Question

An energy of $24.6eV$ is required to remove one of the electrons from a neutral helium atom. The energy (in eV) required to remove both the electrons from a neutral helium atom is

• A. $38.2$
• B. $49.2$
• C. $51.8$
• D. $79.0$

Answer 1

The correct option is D $79.0$

When one electron from neutral helium atom is removed, it becomes hydrogen like. The ionisation energy of hydrogen like atom is given by
$E_{ion}=Z^{2}Rhc$
for helium, $Z=2$ $Rhc=13.6eV$
$\therefore$ Energy required to liberate second electron from helium
$E_2=E_{ion}=2^2\times 13.6eV=-54eV$
$\therefore$ Energy required to remove both the electrons from helium is
$=E_1+E_2=(24.6+54.4)eV=79.0eV$