The charge of a strontium ion is +2. The charge of an ion is determined by the amount of electrons compared to the number of protons, so a charge of +2 indicates that the ion has two more protons than electrons.
An ion of strontium has 38 protons and 36 electrons, while an atom of strontium in an uncharged, nonionic state has 38 electrons and 38 protons. An ion that has fewer electrons than protons, such as a strontium ion, is known as a cation. A strontium ion is a monatomic ion, meaning that it is composed of only one atom.