Question

give reasons only electrons can flow in or out of atoms

Answer 1

Electrons revolve around the atom in energy levels called shells or orbits

Answer 2

if you see the structure of an atom there is a nucleus where the protons are put together with neutrons and there are electrons which revolve around the nucleus due to electrostatic force of attraction. on giving sufficient energy we can make these electron jump from one shell to an higher energy shell or even jump out of the atom against the force of attraction