Question

which is the charge that results when oxygen becomes an ion ?

Answer 1

Oxygen has 6 valence electrons. To fill up the valence shell, which has 8 electrons, an oxygen atom wants to gain 2 electrons. Since every electron has a single negative charge, the addition of two electrons results in an oxygen ion with a charge of −2.

This is true of every other element located beneath oxygen on the periodic table (the oxygen family, or the chalcogens).