Course Outline

General Chemistry

04/25/09

Ionic Compounds

Nomenclature:

To name ionic compounds, name the cation then the anion. Monatomic cations have the same name as the element from which they form. For example, Ca⁺² is calcium ion. The names of the simple monatomic anions are similar to the names of their element but end in an –ide suffix. S^-2 is sulphide ion. The name of the ionic compound containing Ca²⁺ and S^-2 is calcium (II) sulfide. See the periodic table section for common charges of monatomic ions.

Some metals can form more than one ion. The names of these ions include a Roman numeral in parenthesis to indicate the charge. For example, Cu⁺¹ is copper (I) ion and Cu⁺² is copper (II) ion. The name of the ionic compound containing Cu⁺¹ and O^-2 is copper (I) oxide. There is an older style of nomenclature which uses the Latin form of the metal name and a suffix, the higher charged ion has an –ic ending and the lower charged ion has an –ous ending. This means that Cu⁺¹ is also called cuprous ion. This leads to some difficulties (not just learning the Latin), because the higher/lower charges might be +2/+1 or +2/+3 or some other combination. The Roman numeral designation always represents the charge of the ion (except for the polyatomic mercury ion Hg₂⁺² which is mercury (I)—the exceptions are annoying, aren’t they?). The metals that only form one type of ion do not need a Roman numeral in their names, but, when in doubt, put one in. It is not wrong, merely redundant.  

An ion may have more than one atom joined together by bonds to form a single charged unit. These are called polyatomic ions. While there are certain rules governing the names of polyatomic anions, there are so many exceptions that the beginning student is better off just memorizing them. You will see subscripts in the formulas for many polyatomic ions. These represent the number of atoms of the element immediately preceding the subscript:  NO₃^1- is nitrate ion and it has one nitrogen atom and three oxygen atoms bonded together to form an ion with a negative one charge. Note that you must learn the formulae, charges, and names of the common ions. Some of the most common are listed in the table below.

The name of an ionic compound is the name of the cation and anion. For example, CuSO₃ is copper (II) sulfite because, for the compound to be neutral, the copper ion must have a charge of 2+ to match the charge of the sulfite ion, 2-. Cu₂S is copper(I) sulphide because, for the compound to be neutral, the copper ion must have a charge of 1+ for two of them to balance the charge of the sulphide ion, 2-. Ca(NO₃)₂ is calcium nitrate; a Roman numeral is not needed because calcium has only one possible ion. Note that the nitrate is enclosed in parenthesis and given a subscript of 2. This indicates that there are two nitrate ions needed for every one calcium ion, for the compound to be neutral.

Formulae:

To write the formula of an ionic compound from its name, identify the ions, write their symbols, and use subscripts to make the compound neutral. Iron (III) sulfate contains the iron (III) ion and the sulfate ion. The symbol for iron (III) is Fe³⁺, and the symbol for sulfate is SO₄^2-. These charges do not balance in a one to one ratio, so the compound must have multiples of the ions in order to be neutral. If there are two iron (III) ions for every three sulfate ions, the charges will be six positive and six negative, which balance. This gives the ionic formula Fe₂(SO₄)₃. The parentheses are needed to make it clear that this compound has three parts sulfate ion rather than 43 parts oxygen.

4Concept Check:  What is the name of the compound with the formula Cu₃(PO₄)₂? What is the formula for potassium dichromate?

Answer: Cu₃(PO₄)₂ is copper (II) phosphate. The formula for potassium dichromate is K₂Cr₂O₇.

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