An
ion is an
atom or
molecule where the total number of
electrons is not equal to the total number of
protons, giving it a net positive or negative
electrical charge. An
anion (pronounced
/'æna??n/;
an-eye-on), from the Greek word ??? (
ana), meaning 'up', is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged). Conversely, a
cation (pronounced
/'kæta??n/;
cat-eye-on), from the Greek word ?at? (
kata), meaning 'down', is an ion with more protons than electrons.
An ion consisting of a single atom is a monatomic ion. If it consists of two or more atoms, it is a polyatomic ion. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called oxyanions. When writing the chemical formula for an ion, its charge is written as a superscript '+' or '-' following a number indicating the difference between the number of protons and the number of electrons. The number is omitted if it is equal to 1. For example, the sodium cation is written as Na+, the '+' indicating that it has one less electron than it has protons. The sulfate anion is written as SO42-, the '2-' indicating that it has two more electrons than it has protons. If an ion contains unpaired electrons, it is called a radical ion. Just like neutral radicals, radical ions are very reactive.
Monatomic ions are formed by the addition of electrons to the valence shell of the atom, or the losing of electrons from this shell. The inner shells of an atom are filled with electrons that are tightly bound to the positively-charged atomic nucleus, and so do not participate in this kind of chemical interaction. The process of gaining or losing electrons from a neutral atom or molecule is called ionization.
Atoms can be ionized by bombardment with radiation, but the more usual process of ionization encountered in chemistry is the transfer of electrons between atoms or molecules. This transfer is usually driven by the attaining of stable ("closed shell") electronic configurations. For example, a sodium atom, Na, has a single electron in its valence shell, surrounding a stable, closed inner shell of 10 electrons. Since the 10-electron configuration is very stable, sodium "likes" to lose its extra electron so that it can attain to this stable configuration, becoming the sodium cation in the process
