Now you know that atoms consist of protons, electrons, and neutrons. You also know that what makes an atom what it is; it�s the number of protons, indicated by it�s atomic number. For example Oxygen will always have 8 protons, or sodium will always have 11 protons. With electrons, you know that this number can change according to the octet rule. The number of electrons in an atom can vary from time to time. Now, the number of neutrons in the nucleus of atoms can also change. Usually, the number of neutrons are the same as the number of electrons but they can have more or less or both. For example, oxygen always has 8 protons, but it can only have 7 neutrons; it�s still oxygen, just another kind. These kinds are called isotopes. When a person writes a symbol about a particular element and wanted to indicate what type or isotope it was, they would write this: For example if sodium(Na) had 11 protons, but 12 neutrons: 23Na 11 The bottom number (subscript) indicates the atomic number which is always stable. The top number (superscript) indicates the mass number, or total number of protons or neutrons. It has 11 protons and 12 neutrons. This atom in particular would be called "sodium twenty-three" Isotopes have almost identical properties, the only difference is the weight, so it�s very difficult to tell the difference. Usually most elements are mixtures of isotopes. This can pose a problem. Scientists usually determine elements by their mass. Since some atoms have an extra neutron or one less neutron, that can throw off the mass by a lot. Luckily, isotopes in nature are evenly mixed up by a common ratio.