Why does a oxygen atom form a 2- ion

It's because it has 2 electrons on its outer ring

Explanation:

It has a -2 ion due to it having 2 more electrons than protons making it an overall negative. It's well known in chemistry that the first ring can only hold 2 electrons while the second and furthermore can hold up to 8. But what happens when you have 9? a new ring starts and it would only contain 1 electron.. In our case here it has 2 more electrons than protons and those 2 electrons are located on the outer ring

- "the choice that is not an indicator of a chemical change is "state of matter changes". more common than not, chemical reactions produce energy in the form of light or heat. along with energy, they also produce a new substance called the product that could be in any state of matter (solid, gas, or liquid)."

that tool would be a compass