Atoms of the same element must have the same number of protons, but they can have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes. Isotopes of an element have:

Three isotopes of hydrogen

All hydrogen atoms contain one proton (and one electron), but they can contain different numbers of neutrons. Hydrogen-1 is the most common isotope of hydrogen.

Three isotopes of hydrogen, and their symbols, and numbers of protons, neutrons and electronsThe three isotopes of hydrogen

All three isotopes of hydrogen have identical chemical properties. This is because the number of electrons determines chemical properties, and all three isotopes have one electron in their atoms.

An isotope is named after the element and the mass number of its atoms. For example, carbon-12 is an isotope of carbon with a mass number of 12.

An introduction to how protons, neutrons and electrons make up atoms