Question

An electron jumps from n = 4 to n = 1 state in H-atom. The recoil momentum of H-atom (in eV/C) is

• A. 12.75
• B. 6.75
• C. 14.45
• D. 0.85

Answer 1

The correct option is A 12.75

The H-atom before the transition was at rest. Therefore from conservation of momentum
Photon momentum = Recoil momentum of H-atom or
$P_{recoil}=\frac{hv}{c}=\frac{E_4-E_1}{c}=\frac{-0.85eV-(-13.6eV)}{c}=12.75\frac{eV}{c}$