5.3: Naming Covalent Compounds

Learning Objectives

• Write the chemical names of covalent molecules.
• Nomenclature of alkanes

In the previous section, the process for using a Lewis structure to determine the chemical formula of a covalent molecule was presented and applied. The chemical name of a molecule, in turn, is derived based on the information included in its chemical formula. As stated previously, a chemical name should uniquely correspond to a single chemical formula. In other words, no two chemical formulas should share a common chemical name.

Recall that covalent bonds are produced when unpaired electrons found within two atoms interact to form a shared pair of electrons. As a Lewis structure is the result of correctly executing this sharing process, the chemical formula and the chemical name of the corresponding covalent molecule must accurately reflect the information shown in that Lewis structure. The procedure for writing the chemical name of a covalent molecule is explained in the following paragraphs.

Naming Covalent Molecules

For a covalent molecule, the information represented in its chemical name must be a direct reflection of its Lewis structure. Therefore, the chemical name of a covalent molecule must not only contain information that indicates the identities of its constituent elements, but also must reflect how many of each of those elements are present within the molecule. Note that, if written properly, the chemical formula for a covalent molecule also contains this information and, therefore, can be used as the basis for developing a chemical name.

Elemental names are incorporated into a covalent molecule's chemical name in the order in which their corresponding elemental symbols appear in the chemical formula. The suffix on the second elemental term is replaced with "-ide," in order to indicate its secondary placement within the chemical formula. Finally, since the subscripts in a covalent chemical formula are used to indicate how many times each elemental symbol appears in the molecule's Lewis structure, corresponding numerical prefixes, which are listed below in Table \(\PageIndex{1}\), must be incorporated into the molecule's chemical name.

Table \(\PageIndex{1}\): Numerical Prefixes Used When Naming Covalent Molecules

Number of Atoms in the Molecule (Indicated by a Subscript)	Numerical Prefix
1	mono-*
2	di-
3	tri-
4	tetra-
5	penta-
6	hexa-
7	hepta-
8	octa-
9	nona-
10	deca-

Note that the prefix "mono-" is never used to change the first elemental term in a covalent chemical name and should only be used as a modifier on the remaining term if the secondary element is oxygen. Finally, an "a" or "o" at the end of a prefix is usually dropped if the name of the element that is being altered begins with a vowel.

Example \(\PageIndex{1}\)

Write the chemical name of CO.

Solution

These prefix rules are best exemplified when writing the chemical name of "CO," which is the chemical formula of a commonly-known molecule.

This molecule is formed using covalent bonds, as its constituent elements, C (carbon) and O (oxygen), are both non-metals.

Since the elemental symbol "C" appears first in the given chemical formula, "carbon" is the basis of the first word in the molecule's chemical name. The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-." However, this prefix is not used to alter the first elemental term in a covalent chemical name. Therefore, the first word in the chemical name of this molecule is "carbon."

Because the elemental symbol "O" is written second in the given chemical formula, "oxide" becomes the basis of the second word in the molecule's chemical name. The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula. The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-." Because the secondary element is oxygen, this prefix is used as a modifier on the term "oxide." Finally, since "oxide" begins with a vowel, the "o" of "mono-" is dropped. Therefore, the second word in the chemical name of this covalent molecule is "monoxide."

The result of combining these words, "carbon monoxide," is the chemically-correct name for CO.

Example \(\PageIndex{2}\)

Write the chemical name of SF₂, a covalent molecule that is formed when fluorine and sulfur bond with one another.

Solution

Since the elemental symbol "S" appears first in the given chemical formula, "sulfur" is the basis of the first word in the molecule's chemical name. The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-." However, this prefix is not used to alter the first elemental term in a covalent chemical name. Therefore, the first word in the chemical name of this molecule is "sulfur."

Because the elemental symbol "F" is written second in the given chemical formula, "fluoride" becomes the basis of the second word in the molecule's chemical name. The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula. The subscript on this elemental symbol, a "2," corresponds to prefix of "di-." Therefore, the second word in the chemical name of this covalent molecule is "difluoride."

The result of combining these words, "sulfur difluoride," is the chemically-correct name for SF₂.

Exercise \(\PageIndex{1}\)

Write the chemical name that corresponds to each of the following chemical formulas.

1. CCl₄, the covalent molecule that is formed when carbon and chlorine bond with one another.
2. PBr₃, a covalent molecule that is formed when bromine and phosphorus bond with one another.
3. N₃P₆, a covalent molecule that is formed when nitrogen and phosphorus bond with one another.
4. I₂O₅, a covalent molecule that is formed when oxygen and iodine bond with one another.
5. HF, the covalent molecule that is formed when hydrogen and fluorine bond with one another.

Answer a

Since the elemental symbol "C" appears first in the given chemical formula, "carbon" is the basis of the first word in the molecule's chemical name. The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-." However, this prefix is not used to alter the first elemental term in a covalent chemical name. Therefore, the first word in the chemical name of this molecule is "carbon."

Because the elemental symbol "Cl" is written second in the given chemical formula, "chloride" becomes the basis of the second word in the molecule's chemical name. The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula. The subscript on this elemental symbol, a "4," corresponds to prefix of "tetra-." Therefore, the second word in the chemical name of this covalent molecule is "tetrachloride."

The result of combining these words, "carbon tetrachloride," is the chemically-correct name for CCl₄.

Answer b

Since the elemental symbol "P" appears first in the given chemical formula, "phosphorus" is the basis of the first word in the molecule's chemical name. The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-." However, this prefix is not used to alter the first elemental term in a covalent chemical name. Therefore, the first word in the chemical name of this molecule is "phosphorus."

Because the elemental symbol "Br" is written second in the given chemical formula, "bromide" becomes the basis of the second word in the molecule's chemical name. The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula. The subscript on this elemental symbol, a "3," corresponds to prefix of "tri-." Therefore, the second word in the chemical name of this covalent molecule is "tribromide."

The result of combining these words, "phosphorus tribromide," is the chemically-correct name for PBr₃.

Answer c

Since the elemental symbol "N" appears first in the given chemical formula, "nitrogen" is the basis of the first word in the molecule's chemical name. The subscript on this elemental symbol, a "3," corresponds to prefix of "tri-." Therefore, the first word in the chemical name of this molecule is "trinitrogen."

Because the elemental symbol "P" is written second in the given chemical formula, "phosphide" becomes the basis of the second word in the molecule's chemical name. The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula. The subscript on this elemental symbol, a "6," corresponds to prefix of "hexa-." Therefore, the second word in the chemical name of this covalent molecule is "hexaphosphide."

The result of combining these words, "trinitrogen hexaphosphide," is the chemically-correct name for N₃P₆.

Answer d

Since the elemental symbol "I" appears first in the given chemical formula, "iodine" is the basis of the first word in the molecule's chemical name. The subscript on this elemental symbol, a "2," corresponds to prefix of "di-." Therefore, the first word in the chemical name of this molecule is "diiodine."

Because the elemental symbol "O" is written second in the given chemical formula, "oxide" becomes the basis of the second word in the molecule's chemical name. The subscript on this elemental symbol, a "5," corresponds to prefix of "penta-." Since "oxide" begins with a vowel, the "a" of "penta-" is dropped. Therefore, the second word in the chemical name of this covalent molecule is "pentoxide."

The result of combining these words, "diiodine pentoxide," is the chemically-correct name for I₂O₅.

Answer e

Since the elemental symbol "H" appears first in the given chemical formula, "hydrogen" is the basis of the first word in the molecule's chemical name. The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-." However, this prefix is not used to alter the first elemental term in a covalent chemical name. Therefore, the first word in the chemical name of this molecule is "hydrogen."

Because the elemental symbol "F" is written second in the given chemical formula, "fluoride" becomes the basis of the second word in the molecule's chemical name. The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-." However, this prefix is only used as a modifier on the second term in a covalent chemical name if the secondary element is oxygen. Because the secondary element is fluorine, the "mono-" prefix is not applied. Therefore, the second word in the chemical name of this molecule is "fluoride."

The result of combining these words, "hydrogen fluoride," is the chemically-correct name for HF.

Writing the Molecular Formula from the Name

Example \(\PageIndex{3}\)

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 \(\PageIndex{4}\)

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

Methane is the simplest organic compound. Organic compounds are compounds with carbon atoms and are named by a separate nomenclature system.

The prefix of mono are not used for the two molecules below:

HCl: hydrogen chloride

HF: hydrogen fluoride

Oxides of Nitrogen-Applications in Medicine

Nitric Oxide

Nitrogen monoxide (NO) is known more commonly as nitric oxide. It is a colorless gas formed when nitrogen and oxygen in the air combine when gasoline is burned in the engines of cars and trucks. So this compound has a bad reputation regarding atmospheric pollution. However, NO has many biological functions. It regulates blood pressure, dilates arteries and blood vessels, and is used by the immune system to help fight infections.

These effects of NO on the human body were noticed in the mid 19 ^th century soon after the discovery of the explosive nitroglycerin that was used to make dynamite. It was noticed that people who worked with nitroglycerin experienced splitting headaches which was the result of vasodilation (relaxation) of the blood vessels in the brain. This led to the use of nitroglycerin being used as a medication for the treatment of angina which is caused by constriction of the arteries that carry blood to the heart. A century later scientists discovered that the body produces NO from nitroglycerin and it is NO that causes coronary arteries to dilate reducing the symptoms of angina. In 1998 Nobel Prize in Medicine was awarded to three scientists who worked on the nitric oxide research.

Nitrous oxide

Nitrous oxide (N₂O) is prepared by the careful thermal decomposition of ammonium nitrate (NH₄NO₃). Nitrous oxide is known as "laughing gas" due to the euphoric effects of inhaling it, a property that has led to its recreational use as a hallucinogen. However, it is as an anesthetic that it has a legitimate application. The first use of nitrous oxide as anesthetic drug was when dentist Horace Wells with assistance by Gardner Quincy Colton and John Mankey Riggs, demonstrated insensitivity to pain from a dental extraction in December 1844. Wells subsequently treated 12-15 patients, and according to his own record it only failed as an anesthetic in two cases.

The method did not come into general use until 1863, when Colton successfully used it for more than 25,000 patients. As such, the usage of nitrous oxide rapidly became the preferred anesthetic method in dentistry. The gas is mild enough to keep a patient in a conscious and conversational state, and yet in most cases strong enough to suppress the pain caused by dental work. It remains the preferred gas anesthetic in today's dentistry.

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.

Contributors

• Jonathan Mooney (McGill University)
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