Question

An electron of the kinetic energy $10\text{eV}$ collides with a hydrogen atom in $1^{\text{st}}$ excited state. Assuming loss of $K.E$ in the collision to be quantized, which of the following statement is incorrect.

• A. The collision may be perfectly inelastic
• B. The collision may be inelastic
• C. The collision may be elastic
• D. The collision must be inelastic

Answer 1

The correct option is D The collision must be inelastic

In the process of collision, the electron (because of very low mass in comparison to hydrogen atom) can lose upto $10\text{eV}$.
$10\text{eV}$ is sufficient to ionise the hydrogen atom or excite to higher state from first excited state.
So, the collision should be inelastic.

Hence, $\left(D\right)$ is the correct answer.