Here's why:

* Protons determine the element: The number of protons in an atom's nucleus defines what element it is. Both oxygen atoms and oxide ions have 8 protons, which is why they are both oxygen.

* Oxide ions gain electrons: An oxide ion (O²⁻) is formed when an oxygen atom gains two electrons. Gaining electrons changes the ion's charge, but not its number of protons.

In summary:

* Oxygen atom: 8 protons, 8 electrons (neutral charge)

* Oxide ion (O²⁻): 8 protons, 10 electrons (2- negative charge)