Atomic number and mass number

Atomic number

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

  • all the atoms of a given element have the same atomic number
  • the atomic number of each element is unique - no two elements have the same atomic number

An atom contains equal numbers of protons and electrons. Since protons and electrons have equal and opposite charges, this means that atoms are neutral overall.

For example, the atomic number of chlorine is 17. Every chlorine atom has 17 protons and 17 electrons. It has 17 positive charges and 17 negative charges, meaning that it is neutral overall.

Mass number

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

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

Calculating numbers of subatomic particles

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

The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom.

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

An interactive lesson in calculating an atom's mass and atomic number


The atomic number of a chlorine atom is 17 and its mass number is 35. Calculate the number of protons, neutrons and electrons it contains.

  • Number of protons = 17
  • Number of electrons = 17
  • Number of neutrons = (35-17)=18