Question

What is equation balance of Mg + O2 --> Mg O

Answer 1

This is more chemistry than physics, but alas.

Simply notice that you need to have 2 oxygens on the right side since there are 2 on the left. So multiply the right side by 2.

Mg + O2 --> 2 Mg O

But now, you need to have 2 magnesiums on the left side since there are 2 on the right. So multiply that by 2.

2 Mg + O2 --> 2 Mg O

There, all balanced!