Electron Gain Enthalpy | Electron Affinity | Variation of EA in Periodic Table

Electron Gain Enthalpy (Electron Affinity) and their Variation in Periodic Table

Electron Gain Enthalpy MCQs

Electron Gain Enthalpy (Electron Affinity)

The minimum amount of energy released when an electron is added to an isolated gaseous atom so as to convert it into a negative ion.
During the addition of an electron, energy can either be released or absorbed.
Since it is energy so expressed in KJ/mol or in ev/atom.(1ev = 97.5KJ)
X_(s) → X_(g)
X_(g) + e → X_(g)⁻
Electron gain enthalpy is generally negative for all elements except few elements. For example, the electron gain enthalpy for halogens is highly negative because they can acquire the nearest noble gas configuration by accepting an extra electron.
In contrast, noble gases have large positive electron gain enthalpies because the extra electron has to be placed in the next higher energy level thereby producing highly unstable electronic configuration.
After the addition of one electron, the atom becomes negatively charged and the second electron is to be added to a negatively charged ion. But the addition of second electron is opposed by electrostatic repulsion and hence the energy has to be supplied for the addition of second electron. Thus the second electron gain enthalpy of an element is positive.
For example, when an electron is added to oxygen atom to form O^– ion, energy is released. But when another electron is added to O^– ion to form O^–2 ion, energy is absorbed to overcome the strong electrostatic repulsion between the negatively charged O^– ion and the second electron being added.
Electron Gain Enthalpy and Electron Affinity are related to each other by the given formula.
Electron gain enthalpy = electron affinity – 5/2 RT
where, R= universal gas constant and T= temperature in Kelvin scale

Factors Affecting the Electron Gain Enthalpy or Electron Affinity

1. Atomic size
2. Nuclear charge
3. Electronic Configuration

Atomic size

As the size of an atom increases, the distance between its nucleus and the incoming electron also increases and electron gain enthalpy becomes less negative.

Nuclear charge

With the increase in nuclear charge, force of attraction between the nucleus and the incoming electron increases and thus electron gain enthalpy becomes more negative.

Electronic Configuration

The atoms with symmetrical configuration (having fully filled or half filled orbitals in the same sub-shell) do not have any urge to take up extra electrons because their configuration will become unstable.
In that case the energy will be needed and electron gain enthalpy will be positive. For example, noble gas elements have positive electron gain enthalpies.

Variation of Electron Gain Enthalpy or Electron Affinity in Periodic Table

In period from left to right electron gain enthalpy increases while down the group it is decreases.

Exception in Electron Gain Enthalpy

In the case of Chlorine and Fluorine, Chlorine has a higher negative electron gain enthalpy value due to very small size of fluorine
In between Sulphur and Oxygen, Sulphur has a higher negative value than oxygen due to very small size of oxygen.

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