Molecular compounds, also known as covalent compounds, form when atoms share electrons to create strong connections called covalent bonds. These compounds consist of two or more non-metal elements linked together in discrete units called molecules. A standardized naming system is important in chemistry, ensuring clear communication among scientists globally and preventing confusion. Without such a system, differentiating between the vast number of compounds would be difficult.
Identifying Molecular Compounds and Their Components
Recognizing a molecular compound begins with identifying its constituent elements; they are formed from two or more non-metal elements. This contrasts with ionic compounds, which involve a metal and a non-metal. The names of molecular compounds are built using the names of the elements, numerical prefixes indicating the number of atoms of each element, and a suffix for the second element.
The prefixes used in naming molecular compounds are derived from Greek and specify the count of each atom:
Mono- indicates one
Di- signifies two
Tri- denotes three
Tetra- for four
Penta- for five
Hexa- for six
Hepta- for seven
Octa- for eight
Nona- for nine
Deca- for ten
The Step-by-Step Naming Process
Naming binary molecular compounds, which contain only two different elements, follows a clear sequence of steps. First, the name of the first element in the chemical formula is stated as its elemental name.
This element is often the one appearing earlier in the periodic table or further to the left. Next, determine the number of atoms for both the first and second elements. For each, the appropriate numerical prefix is attached to its name.
An exception applies to the “mono-” prefix: it is omitted when there is only one atom of the first element. However, “mono-” is always used for the second element if only one atom is present.
The third step involves naming the second element by taking its root name and adding the “-ide” suffix. For example, oxygen becomes “oxide,” chlorine becomes “chloride,” and nitrogen becomes “nitride.”
Finally, a rule addresses pronunciation: if a prefix ends with an ‘a’ or ‘o’ and the element name begins with a vowel, the final ‘a’ or ‘o’ from the prefix is dropped. For example, “tetraoxide” becomes “tetroxide” for smoother pronunciation.
Practice and Common Examples
Applying these rules helps in systematically naming molecular compounds. Consider CO₂. The first element is carbon, and since there is only one carbon atom, the “mono-” prefix is omitted. The second element is oxygen, and there are two oxygen atoms, so the prefix “di-” is used, and oxygen takes the “-ide” suffix to become “dioxide.” Thus, CO₂ is named carbon dioxide.
Another example is N₂O₄. There are two nitrogen atoms, so “di-” is used for nitrogen, making it “dinitrogen.” For oxygen, there are four atoms, so “tetra-” is the prefix, and with the “-ide” suffix, it becomes “tetroxide,” dropping the ‘a’ from “tetra-” before “oxide.” Therefore, N₂O₄ is named dinitrogen tetroxide.
For PCl₅, the first element is phosphorus, and since there is only one, no prefix is needed. The second element is chlorine, and there are five chlorine atoms, so the prefix “penta-” is used, and “chlorine” becomes “chloride.” This compound is named phosphorus pentachloride.
SF₆ involves one sulfur atom, so it is “sulfur.” There are six fluorine atoms, so the prefix “hexa-” is applied, and “fluorine” becomes “fluoride.” This results in the name sulfur hexafluoride.