O₂, or molecular oxygen, consists of two oxygen atoms bonded together. In its diatomic form, each oxygen atom shares electrons equally with its partner, resulting in a nonpolar molecule that has a neutral charge overall. This is why O₂ has a zero charge in its elemental form.
However, when oxygen forms compounds with other elements, such as in oxides (like H₂O or CO₂), it can gain or share electrons. Oxygen has a high electronegativity, which means it tends to attract electrons more strongly than many other elements. In these compounds, oxygen often gains two additional electrons to achieve a full outer shell, resulting in a -2 charge.
To summarize, O₂ is neutral because the electrons are shared equally. But when involved in compounds, oxygen typically ends up with a -2 charge due to its ability to attract and hold onto additional electrons.