Ions consist of either atoms or groups of atoms that have a charge -- either negative or positive. If they have have gained electrons, they will be negatively charged, due to having more electrons than protons, and are called anions. An anion is usually composed from more than one atom. These are called polyatomic ions and are usually built around a core atom, which in most cases is a non-metal.

Common anions include acetate, bromide, hydroxide, carbonate, chlorate, chloride, chromate, cyanide, fluoride, nitride, nitrate, sulphite, phosphate and oxide. Except for hydroxide and cyanide, all anions that end in –ide are monatomic. Group 17 atoms called halides, which include such elements as fluorine, chlorine, bromine and iodine, form anions with their charge of negative one. Elements such as oxygen and sulfur carry a charge of negative two, while nitrogen and phosphorus carry a negative three charge. All of these combine with an atom to form an anion.

Most polyatomic ions combine with oxygen and/or hydrogen to make an anion. In polyatomic ions, the negative charge, or electron, is shared around the entire ion, not within a specific nucleus in the ion. Anions are also known as negative ions, while positive ions are called cations. Cations are atoms that have lost an electron, and as a result, they have a positive charge. Cations and anions are often found in water because of the nature of the water molecule.

An anion’s physical properties differ from those of a cation. An anion polarizes the electron clouds of molecules in the opposite way that cations do. Also, they have weakly bound electron densities, which leads to a great likelihood of polarization, or the resistance of an electrolytic cell. These atoms have stronger interactions with nearby molecules than neutrals and cations, which are less likely to polarize.

Electron-binding energy in an anion is less than in a neutral or cation. An anion does not typically experience an excited electronic state. Furthermore, an anion’s electrons will move to regions of space occupied by orbitals to experience an attractive potential different from that experienced by neutrals and cations.

An anion tends to bind outer electrons more tightly than neutrals and cations, which leads to difficulty in creating a massive amount of anions. Electronically or geometrically stable anions, or those which will neither detach nor fragment, will usually bind excess electrons in their orbitals. This allows for easy anion analysis, and chemists can determine more about them through either experiment or calculation.