4.9: What Are Chemical Compounds Called?

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The naming of chemical compounds can get a little complicated. This is particularly true of organic compounds, the names of which are discussed in Chapter 9. Some of the simpler aspects of naming inorganic compounds are discussed here.

In naming compounds, prefixes are used to represent the relative numbers of atoms in the formula unit of the compound. These prefixes through number 10 are given below:

1-mono 3-tri 5-penta 7-hepta 9-nona

2-di 4-tetra 6-hexa 8-octa 10-deca

The first class of inorganic compounds to be addressed here are binary molecular compounds. Binary molecular compounds are composed of only 2 kinds of elements and do not contain ions. For these compounds, the first part of the name is simply the name of the first element in the compound formula. The second part of the name is that of the second element in the compound formula modified to have the ending -ide. Prefixes are added to indicate how many of each kind of atom are present in the molecule. Consider as an example the name of N₂O₅. The name of the compound is dinitrogen pentoxide where di indicates 2 N atoms, pent indicates 5 oxygen atoms, and the second element has the ide ending. Other examples of this system of naming are SiCl₄, silicon tetrachloride; S₂F₆, disilicon hexafluoride; PCl₅, phosphorus pentachloride; and SCl₂, sulfur dichloride.

A number of compounds, including binary molecular compounds, have common names that have been used for so long that they are part of the chemical vocabulary. An especially common example is the name of water for H₂O; its official name is dihydrogen monoxide. Another example is dinitrogen monoxide, N₂O, usually called nitrous oxide. Aluminum oxide, Al₂O₃, is commonly called alumina and silicon oxide, SiO₂, is called silica, specific mineral forms of which are sand and quartz.

Recall that ionic compounds are those composed of ions that are held together by ionic bonds, rather than covalent bonds. As noted in the discussion of ionic sodium chloride in Section 4.3, ionic compounds do not consist of discrete molecules, but rather of aggregates of ions whose relative numbers make the compound electrically neutral overall. Therefore, it is not correct to refer to molecules of ionic compounds but rather to formula units equal to the smallest aggregate of ions that can compose the compound. Consider, for example, the ionic compound composed of Na⁺ and SO₄^2- ions. Every ionic compound must be electrically neutral with the same number of positive as negative charges. For the compound in question this requires 2 Na⁺ ions for each SO₄^2-ion. Therefore, the formula of the compound is Na₂SO₄ and a formula unit contains 2 Na⁺ ions and 1 SO₄^2- ion. Furthermore, a mole of Na₂SO₄ composed of 6.02×10²³ formula units of Na₂SO₄ contains 2×6.02×10²³ Na⁺ ions and 6.02×10²³ SO₄^2- ions. Since the ionic charges determine the relative numbers of ions, prefixes need not be used in naming the compound and it is called simply sodium sulfate.

Exercise: Give the formulas and names of compounds formed from each cation on the top row with each anion on the bottom row, below:

1. NH₄⁺ 2. Ca²⁺3. Al³⁺

(A) Cl^- (B) SO₄^2- (C) PO₄^3-

Answers: 1(A) NH₄Cl, ammonium chloride; 1(B) (NH₄)₂SO₄, ammonium sulfate; 1(C), (NH₄)₃PO₄, ammonium phosphate; 2(A) CaCl₂, calcium chloride; 2(B) CaSO₄, calcium sulfate; 2(C), Ca₃(PO₄)₂, calcium phosphate; 3(A) AlCl₃, aluminum chloride; 3(B) Al₂(SO₄)₃, aluminum sulfate; 3(C), AlPO₄, aluminum phosphate.

Prefixes are used in naming ionic compounds where more than 1 cation or more than 1 anion are present in the formula unit. For example, Na₂HPO₄ in which each formula unit is composed of 2 Na⁺ ions, 1 H⁺ ion, and 1 PO₄^3- ion is called disodium monohydrogen phosphate. And KH₂PO₄ is called monopotassium dihydrogen phosphate.