Here's why:

* Electrons are negatively charged. When an atom loses electrons, it loses negative charge.

* Protons are positively charged. The number of protons in an atom determines its atomic number and doesn't change when an atom becomes an ion.

* Ions form when the number of protons and electrons are not equal. Since the atom lost two negatively charged electrons, it now has two more positively charged protons than electrons, resulting in a +2 charge.