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Ions consist of either atoms or groups of atoms that have a charge that is 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. These ions are usually composed from more than one atom, making them polyatomic ions, and they are usually built around a core atom, typically a non-metal.
Common anions include acetate, hydroxide, carbonate, chlorate, cyanide, fluoride, nitrate, phosphate, and oxide. Except for hydroxide and cyanide, all that end in –ide are monatomic. Group 7 atoms called halides, which include such elements as fluorine, chlorine, bromine and iodine, form anions with a 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.
Most polyatomic ions combine with oxygen and/or hydrogen to make an anion. In these ions, the negative charge, or electron, is shared around the entire ion, not within a specific nucleus in the ion. They 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. Both are often found in water because of the nature of the molecule.
An anion’s physical properties differ from those of a cation. It polarizes the electron clouds of molecules in the opposite way that cations do. Also, these ions have weakly bound electron densities, which leads to a great likelihood of polarization, or the resistance of an electrolytic cell. They 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. A negative ion does not typically experience an excited electronic state. Furthermore, its electrons will move to regions of space occupied by orbitals to experience an attractive potential different from that experienced by neutrals and cations.
Anions tend to bind outer electrons more tightly than neutrals and cations, which leads to difficulty in creating a massive amount of them. Electronically or geometrically stable ones, or those that will neither detach nor fragment, will usually bind excess electrons in their orbitals. This allows for easy analysis, and chemists can determine more about them through either experiment or calculation.