A normal atom of oxygen has a mass of 16. The isotope oxygen-18 has 18 protons. true or false?

False.

Mass number is the number of protons + neutrons.

Then a mass of 16 means that the number of protons cannot exceed 16.

Isotopes of an element have the same number of protons, they only differ in the number of neutrons. This is, oxygen-18 has the same number of protons than any other isotope of oxygen and we already shown that this cannot exceed 16.

You can find the number of protons of oxygen in a periodic table. This is the atomic number. Atomic number of oxygen is 8, so all the isotopes of oxygen have 8 protons.