Relative Atomic Mass

Key Stage 3

An element tile showing the mass number.

The Atomic Mass or mass number is the number of nucleons (protons + neutrons) in an atom.

Two atoms of the same element may have the same Atomic Number but a different Atomic Mass depending on the number of neutrons in the nucleus. Elements with different mass numbers are called isotopes.

The atomic mass is not affected by the number of electrons.

Only the particles in the nucleus affect the atomic mass.

Hydrogen	Helium	Lithium	Beryllium


Hydrogen has one nucleon so it has an atomic mass of 1.	Helium has four nucleons so it has an atomic mass of 4.	Lithium has seven nucleons so it has an atomic mass of 7.	Beryllium has nine nucleons so it has an atomic mass of 9.

An element tile showing the mass number.

The relative atomic mass or mass number of an element is the number of nucleons (protons + neutrons) in an atom.

The relative atomic mass in grams is also the mass of one mole or 6.02x10²³ atoms of the element.

The relative atomic mass in grams on the Periodic Table tells the mass of a mole of the element. A mole of the element is 6.02x10²³ atoms of that element.

Two atoms of the same element may have the same Atomic Number but a different Relative Atomic Mass depending on the number of neutrons in the nucleus. Elements with different mass numbers are called isotopes.

The relative atomic mass is not affected by the number of electrons.

Only the particles in the nucleus affect the relative atomic mass.

Hydrogen	Helium	Lithium	Beryllium


Hydrogen has one nucleon so it has an atomic mass of 1 and 6.02x10²³ (or 1 mole of) atoms of Hydrogen have a mass of 1g.	Helium has four nucleons so it has an atomic mass of 4 and 6.02x10²³ (or 1 mole of) atoms of Helium have a mass of 4g.	Lithium has seven nucleons so it has an atomic mass of 7 and 6.02x10²³ (or 1 mole of) atoms of Lithium have a mass of 7g.	Beryllium has nine nucleons so it has an atomic mass of 9 and 6.02x10²³ (or 1 mole of) atoms of Beryllium have a mass of 9g.