Why electron gain enthalpy of chlorine is more negative than fluorine.

The electron gain enthalpy (also known as electron affinity) is the energy change that occurs when a neutral atom gains an electron to form a negatively charged ion. The electron gain enthalpy is influenced by several factors, including atomic size, effective nuclear charge, and electron-electron repulsions.

In the case of chlorine (Cl) and fluorine (F), both belong to Group 17 (Group VIIA) of the periodic table, known as the halogens. Both elements have a high electron affinity because they have a strong tendency to gain an electron to achieve a stable, noble gas electron configuration.

The electron gain enthalpy of chlorine is more negative than that of fluorine primarily due to the smaller size of chlorine compared to fluorine. Chlorine has more electron shells than fluorine, and the outer electrons in chlorine are farther away from the nucleus compared to fluorine. As a result, the incoming electron in chlorine experiences weaker attraction from the nucleus and feels more electron-electron repulsion from the existing electrons.

In fluorine, the smaller atomic size means that the incoming electron is added to a region closer to the nucleus, where the attractive forces are stronger and the electron-electron repulsion is less significant. Therefore, it is easier for fluorine to gain an electron compared to chlorine.

In summary, the smaller size of chlorine and the associated increase in electron-electron repulsion make the electron gain enthalpy of chlorine more negative than that of fluorine.