Atomic number and mass number

The number of protons in the nucleus of an atom is called its atomic number:

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

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

Chlorine 35 square from the periodic table. The mass number (35) is top-left. The atomic number (17) is bottom-left.Chemical symbol of chlorine–35

The atomic number is shown below the chemical symbol, and the mass number is shown above. In this example the atomic number is 17 and the mass number is 35. This means that each of these atoms has:

  • 17 protons
  • 17 electrons
  • 35 – 17 = 18 neutrons


Isotopes are atoms of an element with the normal number of protons (and electrons), but different numbers of neutrons. Isotopes have the same atomic number, but different mass numbers.

Three isotopes of hydrogen

Most hydrogen atoms consist of just one proton and one electron, but some also have one or two neutrons.

Table showing the three isotopes of hydrogen, their symbols, and numbers of protons, neutrons and electrons.