An isotope of an element has a slightly different number of particles to other isotopes of the element. So, the most common isotope of oxygen is oxygen-16, and it has 16 nucleons (both protons and neutrons) in its nucleus. Oxygen-18 has 18 nucleons.
Isotope
Fact of the Day
In 1252 Henry III was given a polar bear by King Haakon of Norway. He kept it in the Tower of London on a long chain so it could swim in the Thames.
Quote of the Day
"Democracy means government by discussion, but it is only effective if you can stop people talking.
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~ Clement Attlee
On This Day
1787 The 'First Fleet' of 11 ships carrying convicts to Australia left England.
1888 Slavery was abolished in Brazil.