(a) Atomic number of Nitrogen is 7 and electronic configuration is 2, 5. Atomic number of Phosphorus is 15 and electronic configuration is 2, 8, 5.
(b) Phosphorus will be more electronegative because phosphorous and nitrogen both are non-metals. Phosphorous is situated in the lower side than Nitrogen. In non-metals, as we go top to bottom the electronegativity is increased.

(a) The positions of certain elements were justified.
(b) The position of isotopes has been justified.
(c) The prediction of properties of elements.

(a) Newlands' Law of Octaves was applicable only up to calcium. After calcium the eighth element did not possess properties similar to that of the first.
(b) Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future.
(c) Newlands' Octaves contains only few elements having similar properties.

(a) They can lose electrons easily due to bigger size, energy required to remove electron is less.
(b) It is because of greater effective nulear charge on Mg, i.e. more number of protons attract more number of electrons than Na.
(c) 'F' can form F^- more easily than Cl due to smaller atomic size. F^- is more stable than Cl^- . Therefore, fluorine is more reactive than chlorine. 

Atomic mass as a physical property and nature and formulae of oxide and hydride formed, and chemical property was used by Mendeleev.
Observations :
(i) Increasing order of atomic weights could not be maintained while matching chemical properties. Chemical properties do not depend atomic mass.
(ii) Isotopes have different atomic mass but same chemical properties.

(a) There are seven periods in the modern periodic table.
(b) On moving from left to right in a period, the valency of elements increases from 1 to 4 and then decreases to 0.
(c) On moving from left to right in a period, the metallic characters of elements decreases as the electropositive character of elements decreases on moving from left to right in a period.
(d) All the elements in a group have same valency, because the number of valence electrons in a group is same.
(e) On going down in a group of the periodic table, the size of atoms increases because a new shell of electrons is added to the atoms at every step on moving down in a group.

(a) Lithium, Sodium and Potassium atoms have same number of electrons in their outermost shell and have same valency.
(b) Helium and Neon both have their outermost shell filled.

The criteria for Mendeleev periodic table was the use of atomic masses. Mendeleev arranged the elements in increasing order of their atomic masses and according to their chemical properties.

(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.

(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) 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.

(a) The properties of all elements in the boron family
      (i) All the elements show valency of 3.
(b) The properties of  all the elements in halogen family
    (i) All the elements possess seven electrons in the valence shell.