Electron Gain Enthalpy (EGE):-It is the amount of energy released a (absorbed in some few cases) when an extra electron is added to isolated gaseous neutral atom to form anion.

X + e^Θ → X^- + EGE

First Electron gain enthalpy is an Any Atom be position or can be negative but

Second Electron gain enthalpy is always positive because after accepling 1e^Θ,

Effective nuclear charge decreases, size increases and due to this e^Θ will experience more repulsion than alliation example:-

O + 1 e^Θ → O- + 141 KJ / mole  ∆ eg₁ H = -1negative)

O^- + 1e^-  → O^2-     (∆ eg₂ H = positive)

Important:-  Though Second Electron Gain Enthalpy of Oxygen is highly C12₊ Positive still it form compounds in O^2- form because of very high lattice energy, released by O^2- ion while making compound.

Lattice Energy:- It is the amount of energy released when 1 mole of ionic compound is formed its constituents ions in the gaseous slate

M⁺_(g) + X^-_(g) → M x_(s) + Energy (Lattice Energy).

Electron Gain Enthalpy of noble gases is positive because Noble gases have fully filled stable configuration to add I extra electron, noble gases have to make one more shell which required lot of energy, so electron gain enthalpy is always positive.

Halogens have more negative electron gain enthalpy because valence shell e^Θ configuration of halogens are ns² np⁵ and due to this they require one more e^Θ to complete their orbit.  As a result, they have Strong tendency to accept an additional election and hence their Electron Gain Enthalpy is more negative.

Factors affecting Electron Gain Enthalpy:-

(1) Size:- Greater the size, more negative is Electron Gain Enthalpy

(2) Electron Gain Enthalpy:- Greater the Electron Gain Enthalpy , more negative is electron gain enthalpy

(3) Stability:- If any species is stable, then lesser negative will be electron gain enthalpy.

Trend of electron gain enthalpy  along period and group

Periods	Groups
Trend:- In a period, electron gain enthalpy become more negative Reason:- Size increases	In a group, electron gain enthalpy becomes less negative size decreases

 

Important Exceptions:-

Though size of F is louver than that of CL still its electron gain enthalpy is less negative than CL because of extra small size of F , the increasing electron experiences inter electronic repulsions due to which some energy is lost in over coming repulsion. Hence lesser energy is released but in case of CL, size is sufficiently larger to accommodate / more e^Θ easily so more negative is its electron gain enthalpy

Similarly, Electron gain enthalpy of   S > O (Negative)

                    Electron gain enthalpy of   p > N (Negative)

Order of increasing more negative Electron gain enthalpy is

                   N < p < O < S < F < CL

             (Most negative Electron gain enthalpy)