An α-particle having a kinetic energy of 7.7 MeV is approaching a fixed gold nucleus (atomic number Z = 79). Find the distance of closest approach.

An electron in a hydrogen atom jumps from the first excited state to the ground state. What will be the percentage change in the speed of the electron?

A free electron of 2.6 eV energy collides with a H+ ion. This results in the formation of a hydrogen atom in the first excited state and a photon is released. Find the frequency of the emitted photon (take h = 6.6 × 10-34 J s).