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 abundant (most common) isotope of hydrogen.

Hydrogen-1 _{1}^{1}\textrm{H}111 - 1 = 0
Hydrogen-2 _{1}^{2}\textrm{H}112 - 1 = 1
Hydrogen-3 _{1}^{3}\textrm{H}113 - 1 = 2

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.

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.

Relative atomic mass

The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes. It takes account of the abundance of each of the isotopes of the element.

Relative atomic masses can be found in the periodic table. They have the symbol Ar.

Take care not to confuse mass numbers and relative atomic masses:

  • mass numbers are always whole numbers (protons or neutrons cannot be split into parts)
  • relative atomic masses are often rounded to the nearest whole number, but are actually not whole numbers

For example, the relative atomic mass of chlorine is 35.5 rather than a whole number. This is because chlorine contains two different isotopes, chlorine-35 and chlorine-37.