Covalent compounds, also known as molecular compounds, are formed when two nonmetal atoms share valence electrons. Unlike ionic compounds, covalent compounds do not form charged particles, allowing their constituent atoms to combine in various whole-number ratios. This flexibility requires a highly systematic naming method to distinguish between compounds made of the same elements, such as carbon monoxide (\(\text{CO}\)) and carbon dioxide (\(\text{CO}_2\)). The nomenclature system for these compounds relies on specific numerical prefixes that precisely indicate the number of atoms for each element present in the molecule.
The Foundation: Prefixes and Element Order
The systematic naming of covalent compounds depends on Greek numerical prefixes that indicate the atom count for each element. These prefixes ensure the chemical name accurately translates to the molecular formula. The prefixes used are:
- Mono- (1)
- Di- (2)
- Tri- (3)
- Tetra- (4)
- Penta- (5)
- Hexa- (6)
- Hepta- (7)
- Octa- (8)
- Nona- (9)
- Deca- (10)
Before applying prefixes, the elements must be sequenced based on electronegativity. Electronegativity describes an atom’s ability to attract electrons toward itself within a chemical bond. The element with the lower electronegativity is generally named and written first. This usually means the element further to the left or closer to the bottom of the periodic table is named first. For instance, in a compound like \(\text{N}_2\text{O}_4\), nitrogen (\(\text{N}\)) is named before oxygen (\(\text{O}\)) because it is further left.
Naming the First Element
The element that appears first in the chemical formula is named using its full, unmodified element name. For example, in the compound \(\text{PCl}_5\), the first element is phosphorus, and its name remains unchanged in the compound’s nomenclature. The numerical prefix is only added to this first element if the atom count is two or greater.
The prefix “mono-” is conventionally omitted when associated with the first element in the sequence. This means a compound like \(\text{CO}\) is named Carbon Monoxide, not Monocarbon Monoxide. This omission streamlines the name while still indicating a single atom of carbon.
For any number of atoms greater than one for the first element, the appropriate numerical prefix is used. For example, the compound \(\text{N}_2\text{O}_5\) would start with the prefix “di-“, resulting in Dinitrogen. Similarly, the compound \(\text{P}_4\text{O}_{10}\) would begin with Tetraphosphorus, clearly indicating the presence of four phosphorus atoms.
Naming the Second Element
The second element in the formula requires the mandatory application of a numerical prefix. Unlike the first element, the prefix “mono-” is always used for the second element if only one atom exists. This means \(\text{CO}\) is named Carbon Monoxide.
The second element’s name must also be modified by dropping the original ending and attaching the suffix “-ide.” For instance, oxygen becomes oxide, chlorine becomes chloride, and sulfur becomes sulfide. This suffix change is universally applied to the second element.
A pronunciation convention dictates that if a prefix ends in ‘a’ or ‘o’ and is followed by an element name starting with ‘o’ (like oxide), the final vowel of the prefix is dropped. This is why \(\text{N}_2\text{O}_4\) is Dinitrogen Tetroxide, not Tetraoxide. The prefixes “di-” and “tri-” are exceptions, as their final vowels are never dropped before a following vowel.
Putting these rules together, the compound \(\text{PCl}_3\) is named Phosphorus Trichloride. \(\text{N}_2\text{O}_4\) is named Dinitrogen Tetroxide, using prefixes for both elements and applying the vowel-dropping rule. Finally, \(\text{CO}_2\) is Carbon Dioxide, as the first element omits “mono-” and the second uses the “di-” prefix and the “-ide” suffix.