Electron gain enthalpy is the energy released when a neutral isolated gaseous atom accepts the electron. Here, we will discuss how the ▲eg H varies in the periodic table.
What is electron gain enthalpy
It is the amount of energy released when a neutral gaseous atom accepts the extra electron to form anion.
X (g) + e- ➡️ X– (g) + ▲eg H
Greater the amount of energy released, higher will be the ▲eg H. For example ▲eg H for halogens is highly negative to attain the nearest noble gas configuration. But noble gases have positive▲eg H as they are already stable.
What is successive electron gain enthalpy
After the addition of one electron, the atom becomes negatively charged. But the addition of further electron in the negatively charged species results in electrostatic repulsion. Therefore, the second ▲eg H2 is positive. For example
O (g) + e- ➡️ O– (g) ▲eg H1 = -141KJ/mol ( Energy is released )
O– (g) + e- ➡️ O2- (g) ▲eg H2 = +780 KJ/mol ( Energy is absorbed )
Factors on which electron affinity depends
- Atomic radii
As the distance between nucleus and valence electron increases, the force of attraction between them decreases. Hence the electron gain enthalpy becomes less negative.
2. Nuclear charge
As the nuclear charge increases, force of attraction for the valence electron increases. ▲eg H becomes more negative.
3. Electronic configuration
Elements having half-filled and completely filled electronic configuration are very stable. Therefore, it is hard to add the electron in them. Hence, ▲eg H is highly positive for them.
Variation of ▲eg H in group
- As we move down the group in the periodic table, ▲eg H becomes less negative. For example it becomes less negative when we move from Cl to Br to I
- There are some exceptional behaviours. For example, O has less negative ▲eg H than S. Because of inter electronic repulsions present in O atom itself due to smaller size.
- Cl has most negative ▲eg H in the PT even compared to F
Variation of ▲eg H in period
▲eg H becomes more negative from left to right in PT
Check out the following post on variation of IE in periodic table here :
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