Explain what is meant by the term isotope

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An atom is made up of 3 different types of sub-atomic particles (just the name for particles that are inside the atom). 

The nucleus, the 'ball' in the middle of the atom where almost all it's mass is found, is made up of protons and neutrons. Electrons circle this nucleus. The number of protons in an atom always equals the number of electrons in that atom. It is this number that defines what the atom is - whether it is gold, or oxygen, or calcium and so on. 

The number of neutrons in an atom will be roughly the same if not equal to the number of protons. Importantly though, changing the number of neutrons in an atom doesn't change what element the atom is. Therefore, we could play around with the number of neutrons in an atom of gold, and as long as we didn't change the number of protons, it would still be am atom of gold. We would be creating different isotopes of gold if we did this. 

Isotopes are atoms of the same element with different numbers of neutrons. 

Therefore isotopes differ in relative atomic mass, but have the same chemical properties, because neutrons have nothing to do with how an atom reacts (it's down to the electrons). 

Annabel W. GCSE Maths tutor, A Level Maths tutor, GCSE Chemistry tutor

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