����ý

Atoms of an element that have the same number of protons, but different numbers of neutrons in their nuclei are called isotopes. Isotopes of an element have:

• the same atomic number
• different mass numbers

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 isotope of hydrogen.

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.

Learn more on isotopes in this podcast.

Listen to the full series on ����ý Sounds.

Relative atomic mass

Relative atomic masses are given in the periodic table. They have the symbol A_r.

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

• mass numbers are always whole numbers (it is not possible to have part of a proton or neutron)
• 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.