5.4: Covalent Compounds - Formulas and Names

The chemical formulas for covalent compounds are referred to as molecular formulas because these compounds exist as separate, discrete molecules. Typically, a molecular formula begins with the nonmetal that is closest to the lower left corner of the periodic table, except that hydrogen is almost never written first (H₂O is the prominent exception). Then the other nonmetal symbols are listed. Numerical subscripts are used if there is more than one of a particular atom. For example, we have already seen CH₄, the molecular formula for methane. Below is the molecular formula of ammonia, NH₃.

NH_3. An arrow points to the N and says, "The lack of a subscript on the N symbol indicates that there is 1 atom of nitrogen in a molecule of ammonia". An arrow points to H and says, "The subscript below the H symbol indicates that there are 3 hydrogen atoms in a molecule of ammonia".

NAMING COVALENT COMPOUNDS

Naming binary (two-element) covalent compounds is similar to naming simple ionic compounds. The first element in the formula is simply listed using the name of the element. The second element is named by taking the stem of the element name and adding the suffix -ide. A system of numerical prefixes is used to specify the number of atoms in a molecule. Table \(\PageIndex{1}\) lists these numerical prefixes. Normally, no prefix is added to the first element’s name if there is only one atom of the first element in a molecule. If the second element is oxygen, the trailing vowel is usually omitted from the end of a polysyllabic prefix but not a monosyllabic one (that is, we would say “monoxide” rather than “monooxide” and “trioxide” rather than “troxide”).

Table \(\PageIndex{1}\): Numerical Prefixes for Naming Binary Covalent Compounds

Number of Atoms in Compound	Prefix on the Name of the Element
1	mono-*
2	di-
3	tri-
4	tetra-
5	penta-
6	hexa-
7	hepta-
8	octa-
9	nona-
10	deca-
*This prefix is not used for the first element’s name.

Let us practice by naming the compound whose molecular formula is CCl₄. The name begins with the name of the first element—carbon. The second element, chlorine, becomes chloride, and we attach the correct numerical prefix (“tetra-”) to indicate that the molecule contains four chlorine atoms. Putting these pieces together gives the name carbon tetrachloride for this compound.

Example \(\PageIndex{2}\)

Write the molecular formula for each compound.

1. chlorine trifluoride
2. phosphorus pentachloride
3. sulfur dioxide
4. dinitrogen pentoxide

Solution

If there is no numerical prefix on the first element’s name, we can assume that there is only one atom of that element in a molecule.

1. ClF₃
2. PCl₅
3. SO₂
4. N₂O₅ (The di- prefix on nitrogen indicates that two nitrogen atoms are present.)

Exercise \(\PageIndex{2}\)

Write the molecular formula for each compound.

1. nitrogen dioxide
2. dioxygen difluoride
3. sulfur hexafluoride
4. selenium monoxide

Answer a:

a. NO₂

Answer b:

O₂F₂

Answer c:

SF₆

Answer d:

SeO

Because it is so unreactive, sulfur hexafluoride is used as a spark suppressant in electrical devices such as transformers.

Example \(\PageIndex{3}\)

Write the name for each compound.

1. BrF₅
2. S₂F₂
3. CO

Solution

1. bromine pentafluoride
2. disulfur difluoride
3. carbon monoxide

Exercise \(\PageIndex{3}\)

Write the name for each compound.

1. CF₄
2. SeCl₂
3. SO₃

Answer a:

carbon tetrafluoride

Answer b:

selenium dichloride

Answer c:

sulfur trioxide

For some simple covalent compounds, we use common names rather than systematic names. We have already encountered these compounds, but we list them here explicitly:

• H₂O: water
• NH₃: ammonia
• CH₄: methane

Key Takeaways

• The chemical formula of a simple covalent compound can be determined from its name.
• The name of a simple covalent compound can be determined from its chemical formula.