This writeup is about the meaning of the word ion as it is definied in physics and chemistry. In these sciences, an ion is essentially a charged atom or molecule. This means the particle has either one or more extra electrons, or is missing one or more electrons. If it is missing electrons, it is called a cation, and is positive; if it has extra electrons, it is called an anion and is negative.




One of the most obvious things about an ion is that it is charged. As such, it is affected by electric fields. This means, for instance, you can accelerate it in a particle accelerator. It also means that cations and anions attract each other. As such, it is almost impossible to have a clump of cations in one place and a clump of anions in another, because the electric force is overwhelmingly strong. If you were to take for instance one gram of table salt, split it in sodium cations and chloride anions, and put them on opposite sides of a spoon, the the electric force between them would be around 1020 Newton, which is comparable to the force that keeps the earth in her orbit. As such, separating significant numbers of charges is pretty much impossible, and ionic matter is largely mixed evenly. This is called quasi-neutrality.

Apart from this rather impressive physical property, there is also a chemical change. Because the chemical properties of an atom or molecule are determined by the electrons surrounding the nucleus, having extra or missing electron makes the particle "behave" as a different atom chemically. For instance, a fluorine cation has one extra electron. As such, it has the same number of electrons a neon, a noble gas. This configuration is chemically quite stable. As such, fluorine, and many other substances, are often or almost always found in this ionic form.




Where do we find ions? Well, pretty much all over the place. I'll summarize three places where ions are commonly found


  1. In a salt: A salt is defined as a substance that consists of ions. Table salt is just one example of a salt. Salts are typically formed when a substance that easily loses electrons, such as a metal, meets a substance that easily accepts them, such as a halogen or an organic compound. Because of the strong ionic bonds, salts are quite stable; most have very high melting points.
  2. In a polar solvent: Some salts can dissolve in polar solvents, such as water. These dissolved ions are called electrolytes. In the human body, these electrolytes play a crucial role.
  3. In a plasma: Plasmas are essentially energetic gases, in which some of the molecules are split in ions and free electrons. Most of these ions are cations; anions are more rare, as the plasma is so energetic the free electron state is preferable.
It is interesting to note that both the solution and the plasma are decent to good conductors of electricity. If a salt is melted, it will conduct electricity as well.




This is no more than a short and condensed summary on what an ion is; one could probably spent a lifetime researching ions. The most salient features are that they have an electrical charge, which changes their physical behavior. Furthermore, the extra electrons change the chemical behavior. Ions are essential for biological processes.