Answer:
See explanation
Step-by-step explanation:
Oxygen has 6 valence electrons. To fill up the valence shell, which (when in oxygen's row) 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
Oxygen is in group 6 of the periodic table meaning it has 6 electrons on its outer shell so when Oxygen atom gains two electrons it results in an Oxygen Ion with a charge of -2