Chemical nomenclature is the formal system scientists use to assign names to chemical compounds, ensuring that a specific name corresponds to one unique chemical structure. Prefixes are a foundational tool within this system, serving to indicate the exact number of atoms of each element that make up a compound. Understanding when these prefixes are applied, and when they are intentionally left out, is essential to correctly interpreting any chemical formula or name.
Identifying Compounds That Require Prefixes
Prefixes are systematically used when naming binary molecular compounds, which are substances formed exclusively from two different nonmetal elements. These compounds are held together by covalent bonds, where atoms share electrons rather than transferring them. The absence of fixed electrical charges means that nonmetal elements can often combine in multiple, distinct whole-number ratios.
For instance, nitrogen and oxygen can form several different compounds, such as \(\text{NO}\), \(\text{NO}_2\), and \(\text{N}_2\text{O}\). Without a clear numerical indicator, referring to any of these simply as “nitrogen oxide” would be chemically meaningless. The prefix system resolves this by providing a precise count of atoms for each element present in the molecule.
This precision ensures that nitrogen dioxide (\(\text{NO}_2\)) refers to a compound containing one nitrogen atom and two oxygen atoms. Dinitrogen tetroxide (\(\text{N}_2\text{O}_4\)) correctly identifies a different compound with two nitrogen atoms and four oxygen atoms. Prefixes are necessary for distinguishing between the numerous possible combinations that can occur between nonmetal elements.
The System of Numerical Prefixes
The prefixes used in this nomenclature are derived from Greek words, each corresponding to a specific number of atoms in the compound. These numerical indicators are placed directly before the name of the element they describe. The most commonly encountered prefixes are:
- Mono- (one)
- Di- (two)
- Tri- (three)
- Tetra- (four)
- Penta- (five)
- Hexa- (six)
- Hepta- (seven)
- Octa- (eight)
- Nona- (nine)
- Deca- (ten)
A key rule for applying this system is the treatment of the prefix mono-, which is used when only one atom of an element is present. The mono- prefix is always included for the second element in the compound name, such as in carbon monoxide (\(\text{CO}\)). However, when the first element in the compound has only one atom, the mono- prefix is omitted for simplicity; thus, \(\text{Si}\text{O}_2\) is silicon dioxide.
To improve pronunciation, an adjustment is made when a prefix ending in a vowel is placed before an element name that also starts with a vowel, particularly oxygen. In these cases, the final vowel of the prefix is dropped. For example, a compound with five oxygen atoms uses the prefix penta- but is named pentoxide, not pentaoxide.
Naming Compounds Without Prefixes
In contrast to molecular compounds, prefixes are not used when naming ionic compounds, which typically form between a metal and a nonmetal. These substances are not composed of individual molecules but exist as large, extended lattices of positively and negatively charged ions.
The ratio of elements in an ionic compound is fixed by the requirement that the total positive charge must perfectly balance the total negative charge, resulting in a neutral compound. For example, sodium (\(\text{Na}\)) always forms an ion with a \(+1\) charge, and chlorine (\(\text{Cl}\)) always forms an ion with a \(-1\) charge. This means they must combine in a one-to-one ratio to form sodium chloride (\(\text{NaCl}\)).
Since the charges of the ions determine one specific, fixed combination, using prefixes would be redundant. Therefore, \(\text{NaCl}\) is named sodium chloride, not monosodium monochloride, and \(\text{Mg}\text{O}\) is simply magnesium oxide. For ionic compounds, the name of the metal ion is stated first, followed by the nonmetal ion with an -ide ending.