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Protons and Neutrons

The two numbers that are labelled on every element in the periodic table are:

• The mass number (the larger number – sometimes called the nucleon number)
• The atomic number (the smaller number – sometimes called the proton number)

The mass number tells us the total number of particles in the nucleus – both protons and neutrons together.

The atomic number tells us the number of PROTONS in the nucleus.

We can find the number of neutrons in the nucleus by subtracting the atomic number from the mass number.

So for this atom of potassium:

Number of neutrons = 41 – 19 = 22

GCSE Keywords: Nucleus, Protons, Neutrons, Neutron number (mass number), Proton number (atomic number)

Course overview