5.8: Binary Molecular Compounds (2024)

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Molecular Compounds

Molecular compounds are inorganic compounds that take the form of discrete molecules. Examples include such familiar substances as water (H₂O) and carbon dioxide (CO₂). These compounds are very different from ionic compounds like sodium chloride (NaCl). Ionic compounds are formed when metal atoms lose one or more of their electrons to nonmetal atoms. The resulting cations and anions are electrostatically attracted to each other.

So what holds the atoms of a molecule together? Rather than forming ions, the atoms of a molecule share their electrons in such a way that abondforms between a pairof atoms. In a carbon dioxide molecule, there are two of these bonds, each occurring between the carbon atom and one of the two oxygen atoms.

Larger molecules can have many, many bonds that serve to hold the molecule together. In a large sample of a given molecular compound, all of the individual molecules are identical.

Naming Binary Molecular Compounds

Recall that a molecular formula shows the number of atoms of each element that a molecule contains. A molecule of water contains two hydrogen atoms and one oxygen atom, so its formula is H₂O. A molecule of octane, which is a component of gasoline, contains 8 atoms of carbon and 18 atoms of hydrogen. The molecular formula of octane is C₈H₁₈.

Naming binarymolecular compounds is similar to naming binary ionic compounds.The convention for naming binary molecular compounds:

The first element in the formula retainsits name.
The second element retainsitsroot name followed by a suffix of -ide.
Greek prefixes (seeTable \(\PageIndex{1}\))are added to each name* to indicate the number of atoms of each element present in the formula.

Table \(\PageIndex{1}\): Greek Prefixes for Naming Binary Molecular Compounds
Number of Atoms	Greek Prefix	Number of Atoms	Greek Prefix
1	mono-*	6	hexa-
2	di-	7	hepta-
3	tri-	8	octa-
4	tetra-	9	nona-
5	penta-	10	deca-

*The prefix mono- is not used for the first element’s name.

⛓Note

Generally, the element that is above and/or to the left in the Periodic Table is written first in the formula, though there are a few exceptions. The order of common nonmetals in formulas of binary molecular compounds:
- C, P, N, H, S,I,Br, Cl, O, F
Ana or o at the end of a prefix is dropped when it precedes oxide. Examples:
- carbon monoxide (rather than carbon monooxide)
- dinitrogen pentoxide (rather than dinitrogen pentaoxide)
The prefix is mono-is usually not added to the first element’s name even if there is only one atom of the first element in a formula.

Some examples of molecular compounds are listed in Table \(\PageIndex{2}\).

Table \(\PageIndex{2}\): Examples of Molecular Compounds
Formula	Name
NO	nitrogen monoxide
N₂O	dinitrogen monoxide
S₂Cl₂	disulfur dichloride
Cl₂O₇	dichlorine heptoxide

Notice that the mono- prefix is not used with the nitrogen in the first compound, but is used with the oxygen in both of the first two examples. The formula S₂Cl₂emphasizes thatformulas for molecular compounds are not reduced to their lowest ratios. The o of the mono- and the a of hepta- are dropped from the name when paired with oxide.

✅Example \(\PageIndex{1}\): Writing Chemical Names

Write the name for each compound.

ClF₃
P₂F₄
BaBr₂

Solution

Cl =chlorine. F = fluorine. The root name of fluorineis fluor-. The second elementhas a suffix of -ide. Cl does not need a prefix, since the formula shows one atom. The prefix that means "3" is tri-. ClF₃is chlorine trifluoride.
P =phosphorus. F = fluorine. The root name of fluorineis fluor-. The second elementhas a suffix of -ide. The prefix that means 2 is di-. The prefix that means "4"is tetra-. P₂F₄is diphosphorus tetrafluoride.
Be careful. BaBr₂is a binary ionic compound. Rules for naming binary ionic compound were discussed in Section 5.5. Greek prefixes are not used for binary ionic compounds. Ba = barium. Br = bromine. The root name of bromineis brom-. The nonmetal has a suffix of -ide.BaBr₂is barium bromide.

✅Example \(\PageIndex{2}\): Writing ChemicalFormulas

Write the chemical formulafor each compound.

sulfur hexafluoride
dinitrogen pentoxide
aluminum oxide

Solution

Writing formulas for binary molecular compounds is simple. The prefixes tell exactly how many atoms of each element are present in one molecule. There is no need to look up charges based on the position in the periodic table. Nonmetals bond to each other with covalent bonds, where electrons are shared rather than transferred.

S =sulfur. F = fluorine. S has no prefix, so there is only one atom. The prefix hexa- means "6".Sulfur hexafluoride is SF₆.
N = nitrogen. O = oxygen. The prefix di- means "2".The prefix pent- means "5" (ais left off when paired with oxide).Dinitrogen pentoxide is N₂O₅.
Be careful. Aluminum oxideis a binary ionic compound. Rules for writing formulas for binary ionic compound were discussed in Section 5.5. Greek prefixes are not used for binary ionic compounds. The chargeonan aluminum ion is 3+, Al³⁺.The chargeonan oxideion is 2–, O^2–.Charge is balanced with two Al³⁺and threeO^2–. Aluminum oxide is Al₂O₃.

✏️Exercise \(\PageIndex{1}\)

If the chemical formula is provided, write the name. If the name is provided, write the formula.

CF₄
diarsenic pentaiodide
SO₃

Answer A: carbon tetrafluoride

Answer B: As₂I₅

Answer C: sulfur trioxide

Simple Molecular Compounds with Common Names

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
H₂O₂: hydrogen peroxide

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

Some Compounds Have Both Covalent and Ionic Bonds

If you recall the introduction of polyatomic ions, you will remember that the bonds that hold the polyatomic ions together are covalent bonds. Once the polyatomic ion is constructed with covalent bonds, it reacts with other substances as an ion. The bond between a polyatomic ion and another ion will be ionic. An example of this type of situation is in the compound sodium nitrate. Sodium nitrate is composed of a sodium ion and a nitrate ion. The nitrogen atom and oxygen atoms in a nitrate ion are held together by covalent bonds and the nitrate ion is attached to the sodium ion by an ionic bond.

Summary

A molecular compound is usually composed of two or more nonmetal elements.
Molecular compounds are named with the first element first and then the second element by using the stem of the element name plus the suffix -ide. Numerical prefixes are used to specify the number of atoms in a molecule.

This pageis shared under a CK-12 license and was authored, remixed, and/or curated byLance S. Lund (Anoka-Ramsey Community College), MelissaAlviar-Agnew, and Henry Agnew. Original source:https://www.ck12.org/c/chemistry/.

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