* Protons: The number of protons defines the element. Oxygen always has 8 protons.
* Neutrons: The mass number (18 in this case) represents the total number of protons and neutrons. Therefore, an oxygen-18 atom has 10 neutrons (18 - 8 = 10).
* Electrons: Oxygen normally has 8 electrons to balance its 8 protons. However, to form an oxide ion (O²⁻), it gains two electrons. This gives it a total of 10 electrons.
In summary:
* Protons: 8
* Neutrons: 10
* Electrons: 10
