How can elements have different isotopes?
First of all, the atoms of a chemical element can exist in different types and these are called isotopes.
Here is a quick explanation:
For example, let's talk about oxygen atom. All oxygen atoms on Earth (and Universe) have the same number of protons (and electrons), But they can have different numbers of neutrons. Different isotopes of the same element have different masses. Oxygen atom has 8 protons, 8 electrons, but it can have 8, 9, or 10 neutrons.
Let's take Carbon for example. Carbon 12 is the most common isotope of carbon and it has 6 protons and 6 neutrons in the nucleus of the atom. Carbon 13 also exists but it has 6 protons and 7 neutrons in the nucleus. Therefore, elements have different isotopes due to the varying number of neutrons found in the nucleus of the atom.