Explain term electron gain enthalpy ? How does it vary along a group and across a period.

Explanation

### Step-by-Step Solution:

1. **Definition of Electron Gain Enthalpy**:
- Electron gain enthalpy is defined as the amount of energy released (or absorbed) when an electron is added to an isolated gaseous atom in its ground state. It is usually expressed in kilojoules per mole (kJ/mol). A negative value indicates that energy is released when the electron is added, while a positive value indicates that energy is absorbed.

2. **Understanding the Concept**:
- When an electron is added to an atom, it can either be attracted to the nucleus or experience repulsion from other electrons. The net effect determines whether energy is released or absorbed. Atoms with a high tendency to gain electrons (like halogens) have a high negative electron gain enthalpy.

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