化學反應 問題

氧氣的化學式是 O2。

My chemistry teacher told me that in chemical formula , oxygen is O^2- as oxygen have 2 negative change .Therefore it must be O^2- .
When will oxygen be O?-----when oxygen (non -metal )combine with a substance that was a metal ,for example magnesium(Mg) ,Mg have 2 positive change.
While oxgen have 2 negative change(tell u again) ,the compound is Mg^2+O^2-(positive first ) ,so the name of compound is magnesium oxide and the chemical formula is MgO (one plus one minus,it is equal )
In this time oxygen is needed to writing as O.
Holp can help u