3.6: Molecular Compounds: Formulas and Nomenclature
Molecular compounds or covalent compounds result when atoms share electrons to form covalent bonds. Since there is no electron transfer, molecular compounds do not contain ions; instead, they consist of discrete, neutral molecules.
Since covalent compounds are formed from the combination of nonmetals, the periodic table can help recognize many of them. The position of a compound’s elements in the periodic table can predict whether the compound is ionic or covalent (although there are exceptions).
The bonding characteristics of molecular compounds are different from ionic compounds, and they are named using a different system as well. The charges of cations and anions dictate their ratios in ionic compounds, so specifying the names of the ions provides sufficient information to determine chemical formulas. However, because covalent bonding allows for significant variation in the combination ratios of the atoms in a molecule, the names for molecular compounds must explicitly identify these ratios.
Molecular Compounds Composed of Two Elements
When two nonmetallic elements form a molecular compound, several combination ratios are often possible. For example, carbon and oxygen can form the compounds CO and CO2. Since these are different substances with different properties, they cannot both have the same name (they cannot both be called carbon oxide). To account for this, prefixes specifying the numbers of atoms of each element are used. The name of the more metallic element (the one farther to the left and/or bottom of the periodic table) is first, followed by the name of the more nonmetallic element (the one farther to the right and/or top) with its ending changed to the suffix –ide. Greek prefixes designate the numbers of atoms of each element.
When only one atom of the first element is present, the prefix mono- is usually deleted from that part. Thus, CO is named carbon monoxide, and CO2 is called carbon dioxide. When two vowels are adjacent, the Greek prefix is usually dropped. Sulfur dioxide (SO2), iodine heptafluoride (IF7), and nitrogen dioxide (NO2) are names of some molecular compounds composed of two elements.
In chemistry, certain molecular compounds are generally represented by using common names, instead of chemical names. For example, although NO is often called nitric oxide, its proper name is nitrogen monoxide. Similarly, N2O is known as nitrous oxide, even though it is dinitrogen monoxide. H2O is usually called water, and not dihydrogen monoxide.
Some compounds containing hydrogen are members of an important class of substances known as acids. Many of these compounds release hydrogen ions, H+, when dissolved in water. To denote this distinct chemical property, a mixture of water and an acid is given a name derived from the compound’s name.
If the compound is a binary acid (composed of hydrogen and one other nonmetallic element), first, the word ‘hydrogen’ is changed to the prefix hydro-. The nonmetallic element name is modified by adding the suffix -ic, followed by the addition of the word ‘acid’. For example, when the gas HBr (hydrogen bromide) is dissolved in water, the solution is called hydrobromic acid.
Oxyacids are compounds that contain hydrogen, oxygen, and at least one other element, and are bonded in such a way as to impart acidic properties to the compound. Typical oxyacids consist of hydrogen combined with a polyatomic, oxygen-containing ion.
To name oxyacids, omit ‘hydrogen’ to start with the root name of the anion. Replace –ate with –ic, or –ite with –ous and add the term ‘acid’ at the end. For example, to name H2CO3, ‘hydrogen’ is omitted, the –ate of carbonate is replaced with –ic, and acid is added. Thus, H2CO3 is carbonic acid.