CBSE Class 11-science Answered

Electron gain enthalpy is the amount of energy released when an isolated gaseous atom accepts an electron to become a monovalent gaseous anion.
Eq: atom (gas) +electron? anion (gas) +energy (electron gain enthalpy)

It is the molar enthalpy change accompanying the addition of an electron to an atom (or ion).Since we know that enthalpy is negative when we form something that is more stable, less energetic. All atoms have proton/s within them and the attraction between a positively charged proton and a negatively charged electron is an energetically favorable process, thus the electron gain enthalpy is negative.