Here's how to determine the number of electrons in an O²⁻ ion:

* Start with the neutral atom: Oxygen (O) has 8 electrons in its neutral state.

* Consider the charge: The "2-" charge means the oxygen atom has gained two extra electrons.

* Calculate the total: 8 (original electrons) + 2 (gained electrons) = 10 electrons

Therefore, an O²⁻ ion has 10 electrons.