What is the difference between an element and an isotope?

Iron, Hydrogen and Carbon are all examples of elements. Each element has a distinct atomic number, which is the number of protons in the nucleus of the atoms of that element. Protons are defined as subatomic (smaller than an atom) particles with a positive charge of +1. All Hydrogen atoms have 1 proton, all Carbon atoms have 6 protons.

Another particle found in the nucleus of an atom is the neutron. It has no charge. The number of these can vary for an element, but doesn't change the chemical behaviour of the element. It does change the nuclear behaviour though. The number of neutrons in a nucleus affects its stability. The different numbers of neutrons in the nucleus determine the isotope of that element.

Some isotopes are more common than others. Hydrogen has three naturally occurring isotopes. They are known as Protium, Deuterium and Tritium, and have 0, 1 and 2 neutrons respectively. The most common is Protium.

Because neutrons have mass, the atomic mass of an element (the mass of the atom) varies for each isotope. The average atomic mass takes into account the naturally occurring proportion of each isotope.