Here's why:
* Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
* Oxygen (O) always has 8 protons. This defines it as oxygen.
* 16O has 8 protons and 8 neutrons (16 - 8 = 8).
* 18O has 8 protons and 10 neutrons (18 - 8 = 10).
Therefore, the key difference between these isotopes is the number of neutrons, which affects their atomic mass.
