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An anion is a negatively charged atom or group of atoms. Anions are attracted to the anode, or positive electrode, in an electrolytic cell. Some common anions are the hydroxide ion (OH-), the chloride ion (Cl-), the nitrate ion (NO3- ), and the bicarbonate ion (HCO3- ). The single minus signs indicate that these ions carry one electron's worth of negative charge. The carbonate ion (CO32- ), for example, carries two units of negative charge.

The names of anions consisting of single atoms (monatomic ions) end in the suffix -ide. Fluoride (F-), sulfide (S2-), and oxide (O2-), are examples of such ions. A few polyatomic ions (ions with more than one atom) also have an -ide ending. The cyanide ion (CN-) is an example.

The names of most polyatomic anions end in either -ate or -ite. For example, the most common polyatomic anions of sulfur are the sulfate (SO42- ) and sulfite (SO3- ) ions. In pairs such as this one, the -ate suffix is used for the ion that contains sulfur in the higher oxidation number, and the -ite suffix for the ion with the lower oxidation number. The oxidation number of sulfur is six in the sulfate ion and four in the sulfite ion.

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