(a) 2, 8, 2
(b) Group 2
(c) 'M' is a metal
(d) 
MO is formula of its oxide

(a) X(17) : 2, 8, 7
(b) Valency: 1
It has atomic number 17 and therefore, electronic configuration will be 2, 8, 7. It can gain one electron to become stable. Therefore, its valency is equal to 1. It belongs to third period, as it has three shells. It belongs to group 17 because it has 7 valence electrons.

'X'
Mass Number X = 35
No. of neutrons = 18
Atomic number of X = Mass number - Number of neutrons = 35-18-=17
KLM
Thus, electronic configuration of X - 2, 8, 7
Group number = 17th  Period number = 3rd
Valency = 9-7=1

Electronic configuration of X,Y and Z will be:
X(9) : 2,7
Y(11) : 2,8,1
Z(17) : 2,8,7
X and Z will show similar chemical properties due to same number of valence electrons.

(a) It belongs to the 4th period because it has four energy shells and period number of an element is equal to the number of energy shells.
(b) It belongs to 2^nd group because it has 2 valence electrons and the group number of an element having up to two valence electrons is equal to the number of valence electrons.
Its valency is '2'. To acquire inert gas configuration 'X' either loses 2 electrons or gains 6 electrons. Since the shorter route is to lose 2 electrons for obtaining inert gas configuration, hence its valency is 2.

(a) (i) Valency : The combining capacity of an element is called its valency.
   (ii) Atomic size : The atomic size of an atom is the distance between the centre of the nucleus and outermost electron shell of an isolated atom.
(b) On moving from left to right along a period in the modern periodic table, the valency of elements increases from 1 to 4 and then decreases to zero.
On moving from left to right in a period of the periodic table, the size of the atoms decreases.

Since species X has 12 protons and 12 electrons, it is electrically neutral. Since species Y has 12 protons and 10 electrons, therefore, it has two units of positive charges. The electronic configurations of the two species are

Species X has three shells and species Y has two shells. Y is cation of X. So, the the radius of Y is less than X.

Hydrogen occupies a unique position in the modern periodic table because of the following reasons:
(i) Both hydrogen and halogens have similar outer electric configuration. Therefore some of the properties of hydrogen are similar to those of alkali metals and hence it can be placed in group 1 along with alkali metals.
(ii) Both hydrogen and halogens have similar outer electric configuration. Therefore some of the properties of hydrogen are similar to those of halogens and hence it can be placed in group 17 along with halogens.
(iii) In some properties, it differs from both hydrogen and halogens. For examples, the oxide of hydrogen (H ₂O) is neutral but the oxides of alkali metals (Na ₂O, K₂O) are basic while those of halogen (Cl₂O₇, Br₂O₅) are acidic.

SiO₂

The electronic configuration of the element A is 2, 8, 6. Therefore, it belongs to group 6A.

Lithium, Beryllium, Boron are the metals in Modern Periodic Table among the first ten elements.

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.

(i) Scandium
(ii) Germanium

(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.

(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.