5.4: Classification of Compounds

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⚙️ Learning Objectives

Classify compounds based on their composition.

Nomenclature is the process of naming chemical compounds so they may be easily identified as separate chemicals. The primary function of chemical nomenclature is to ensure that a spoken or written chemical name leaves no ambiguity as to which chemical compound the name refers – each chemical name should refer to a single substance. A less important aim is to ensure that each substance has a single name, although a limited number of alternative names is acceptable in many cases. Preferably, the name also conveys some information about the structure or chemistry of a compound. A common name will often suffice to identify a chemical compound in a particular set of circumstances. To be more generally applicable, the name should indicate the chemical formula at a very minimum. To be more specific still, the three-dimensional arrangement of the atoms may need to be specified.

Ionic Compounds vs. Molecular Compounds

The distinction between ionic compounds and molecular compounds was discussed in Section 5.3 and is summarized in Table \(\PageIndex{1}\) below.

Table \(\PageIndex{1}\): Distiguishing Between Ionic Compounds and Molecular Compounds
Ionic Compounds	Molecular Compounds
Consist of metals bonded to nonmetals with one class of exceptions – ammonium compounds	Consist of nonmetals and/or metalloids bonded to other nonmetals and/or metalloids
Atoms joined by ionic bonds	Atoms joined by covalent bonds

Further Classification of Compounds

Students typically become more successful at writing chemical formulas and chemical names when they are able to properly classify compounds. Many compounds are said to be binary, which means they consist of two different elements. Other compounds contain polyatomic ions, previously identified as groups of covalently bonded atoms that often act as single units in chemical reactions. Acids are compounds that contain the element hydrogen, H, typically written at the front of a chemical formula. Oxyacids are acids that also contain the element oxygen.

With this in mind, many compounds may be classified into five different categories, each of which uses a slightly different, but related, set of rules. These five common classes of compounds are:

Binary ionic compounds
Ionic compounds containing polyatomic ions
Binary molecular compounds
Binary acids
Oxyacids

✏️ Exercise \(\PageIndex{1}\)

Classify each compound into one of the five categories listed above. Provide a short explanation for your choice.

H₃PO₄
Ag₂S
HI
SF₆
Na₂SO₄
Ti(C₂H₃O₂)₄

Answer A: H₃PO₄ is an oxyacid. H is written at the front of the formula, making it an acid. Since it also contains O, it is an oxyacid.

Answer B: Ag₂S is a binary ionic compound. Ag₂S has two elements, which makes it binary. Ag is a metal and S is a nonmetal, making the compound ionic.

Answer C: HI is a binary acid. HI has two elements, which makes it binary. H is written at the front of the formula, making it an acid.

Answer D: SF₆ is a binary molecular compound. SF₆ has two elements, which makes it binary. S and F are both nonmetals, making the compound molecular.

Answer E: Na₂SO₄ is an ionic compound containing a polyatomic ion. Na₂SO₄ has three elements, so it cannot be binary. Na is a metal and both S and O are nonmetals, making the compound ionic.

Answer F: Ti(C₂H₃O₂)₄ is an ionic compound containing a polyatomic ion. Ti(C₂H₃O₂)₄ has four elements, so it cannot be binary. Ti is a metal, while C, H, and O are all nonmetals, making the compound ionic.

This page is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Lance S. Lund (Anoka-Ramsey Community College), Wikipedia, Marisa Alviar-Agnew, and Henry Agnew.