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Atoms and isotopes


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.

This is the full chemical symbol for chlorine-35.

The proton number is shown below the chemical symbol, and the mass number is shown above. In the example below, 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 the atoms of an element with different numbers of neutrons. They have the same proton number, but different mass numbers.

Three isotopes of hydrogen



1 proton, 0 neutron, 1 electron

H 1,1


1 proton, 1 neutron, 1 electron

H 2,1


1 proton, 2 neutrons, 1 electron

H 3,1

Back to Radiation index

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