Question

If an electron gains energy, what may happen to it

Answer 1

When an electron gains energy, it moves into a higher energy level (shell) in an atom, which is farther from the nucleus. If enough energy is added, the electron may completely escape from the nucleus.

Answer 2

@insa good answer

Answer 3

But it can only move within successive subshells.
A 2s electron will not drop down to a 1s subshell