Question

An electron with kinetic energy $E$ collides with a hydrogen atom in the ground state. The collision will be elastic

• A. for all values of E
• B. for E < 10.2 eV
• C. for 10.2 eV < E < 13.6 eV only
• D. for 0 < E < 3.4 eV only

Answer 1

Answer: (B)

First excitation energy of a hydrogen atom in ground state is $10.2 \,eV$.

So, if energy of colliding electron is more than $10.2\, eV$, then hydrogen atom absorbs $10.2\, eV$ from kinetic energy of electron and collision will be inelastic.
If energy of electron is less than $10.2\, eV$, then hydrogen atom does not absorbs energy and total energy before and after collision remains same and collision will be elastic.