An ion is a charged atom or molecule. It is charged because the number of electrons  do not equal the number of protons in the atom or molecule. An atom can acquire  a positive charge or a negative charge depending on whether the number of electrons  in an atom is greater or less then the number of protons in the atom.

When an atom is attracted to another atom because it has an unequal number  of electrons and protons, the atom is called an ION. If the atom has more electrons  than protons, it is a negative ion, or ANION. If it has more protons than electrons,it  is a positive ion.