How does an atom of strontium 88 become a strontium ion with a+2 charge ?

Strontium atom loses 2 electrons to become an ion.

Explanation:

Strontium (an Alkaline Earth metal) is the 38th element found in the Group 2 of the periodic table.

A Strontium atom has same number of electrons as its protons. It has a +2 electronegativity because it belongs to group 2 of the periodic table so it has the tendency to lose 2 electrons to form a positive ion.

Therefor, Strontium atom loses 2 electrons to become an ion with 2 electrons lesser than its atom.