Question

Halogens have maximum negative electron gain enthalpy in the respective periods of the periodic table. Why?

Answer 1

Electron gain enthalpy

Electron gain enthalpy is the change in enthalpy when an electron is added to the neutral gaseous atom.

Electron gain enthalpy of halogen

Since we know, halogens have the smallest size in their respective periods and due to this they have high effective nuclear charge.
Hence, they can readily accept one electron to get noble gas electronic configuration.

Due to this they have maximum negative electron gain enthalpy (because the energy is being released in the process) in their respective periods of the periodic table.