Question

what is the difference between an atom and an ion ???

Answer 1

In an atom, the number of electrons is equal to the number of protons. In an ion, the number of electrons is not equal to the number of neutrons. A positive ion has more protons than electrons whilst a negative ion has more electrons than protons.

Answer 2

Another way of looking at it--an ion is an atom (sometimes more than one atom) with a + or - charge on it.
example of one atom--Na atom; Na^+ ion
examle of more than one atom--SO4^2-