Question

Why do halogens have maximum negative electron gain enthalpy?

Answer 1

Nonmetals are halogens. Fluorine and chlorine are gases at room temperature, but bromine is a liquid. Solids are iodine and astatine. The reactivity of halogens diminishes as they progress from fluorine to astatine. In nature, halogens do not exist in their elemental form. Astatine isotopes have a limited half-life and are radioactive. At ordinary temperature and pressure, the halogens are the only periodic table group that contains elements in all three known states of matter.

Complete answer:
All halogen molecules are diatomic. This means that each of their molecules has two atoms. Because halogens have one electron missing from their valence shell, they produce negative ions and are extremely reactive. By interacting with atoms of other elements, they can obtain an electron.
The amount of energy released when an electron is connected to a neutral atom or molecule in the gaseous state to produce a negative ion is described as the electron affinity of an atom or molecule.
$X\left( g \right)\text{ }+\text{ }{{e}^{-}}~\to \text{ }{{X}^{-}}\left( g \right)\text{ }+\text{ }energy$
This is not to be confused with the enthalpy change of electron capture ionisation, which is negative when energy is released. In other words, there is a negative difference between this enthalpy change and the electron affinity. The electron affinity for a surface is defined differently in solid state physics.
In terms of acquiring electrons and reaching noble gas electrical configuration, halogens are the closest elements to noble gases.
The amount of energy released when an electron is added to a neutral gaseous atom is known as electron gain enthalpy.
Because halogens have a strong inclination to absorb electrons due to their tiny size and effective nuclear charge, they have the highest negative electron gain enthalpy (because energy is lost) in their respective periodic table periods.

Note:
In significant concentrations, halogens can be toxic or fatal to living organisms due to their strong reactivity. Because of their large effective nuclear charge, atoms have a high electronegativity, resulting in strong reactivity. Because halogens contain seven valence electrons in their outermost energy level, they can meet the octet rule by interacting with atoms of other elements.