**Electron Gain Enthalpy**

The electron gain enthalpy refers to the amount of energy released or absorbed when an atom gains an electron to form a negative ion. It can be either exothermic (energy is released) or endothermic (energy is absorbed) depending on various factors.

**Factors Influencing Electron Gain Enthalpy:**

1. Atomic Size: The size of an atom plays a crucial role in determining the electron gain enthalpy. Generally, smaller atoms have a higher electron gain enthalpy because the incoming electron experiences a stronger attractive force from the nucleus.

2. Electron Configuration: The stability of the electron configuration also affects the electron gain enthalpy. If the addition of an electron leads to a stable, filled electron configuration (such as a noble gas configuration), the electron gain enthalpy will be exothermic. However, if the electron addition leads to an unstable configuration, it will be endothermic.

3. Effective Nuclear Charge: The effective nuclear charge experienced by the incoming electron determines the electron gain enthalpy. Higher effective nuclear charge leads to a higher attraction between the nucleus and the incoming electron, resulting in a more exothermic electron gain enthalpy.

4. Electron Repulsion: When an electron is added to an atom, it experiences repulsion from the existing electrons. This electron-electron repulsion can make the process endothermic.

5. Electron Affinity: The electron affinity, which is a measure of the attraction an atom has for an additional electron, also affects the electron gain enthalpy. If the atom has a high electron affinity, it will tend to release energy when gaining an electron (exothermic). On the other hand, if the electron affinity is low, it will require energy to gain an electron (endothermic).

**Ionisation Enthalpy**

The ionisation enthalpy refers to the energy required to remove an electron from an atom to form a positive ion. It is always an endothermic process because energy is needed to overcome the attractive forces between the electron and the nucleus.

When an electron is removed, the remaining electrons experience a decrease in electron-electron repulsion, which stabilizes the system. However, energy is still required to overcome the attractive forces and remove the electron from its orbital.

**In Summary:**

The electron gain enthalpy can be exothermic or endothermic depending on factors such as atomic size, electron configuration, effective nuclear charge, electron repulsion, and electron affinity. On the other hand, the ionisation enthalpy is always endothermic because energy is needed to remove an electron from an atom.

Electron Gain Enthalpy or Electron Affinity is the amount of energy released when an isolated, neutral, gaseous atom takes up an extra electron to form uninegative gaseous ions.   X(g) + e− ----→ X− . A reaction with negative energy of reaction is termed as exothermic and those with positive energy of reaction is termed as endothermic. As a general rule, electron gain enthalpy becomes more negative with increase in atomic number across a period and less negative as we go down a group. Now this energy will be negative for those elements which have a tendency to gain electrons. The negative value of electron gain enthalpy denotes that energy is given out and a positive value denotes that energy is absorbed when an electron is gained by an atom. More is the electronegativity of an atom, more is its tendency to accept electrons and more is the energy released when it accepts electrons. Therefore for elements with high electronegativity, the electron gain enthalpy is negative. As an electropositive element has more tendency to lose electrons than to gain electrons, therefore for these elements (like Na, K, Ca etc) the electron gain enthalpy will either be positive or will have small negative values.