Mass number

Of the three subatomic particles, the electron is extremely light: about 1/1840 of the mass of a proton.

A proton and a neutron have approximately the same mass as each other. Set beside them, the mass of the electrons in an atom is small enough to ignore entirely. So the mass of an atom is decided by its protons and neutrons alone.

A worked example

• The atomic number of sodium is 11.
• So a sodium atom has 11 protons.
• If it contains 12 neutrons, then:

mass number of the sodium atom = 11 + 12 = 23

How the two numbers are written

There is a standard way of showing both at once. The mass number goes above and the atomic number below, on the left of the symbol:

²³₁₁Na

A = 23 (mass number) · Z = 11 (atomic number)