Once an atom has an electrical charge it is called an ion. In an ion the atomic number and atomic mass do not change from the original. If an atom were to gain or lose neutrons it becomes an isotope. If it gains a neutron it become an isotope called deuterium.

For example: O2- indicates an ion of oxygen having two extra electrons. I.e., since an oxygen atom normally has 8 protons and 8 electrons, this ion has 10 electrons (-10 charge) and 8 protons (+8 charge) giving it a charge of -2 (-10 + 8 = -2)….Table 4.1.

Subtract the negative charge total from the positive charge total to get the net charge.

If the atom had 8 protons and 8 electrons (oxygen for example), knocking one of the electrons out, makes 8 protons and 7 electrons. The atom now has a net charge of +1 because the 7 electrons cancil the positive charges of 7 of the protons in the nucleus, but there is one remaining proton with a positive charge.

oxygen 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.

alkali earth metals

Magnesium bromide

Always name the cation before the anion; in the chemical formula, the cation will always appear first as well. When naming the cation within an ionic compound, we don’t include the word ion or the charge unless it is a polyvalent cation.