Common Anions Table and Formulas List

An anion is an ion that has a negative charge. Here is a table listing common anions and their formulas:

Table of Common Anions

Writing Formulas of Salts

Salts are compounds composed of cations bonded to anions. The resulting compound carries a neutral electrical charge. For example, table salt, or sodium chloride, consists of the Na⁺ cation bonded to the Cl^- anion to form NaCl. Salts are hygroscopic, or tend to pick up water. This water is called water of hydration. By convention, the cation name and formula are listed before the anion name and formula. In other words, write the cation on the left and the anion on the right.

The formula of a salt is:

(cation)_m(anion)_n·(#)H₂O

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Anion Definition and Examples

By Anne Marie Helmenstine, Ph.D.

where the H₂O is omitted if the # is zero, m is the oxidation state of the anion, and n is the oxidation state of the anion. If m or n is 1, then no subscript is written in the formula.

The name of a salt is given by:

(cation)(anion) (prefix)(hydrate)

where the hydrate is omitted if there is no water.

Prefixes indicate the number of water molecules or can be used in front of the cation and anion names in cases where the cation (usually) can have multiple oxidation states. Common prefixes are:

For example, the compound strontium chloride consists of the cation Sr²⁺ combined with the anion Cl^-. It is written SrCl₂.

When the cation and/or the anion is a polyatomic ion, parentheses may be used to group the atoms in the ion together to write the formula. For example, the salt ammonium sulfate consists of the cation NH₄⁺ and the sulfate anion SO₄^2-. The formula of the salt is written as (NH₄)₂SO₄. The compound calcium phosphate consists of the calcium cation Ca²⁺ with the anion PO₄^3- and is written as Ca₃(PO₄)₂.

An example of a formula that includes water of hydrate is that of copper(II) sulfate pentahydrate. Note that the name of the salt includes the oxidation state of copper. This is common when dealing with any transition metal or rare earth. The formula is written as CuSO₄·5H₂O.

Formulas of Binary Inorganic Compounds

Combining cations and anions to form binary inorganic compounds is simple. The same prefixes are applied to indicate the quantities of cation or anion atoms. Examples include the name of water, H₂O, which is dihydrogen monoxide, and the name of NO, which is nitrogen dioxide.

Cations and Anions in Organic Compounds

The rules for naming and writing the formulas of organic compounds are more complex. In general, the name follows the rule:

(group prefixes)(longest carbon chain prefix)(highest root bond)(most important group suffix)