belong to group 1 of the periodic table.


Answer 61:
(i) Element C belongs to group 18.
Electronic configuration of element C (10) = 2,8
Therefore, group number = valence shell + 10
= 8+10 = 18.
(ii) Element B belongs to group 15.
Electronic configuration of element B (15) = 2,8,5
Therefore, group number = valence shell + 10
= 5+10 = 15.
(iii) Element A belongs to group 13.
Electronic configuration of element A (5) = 2,3
Therefore, group number = valence shell + 10
= 3+10 = 13.
(iv) The elements A, B and C all belong to period 2 because all three have the same valence shell, i.e, L.


Answer 62:
(a) The element X has 2 valence electrons because it belongs to group 2.
(b) Since the number of valence electrons and the valency of group 2 elements is 2, the valency of element X is 2.
(c) Group 2 is on the left side of the periodic table which consists of metals. Therefore X is a metal.
(d) The name of the element is Magnesium (Mg), since X belongs to group 2 and the 3rd period.


Exercise : Solution of Questions on page Number : 307


Answer 63:
(i) Element a has a bigger atom. In the periodic table, the atomic radius of the element decreases on moving from left to right in a period.
(ii) k has a higher valency.
Electronic configuration of k : K L M
2 8 3
Therefore, valency for k = valence electron of K = 3
Electronic configuration of o: K L M
2 8 7
Therefore, valency for o = 8 – valence electron of o
= 8 – 7 = 1
(iii) The element i is more metallic because i belongs to the left side and group 1 of the periodic table. On moving from left to right in a period, the metallic character decreases.
(iv) The element g is more non-metallic, because the non-metallic character increases on moving from left to right in a period.
(v) The elements b or j are metals with a valency of 2 as they have 2 valence electrons and are placed on the left side of periodic table.
(vi) The elements f or n are metals with a valency of 2 .
Electronic configuration of f : K L
2 6
Valency of f = 8 – valence electron
= 8 – 6 = 2
Electronic configuration of n : K L M
2 8 6
Valency of n = 8 – valence electron
= 8 – 6 = 2.


Answer 64:
(a) An element X in group 2 has a valency of 2. We know that chlorine has 7 valence electrons. Therefore, the valency of chlorine Cl is 1, that is, (8 – 7).
Now, when one atom of X combines with two atoms of Cl, an ionic compound of the formula XCl2 is formed.
(b) We know that oxygen has 6 valence electrons and needs 2 more electrons to complete its octet. Therefore, the valency of oxygen O is 2, that is, (8 – 6). Both X and O have the same valency. When one atom of X combines with one atom of O, an ionic compound of the formula XO is formed.


Answer 65:
(i) The element Y is a non-metal as all the elements of group 16 are non-metals.
(ii) The number of valence electrons of the element = Group number – 10
= 16-10 = 6.
(iii) Valency of Y = 8- number of valence electrons
= 8 – 6 = 2.
(iv) The name of the element is oxygen as it is the first element in group 16.
(v) Sodium is a group 1 element with a valency of 1. The valency of Y is 2. Therefore, when two atoms of sodium (Na) combine with one atom of Y, the formula of the resulting ionic compound is Na2Y.


Answer 66:
(a) X belongs to Group 1. The mass number of element X is 40 and the number of neutrons in its atom is 21. In order to find out the group to which the element belongs, we need to know the atomic number of the element. Once we know the atomic number of the element, the electronic configuration and valency can be determined.
Atomic number = Mass number – Number of Neutrons.
Atomic number = 40-21 =19
The electronic configuration of X is 2,8,8,1. The number of valence electrons in X is 1. Therefore, the valency of element X is 1. We know that group 1 elements have a valency of 1.
(b) Y can be oxygen because oxygen has a valency of 2. As X is monovalent, Y has to be divalent in order to form the compound X2Y.


Answer 67:
(a) The element X is electrically conductive. Therefore, it is a metal. In the case of compound XO, both the elements are divalent, since the valency of oxygen is 2. Therefore, the valency of X should also be 2.
(b) We can conclude that X belongs to Group 2. Since the valency of the element X is 2 and it is showing electrical conductivity, therefore, it is a metal.
(c) The formula of the compound formed is XCl2, since chlorine is a group 17 element with 7 valence electrons. It has a valency of 1. Therefore, when one atom of X combines with two atoms of chlorine (Cl), an ionic compound of the formula XCl2 is formed.


Answer 68:
(a) Element A has an atomic number of 6, so its electronic configuration is 2,4 (having 4 valence electrons).
Therefore, the group number for element A = valence shell + 10
= 4+10 =14.
Hence, the group number for element A is 14.
Element B has 17 electrons in its neutral atom. Its electronic configuration is 2,8,7.
Therefore, the group number of element B = valence shell + 10
= 7 + 10 = 17.
Hence, the group number for element B is 17.(b) When two non-metals A and B combine, a covalent bond is formed, since group 14 elements and group 17 elements are all non-metallic in nature.
(c) When one atom of A combines with four atoms of B, a covalent compound with the formula AB4 is formed. This is because the element A has a valency of 4 and element B has a valency of 1.


Answer 69:
(iii) C and D will produce a covalent bond.
The elements C and D belong to group 14 and group 17, respectively. We know that groups 14 and 17 consist of elements that are non-metallic in nature. When two non-metals combine, sharing of electrons takes place, which further results in the formation of a covalent bond.


Answer 70:
(a) An element X from group 2 has a valency of 2. An element Y from group 16 has a valency of 2. As both the elements are divalent, an atom of X combines with an atom of Y to form a compound with the formula XY.
(b) The nature of the bond in the compound is ionic, since the element X is a metal as it is from group 2. The element Y is a non-metal as it is from group 16. When a metal combines with a non-metal, a transfer of electrons takes place and an ionic bond is formed.


Answer 71:
The elements in the first group of the periodic table have a valency of 1. As the metal X is in the first group, its valency is 1. We know that the valency of oxygen is 2 because it needs two electrons to complete its outer shell of 8. From this, we can conclude that two atoms of metal X will combine with one atom of oxygen to form an oxide X2O. Therefore, the formula of the oxide of metal X is X2O.


Answer 72:
(i) The resulting compound is a covalent compound since the elements of group 14 and group 16 are non-metals. Therefore, when a non-metal A from group 14 combines with a non-metal B from group 16, electrons are shared between their atoms and  a covalent bond is formed.
(ii) Element A belongs to group 14 and has 4 valence electrons, and element B belongs to group 16 and has 6 valence electrons. Therefore, when 1 atom of element A combines with 2 atoms of element B, sharing of electrons takes place and the resultant compound will have the formula AB2.


Answer 73:
(a) The resulting compound is an ionic compound since the elements of group 2 are metals and those of group 17, non-metals. When a metal reacts with a non-metal, electrons get transferred from the metallic atoms to the non-metallic atoms, and an ionic bond is formed.
(b) Yes, it will conduct electricity because an ionic compund has formed.
(c) Element X belongs to group 2 and its valency is 2. Element Y is from group 17 and its valency is 1. So, one atom of X combines with two atoms of Y to give a compound of the formula XY2.
(d) The valency of the element X is 2 because it belongs to group 2.
(e) Group 17 elements have seven valence electrons in their atoms.
Group number = valence electron+10
Valence electron = Group number – 10
= 17-10 = 7.


Exercise : Solution of Questions on page Number : 308


Answer 74:
(i) The letter ‘d‘ represents an alkali metal. This is because group 1 elements of the periodic table, except hydrogen, are all alkali metals having 1 valence electron.
(ii) The letter ‘c‘ represents a noble gas. All the group 18 elements are noble gases or inert gases whose valence shells are completely filled with electrons.
(iii) The letter ‘e‘ represents a halogen. Group 17 elements are known as halogens and they have 7 valence electrons.
(iv) A covalent bond is formed between ‘a‘ and ‘e’. ‘a‘, the top most element represents hydrogen which is the only non-metal in group 1. Therefore, when a non-metal ‘a‘ forms a compound with another non-metal ‘e‘, sharing of electrons takes place resulting in the formation of a covalent bond.
(v) An ionic bond is formed between ‘d‘ and ‘e‘. When a metal reacts with a non-metal, a transfer of electrons takes place from the metal to the non-metal and results in the formation of an ionic bond.


Answer 75:
(a) The element B combines with the element C to form a covalent comp1ound. The elements B and C belong to group 14 and group 17, respectively, which are non-metals. Therefore, when two non-metals react with each other, sharing of electrons takes place, which further results in the formation of a covalent bond.
(b) The element A combines with the element C to form an ionic compound. The element A belonging to group 1 is a metal. The element C belonging to group 17 is a non-metal. Therefore, when a metal combines with a non-metal, electrons are transferred from the metal to the non-metal, resulting in the formation of an ionic bond.


Answer 76:
Argon is a neutral atom in the periodic table and has the same number of electrons as Kand Cl. The atomic number of argon is 18. It has 18 electrons. Potassium (K) has 19 electrons but because it loses one electron, it becomes K+ and has 18 electrons. On the other hand, chlorine (Cl)  has 17 electrons but because it gains one electron, it forms Cl– and has 18 electrons.


Answer 77:
(a) The elements belong to the third period of the periodic table, and have the same number of electron shells but different number of electrons in their outermost shells. The number of valence electrons in these elements increases from 1 to 8, on moving from left to right in this period.
(b) When metal B combines with a non-metal F, an ionic compound is formed due to the transfer of electrons from the metal to the non-metal. This is because in the third period, elements A, B and C are metals, D is a metalliod, and E, F, G and H are non-metals.
(c) A and B are definitely metals because in the 3rd period, at the extreme left, only metals are placed in the periodic table .
(d) The element H is most likely to be found in a gaseous state at room temperature. This is because the last group elements in the periodic table are all in a gaseous state at room tempearture.
(e) If the number of valence electrons of element C is 3 and those of element G is 7, then 1 atom of element C will combine with 3 atoms of element G to form a compound with the formula CG3.


Answer 78:
Sodium (Na) and Potassium (K) are the two very reactive metals belonging to group 1 of the periodic table.
Let us see the formation of an ionic compound, namely Sodium Fluoride (NaF), when Sodium (Group 1 element) reacts with Fluorine ( Group 17 element). Sodium has 1 electron in its outermost shell whereas Fluorine has 7 electrons. When Sodium and Fluorine react, a transfer of electrons takes place from Sodium to Fluorine and an ionic compound is formed.
The following drawing clearly explains the reaction of Sodium and Fluorine:

Halogens are the non-metal elements which belong to group 17 of the periodic table. Metals are electropositive and have a tendency to lose electrons while non-metals are electronegative and have a tendency to accept electrons. Therefore, when a metal reacts with a non-metal, a transfer of electrons takes place, resulting in the formation of an ionic bond. The resulting compound is called an ionic compound.
Physical properties of Ionic Compounds:
1. Ionic compounds are solids.
2. They have high melting and boiling points.
3. They are generally soluble in water.
4. They conduct electricity in molten state.


Answer 79:
(a) The non-metal A which is an important constituent of our food and most of the fuels around us is carbon. Carbon combines with oxygen in two ways to form two oxides. When one atom of carbon combines with one atom of oxygen, carbon monoxide (CO), a poisonous gas is formed. And when one atom of carbon combines with two atoms of oxygen, carbon dioxide (CO2) is formed which causes global warming.
(b) Carbon belongs to group 14 of the periodic table.
(c) Silicon (Si) is another element which is placed in the same group 14 of the periodic table.


Answer 80:
(a) Element X is Nitrogen gas (N2) since the largest constituent of air is nitrogen. When nitrogen reacts with hydrogen in the presence of iron as a catalyst, ammonia (NH3) is formed. Therefore, Y is ammonia (NH3). When ammonia reacts with sulpuric acid, ammonium sulphate is formed which is used as a chemical fertilizer. Therefore, Z is ammonium sulphate (NH4)2SO4.
(b) Nitrogen belongs to group 15 of the periodic table.
(c) Nitrogen is placed in the second period of the periodic table.
(d) Carbon (C) is placed just before nitrogen in the period.1
(e) Oxygen (O) is placed just after nitrogen in the period.


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