The ionisation energy of hydrogen atom is 13.6eV. What is the ionisation energy of He ?

Options

A. `-27.2 eV`
B. `+27.2 eV`
C. `+54.4 eV`
D. `-54.4 eV`

Answer

C

Explanation

Ionisation energy is the energy required to completely knock out an electron from the atom and it is proportional to `Z^2` (where Z is the atomic number of the atom).
`therefore " For helium Z =2 and for " H_2, Z=1`
`therefore E_2/E_1 = 2^2/1^2 = 4`
`therefore E_2 = 4E_1 = 4xx13.6=54.4 eV`