Question

Why are the electron gain enthalpies of $Be$ and $Mg$ are positive?

Answer 1

"Electron gain enthalpy is energy released when one mole of electrons is added to gaseous atoms of an element".

Positive electron gain enthalpy means energy needs to be supplied so that an electron can enter the atom.

$Be$ and $Mg$ are electropositive metallic elements, which become unstable on adding electrons.

$\therefore$ Electron gain enthalpy of $Be$ and $Mg$ care positive.