The first ionisation potential of Na is 5.1 eV. The value of...

Electron affinity is defined as the change in energy (in kJ/mole) of a neutral atom (in the gaseous phase) when an electron is added to the atom to form a negative ion.

In other words, the neutral atom's likelihood of gaining an electron.
${Na}_{( g )} \to {Na}^{+}( g ) + {e}^{-};$
${1}^{st}$ ionisation energy ; ${\Delta H}_{IE ( 1 )} = + 5 .1 eV$
Electron gaining is the reversed reaction of ionization. Hence, electron gain enthalpy is ;

$\left|{IE}_{Na}\right| = \left|EA{Na}^{+}\right|<br />${Na}^{+}( g ) + {e}^{-} \to Na$<br />So,${\Delta H}_{eg} = - 5 .1 eV$</p></blockquote>$