 As you should recall from a previous video, atoms consist of three sub-atomic particles. Protons, which have a positive charge, neutrons, which are neutral, and electrons, which have a negative charge. Protons and neutrons are in the center of the atom, we call this the nucleus, and the electrons surround them. The number of protons determines the identity of an atom. An oxygen atom always has eight protons. If it doesn't have eight protons, it's no oxygen. A magnesium atom always has 12 protons, never more, never less. Because protons determine the identity, if you were able to add a proton to an atom or take a proton away from an atom, the identity of that atom would change. For example, if we took a proton off oxygen, it wouldn't be oxygen anymore. It would be nitrogen, since any atom with seven protons is nitrogen. However, neutrons and electrons can be added to or removed from an atom without changing the identity of the atom. Isotopes are atoms of the same element that have the same number of protons, but different numbers of neutrons. Most elements have more than one isotope that is found in nature. Isotopes have identical chemical properties and can participate in the same chemical reactions, however they have different masses. Hydrogen, for example, has three main isotopes. Proteam, which has one proton and no neutrons. Deuterium, which has one proton and one neutron. And Tritium, which has one proton and two neutrons. Carbon also has several isotopes. The one that is most common is carbon-12, with six protons and six neutrons. It's called carbon-12 because it has 12 particles in its nucleus. Carbon-13 has six protons and seven neutrons. One more neutron than it has protons. Carbon-14 has six protons and eight neutrons. Two more neutrons than protons. All three types of carbon are found in nature, but not all are found in the same proportion. This is because some isotopes are more stable than others. Let's see what happens in this simulation when we add neutrons to carbon. The more neutrons we add, the less stable the isotope becomes, and the more the atom wants to lose that extra neutron. The isotopes that are the most common are the ones that are the most stable. In summary, isotopes are atoms of the same element that have the same number of protons, but different numbers of neutrons. Most elements have more than one isotope that is found in nature. The isotopes that are the most common in nature are the ones that are the most stable.