The first ionisation potential of $Na$ is $5.1\;eV$. The value of electron gain enthalpy of $Na^+$ will be :

Question

$\begin{array}{1 1} (1) -2.55\;eV \\ (2) -5.1\;eV \\ (3) -10.2\;eV\\ (4)+2.55\;eV \end {array}$