The energy which is released by an atom in gaining an electron from outside atom or ion to form negative ion (or anion) is called electron gain enthalpy (\(\triangle\)_eg H). Unit of electron gain enthalpy is kJ/mol. In some cases, like in noble gas, atoms do not have any attraction to gain an electron. In that case energy has to be supplied.
For example :
Ne(g) + e^- ⟶ Ne^-(g)
\(\triangle\)_egH = +116 kJ mol^-1

Dobereiner arranged certain elements with similar properties in groups of three in such a way that the atomic mass of the middle element was nearly the same as the average atomic masses of the first and third elements.
For example:
Triad:       lithium  sodium Potassium
Atomic mass 7        23         39
Atomic mass of Na =\(\frac{39+7}{2}=23\)
Triad:        Chlorine Bromine Lodine
Atomic mass: 35.5        80         127
Atomic mass of Br = \(\frac{127+35.5}{2}=81.25\)

(i) Atomic size. With the increase in the atomic size, the number of electron shells increases. Therefore, the force that binds the electrons with the nucleus decreases. Thus, the ionization enthalpy decreases with increase in atomic size.
(ii) Nuclear charge. As the magnitude of the positive charge on the nucleus of an atom increases, the attraction with the electrons also increases. Therefore, the ionization enthalpy increases with the increase in the magnitude of the nuclear charge.
(iii) Screening or shielding effect. Greater the magnitude of the screening effect, less will be the value of ionization enthalpy or potential.

Elements in the long form of the periodic table have been divided into four blocks i.e., s, p, d and f This division is based upon the name of the orbital which receives the last electron. General electronic configuration of
s-block elements: ns^{1- 2} where n = 2 - 7
p-block elements: ns² np^{1 - 6} where n = 2 - 6
d-block elements: (n -1) d^1-10 ns^{0 - 2} where n = 4 - 7
f-block elements: (n - 2) f^{0 -14} (n -1)d^0-1 ns² where n = 6 - 7

The elements of s and p-block are collectively called representative or main group elements. These include elements of group I (alkali metals), group 2 (alkaline earth metals).

Non-metallic character across a period (left to right) increase but on moving down the group it decreases. So, the increasing order of non0metalic character is P.

Mg is more metallic due to lower ionisation energy.

(a) Isoelectronic species have the same number of electrons but different nuclear charges.
In case of isoelectronic species, as the nuclear charge increase, their size decreases.
F^- has 10 electrons (9 + 1)
(b) Ar has 18 electrons.
(c) Isoelectronic species have the same number of electrons but different nuclear charges.
In case of isoelectronic species, as the nuclear charge increase, their size decreases.
Mg²⁺ has 10 electrons (37 - 1).
(d) Isoelectronic species have the same number of electrons but different nuclear charges.
In case of isoelectronic species, as the nuclear charge increase, their size decreases.
Rb⁺ has 36 electrons (37-1)
N^3-,O^2-,Ne, Na⁺ and Al³⁺ are some species which are isoelectronic with F and Mg
P^3-,S^2-,Cl^-,K⁺ and Ca are some species which are isoelectronic to Ar.
Similarly Br^-, Kr and Sr²⁺ are isoelectronic with Rb.⁺

Electropositive character means tendency to give an electron to form cation. It varies directly with atomic radii. Thus, the correct order is Mg > fe > Cu.

First member of each group of representative elements ( i.e. s and p-block elements ) shows anomalous behaviour due to
(i) small size
(ii) high ionisation enthalpy
(iii) high electronegativity and
(iv) absence of d- orbitals.
For example in s-block elements, lithium shows anoimalous behaviour from rest of the alkali metals.
(i) Compounds of lithium have significant covalent character. While compounds of other alkali metals are predominantly ionic.
(ii) Lithium reacts with nitrogen to form lithium nitride while other alkali metals do not form nitrides.
In p-block elements, first member of each group has four orbitals, one 2s- and three 2p-orbitals in their valence shell. So, these elements show a maximum covalency of four ehile other members of the same group or different group show a maximum covalency beyond four due to availability of vacant d-orbitals.

No, the maximum number of elements which can be accommodated in the present set up of the long form of the periodic table is 118. Thereafter, filling of 8s-orbital shell begin which all accommodate only two electrons. After 8s-oorbitals, the filling of 5g-orbitals will begin. Since we do not have any provision for g-block elements in the present set up of the long form of periodic table, therefore, an element with atomic number 126, if discovered, cannot be accommodated in the present set up of the long form of periodic table.

The first inert gas which contains d electrons is krypton. Its atomic number is 36 and its electronic configuration is 1s² , 2s² , 2p⁶, 3s², 3p⁶ , 3d¹⁰ , 4s² , 4p⁶
Total number of d-electrons = 10
Total number of p-electrons = 6 + 6 + 6 = 18
Total number of s-electrons = 2 + 2 + 2 + 2 = 8
\(\therefore \) Difference in total number of p and s electrons = 18 - 8 = 10
Thus, the inert gas is krypton.