Atomic number and mass number

Atomic number

The number of protons in an atom of an element is its atomic number. Remember that:

  • all atoms of a given element have the same number of protons
  • atoms of different elements have different numbers of protons

An atom contains equal number of protons and electrons. Since protons and electrons have equal and opposite charges, this means that atoms have no overall electrical charge.

For example, the atomic number of sodium is 11. Every sodium atom has 11 protons and 11 electrons. It has 11 positive charges and 11 negative charges.

Mass number

The mass number of an atom is its total number of protons and neutrons.

Atoms of different elements usually have different mass numbers, but they can be the same. For example, the mass number of both argon atoms and calcium atoms is 40.

Calculating numbers of subatomic particles

Atoms can be represented as shown below. The mass number is at the top, and the atomic number at the bottom.

Symbol for sodium; a yellow square with Na written in the middle, 23 in the top left corner and 11 in the bottom left.The chemical symbol for sodium

To calculate the numbers of subatomic particles in an atom, use its atomic number and mass number:

  • number of protons = atomic number
  • number of electrons = atomic number
  • number of neutrons = mass number – atomic number

Worked example

The atomic number of a sodium atom is 11 and its mass number is 23. Calculate the number of protons, neutrons and electrons it contains.

number of protons = 11

number of electrons = 11

number of neutrons = (mass number - atomic number) = (23 - 11) = 12

A look at how protons, neutrons and electrons make up atoms, and how they tell us the number of subatomic particles that are present in atoms