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Different Atoms of the Same Element
Atoms of the same element always have the same number of PROTONS in their nucleus. This means that a particular element will always have a specific ATOMIC number (sometimes called the proton number) that identifies it.
So if the number of protons changes (this can happen during radioactive decay of a nucleus), then we will have an entirely different element.
However, if the number of NEUTRONS in the nucleus of an element changes, then the element stays the same but just gets a little more or less massive. It will have a different MASS number (sometimes called the nucleon number) because the mass number counts the total number of particles (protons and neutrons) in the nucleus.
Atoms that have a different number of neutrons, but the SAME number of protons are called ISOTOPES. These are different forms of the same element, just slightly more or less massive than each other
GCSE Keywords: Protons, Neutrons, Isotopes, Mass Number (Nucleon Number), Atomic Number (Proton number)