What is the difference between an isotope and an element?

Isotopes are forms of the same element that differ in their nuclei.

Isotopes have the same number of protons in their nuclei, but have different numbers of neutrons, so their mass numbers differ. The name of an isotope includes its mass number.

For example, oxygen has three stable isotopes; oxygen-16, oxygen-17, and oxygen-18. The atomic number of oxygen is 8, so all of its atoms contain 8 protons. Subtract the number of protons (8) from the mass numbers to determine the number of neutrons in the different isotopes.

So oxygen-16 has 8 neutrons, oxygen-17 has 9 neutrons, and oxygen-18 has 10 neutrons.