(a) As different elements were being discovered, scientists gathered more information about the properties of these elements. It was observed that it was difficult to organise all the information or properties of these elements. So scientists started discovering some pattern in their properties to classify all the known elements to make their study easier.
(b) Atomic mass and similarity of chemical properties (compounds formed by elements with oxygen and hydrogen) were the two criteria used by Mendeleev in his Periodic Table.
(c) Mendeleev left some gaps in his Periodic Table as he predicted the existence of some elements that had not been discovered at that time.
(d) Noble gases like helium, neon, argon etc. were not mentioned in Mendeleevs Periodic Table because these gases were discovered later as they are very inert and present in extremely low concentrations in our atmosphere. After the discovery of noble gases they could be placed in a new group without disturbing the existing order of the Periodic Table.
(e) The two isotopes of chlorine CI-35 and CI-37 would be placed in different slots of Mendeleevs Periodic Table as both have different atomic masses. But the two isotopes have same chemical properties so would also be placed in the same slot. This is a contradiction. Thus position of isotopes of an element is not defined in Mendeleevs Periodic Table.

(a) Gaps were left for undiscovered elements in the Mendeleevs Periodic Table.
(b) (i) Position of hydrogen was not justified.
   (ii) Increasing order of atomic mass could not be maintained.
  (iii) Isotopes have similar chemical properties but different atomic masses, they cannot be given separate places.
(c) Number of shells remains the same, number of valence electrons goes on increasing from left to right in a period till octet is complete.

(1) Valency. Elements belonging to same group have same number of valence electrons and thus same valency.
Valency in a particular period from left to right first increases as positive valency and then decreases as negative valency.
Example, in elements of 2nd period:
Li has 1⁺ valency, then Be2⁺ , C4⁺ covalency, N3- valeI!cy, then 0^2- and F^(-) valency.
(2) The atomic size or atomic radius increases as we move down in a group and it decreases as we move from left to right in a period. Atomic size increases down a group due to increase in the number of shells. Atomic size decreases along a period due to increase in the nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
(3) Metallic and non -metallic properties. In the modern periodic table metals are on the left size and non-metals are on the right size. A zigzag line of metalloids separates metals from non-metals. Metallic character decreases from left to right in a period and increases while going down in a group. Non-metallic character increases from left to right in a period due to increase in the electron-negativity and this character decreases from topo to bottom in a group due to decrease in the electro-negativity of atoms while going down in a group.

When the various elements were arranged in order of their increasing atomic masses, elements having similar properties fell under on another in the same vertical column called the group. He further observed that repetition of elements with similar physical and chemical properties occurred after certain regular intervals. On the basis of these observations, Mendeleev proposed his periodic law which states that the physical and chemical properties of elements are a periodic function of their atomic masses.

At the time when Mendeleev's gave his periodic table, noble gases such as helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) were not known. When these gases were discovered much later, these were placed in a new group called the zero group without disturbing the original order.

(a) Since the element X of group 15 exists as a diatomic molecule, combines with hydrogen at 773 K in presence of a catalyst to form ammonia which has a characteristic smell, therefore the element X is nitrogen (N).
   \(\underset { \quad Nitrogen\\ (diatomicmolecule) }{ { N }_{ 2 } } +\underset { Hydrogen }{ { 3H }_{ 2 } } \underrightarrow { 773k/Catalyst } \underset { Ammonia\\ (pungentsmell) }{ { 2NH }_{ 3 } } \)
The atomic number of element X is 7, so its electronic configuration i 2,5. Thus it has five valence electrons.
(b) Nitrogen has 5 valence electrons. Therefore, it needs 3 more electrons to complete its octet. To do so, it shares three of its electrons with three electrons of the other nitrogen atom to form a diatomic molecule of N₂ gas. Thus, three covalent bonds are formed between two nitrogen atoms and each nitrogen atom is left with one
lone pair of electrons.

(c) Nitrogen atom has 5 valence electrons. Therefore it needs 3 more electrons to complete its octet. To do so, it shares three of its electrons with one electron each of three hydrogen atoms to form a molecule of ammonia .

In NH₃ molecule, there are three N - H single covalent bond and one lone pair of electrons on the nitrogen atom.

(a) The element A which is a yellow solid at room temperature and shows catenation and allotropy is sulphur (S).
(b) The atomic number of sulphur is 16. Therefore, its electronic configuration is 2, 8, 6.
(c) \(\underset { Ferroussulphate }{ { 2FeSO }_{ 4 } } \underrightarrow { Heat } \underset { Ferricoxide }{ { Fe }_{ 2 }{ O }_{ 3 } } +\underset { Sulphurdioxide }{ { SO }_{ 2 } } +\underset { Sulphurtrioxide }{ { SO }_{ 3 } } \)
    Both SO₂ and SO₃ are major air pollutions.
(d) Since sulphur is a non-metal, therefore both S02 and S03 are acidic oxide since they dissolve in water to form the corresponding acids.
        \(\underset { Sulphurdioxide }{ { SO }_{ 2 } } +\underset { Water }{ { H }_{ 2 }O } \rightarrow \underset { Sulphurousacid }{ { H }_{ 2 }{ SO }_{ 3 } } \\ \underset { Ferroustrioxide }{ { SO }_{ 3 } } +\underset { Water }{ { H }_{ 2 }O } \rightarrow \underset { Sulphuricacid }{ { H }_{ 2 }{ SO }_{ 4 } } \)
(e) Since sulphur contains six valence electrons, therefore it lies in group 6 + 10 = 16.Further, since atomic number of sulphur (S) is 16. It lies in the 3rd period.

The names and symbols of elements having atomic numbers 3 - 9 are :

a) Lithium (Li)
b) Fluorine (F)
c) Fluorine (F)
d) Boron (B)
e) Carbon (C)

(a) Since the element lies in group 2, it must be an alkaline earth metalj Since it lies in the third period, it must be magnesium (Mg).
(b) Atomic number of Mg is 12, therefore, its electronic configuration is
    KLM
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(c) \(\underset { Magnesium }{ 2Mg(s) } +\underset { Oxygen }{ { O }_{ 2 }(g) } \underrightarrow { Heat } \underset { Magnesiumoxide }{ 2MgO(s) } \)
(d) \(\underset { Magnesiumoxide }{ MgO(s) } +\underset { Water }{ { H }_{ 2 }O(1) } \rightarrow \underset { Magnesiumhydroxide }{ { Mg(OH) }_{ 2 }(aq) } \)
(e)