Formulas of Ionic Compounds

Ionic Compounds

An ionic bond is formed through the transfer of one or more valence electrons, typically from a metal to a non-metal, which produces a cation and an anion that are bound together by an attractive electrostatic force. On a macroscopic scale, ionic compounds, such as sodium chloride (NaCl), form a crystalline lattice and are solids at normal temperatures and pressures.

Crystalline Lattice

Sodium chloride crystal lattice

The charge on the cations and anions is determined by the number of electrons required to achieve stable noble gas electronic configurations. The ionic composition is then defined by the requirement that the resulting compound be electrically neutral overall.

For example, to combine magnesium (Mg) and bromine (Br) to get an ionic compound, we first note the electronic configurations of these atoms (valence level in indicated in italics):

Mg: 1s²2s²2p⁶3s2

Br: 1s²2s²2p⁶3s²3p⁶3d¹⁰4s24p5

In order to achieve noble gas configurations, the magnesium atom needs to lose its two valence electrons, while the bromine atom, which has 7 valence electrons, requires one additional electron to fill its outer shell. Therefore, for the resulting compound to be neutral, two bromine anions must combine with one magnesium cation to form magnesium bromide (MgBr₂). In addition, though any ratio of 2 bromine atoms to 1 magnesium atom will satisfy the two requirements above, the formula for ionic compounds is typically presented as the empirical formula, or the simplest whole-number ratio of atoms with positive integers.

Note that the cation always precedes the anion both in written form and in formulas. In the written form, while the cation name is generally the same as the element, the suffix of single-atom anions is changed to -ide, as in the case of sodium chloride. If the anion is a polyatomic ion, its suffix can vary, but is typically either -ate or -ite,as in the cases of sodium phosphate and calcium nitrite, depending on the identity of the ion.

More examples:

• lithium fluoride: Li⁺ and F^- combine to form LiF
• calcium chloride: Ca²⁺ and Cl^- combine to form CaCl₂
• iron (II) oxide: Fe²⁺ and O^2- combine to form FeO
• aluminum sulfide: Al³⁺ and S^2- combine to form Al₂S₃
• sodium sulfate: Na⁺ and SO₄^2- combine to form Na₂SO₄
• ammonium phosphate: NH⁴⁺ and PO₄^3- combine to form (NH₄)₃PO₄
• potassium chlorite: K⁺ and ClO₂^- combine to form KClO₂

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