The image below is an image created by a PET scan, which involves injecting radioactive isotopes into a person.

So what is an isotope? Let’s find out.



We learned earlier that all of the atoms of the same element contain the same number of protons. But the number of neutrons in the atoms of the same element can vary. The isotopes of a particular element contain the same number of protons but have different numbers of neutrons.

Let’s illustrate the concept of isotopes with an example.

There are three isotopes of the element, hydrogen. All of the atoms of hydrogen contain one proton but some atoms of hydrogen have no neutrons, some have one neutron, and some have two neutrons.

204 Isotopes of Hydrogen-01


Just to reinforce the idea of isotopes, as you can see from the diagram, these are isotopes of hydrogen because these atoms of hydrogen have different number of neutrons. (The number of neutrons in an atom doesn’t affect its chemical reactivity. You’ll remember that earlier we learned that it is the number and arrangement of the electrons in an atom that determines an atom’s reactivity.)

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