7.4: Binary Covalent Compounds- Formulas and Names

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Learning Objectives

Given the formula of a binary covalent inorganic compound, write the systematic name.
Given the systematic name for a binary covalent inorganic compound, write the formula.

Covalent and Ionic Compounds

What elements make covalent bonds? Covalent bonds form when two or more nonmetals combine. For example, both hydrogen and oxygen are nonmetals, and when they combine to make water, they do so by forming covalent bonds. Compounds that are composed of only non-metals or semi-metals with non-metals will display covalent bonding and will be classified as molecular compounds.

As a general rule of thumb, compounds that involve a metal binding with either a non-metal or a semi-metal will display ionic bonding. Thus, the compound formed from sodium and chlorine will be ionic (a metal and a non-metal). Nitrogen monoxide (NO) will be a covalently bound molecule (two non-metals), silicon dioxide (SiO₂) will be a covalently bound molecule (a semi-metal and a non-metal) and MgCl₂ will be ionic (a metal and a non-metal).

A polyatomic ion is an ion composed of two or more atoms that have a charge as a group (poly = many). The ammonium ion (see figure below) consists of one nitrogen atom and four hydrogen atoms. Together, they comprise a single ion with a 1+ charge and a formula of NH₄⁺. The carbonate ion (see figure below) consists of one carbon atom and three oxygen atoms and carries an overall charge of 2−. The formula of the carbonate ion is CO₃²⁻.

The atoms of a polyatomic ion are tightly bonded together and so the entire ion behaves as a single unit. Nonmetal atoms in polyatomic ions are joined by covalent bonds, but the ion as a whole participates in ionic bonding. For example, ammonium chloride (NH₄Cl) has ionic bonding between a polyatomic ion, \(\ce{NH_4^{+}}\), and \(\ce{Cl^{−}}\) ions, but within the ammonium ion (NH₄⁺), the nitrogen and hydrogen atoms are connected by covalent bonds (shown above).

Both ionic and covalent bonding are also found in calcium carbonate. Calcium carbonate (CaCO₃) has ionic bonding between calcium ion \(\ce{Ca^{2+}}\) and a polyatomic ion, \(\ce{CO_3^{2-}}\), but within the carbonate ion (CO₃²^-), the carbon and oxygen atoms are connected by covalent bonds (shown above).

Example \(\PageIndex{1}\)

Is each compound formed from ionic bonds, covalent bonds, or both?

\(\ce{Na_2O}\)
\(\ce{Na_3PO_4}\)
\(\ce{N_2O_4}\)

Answer a: The elements in \(\ce{Na_2O}\) are a metal and a nonmetal, which form ionic bonds.
Answer b: Because sodium is a metal and we recognize the formula for the phosphate ion, we know that this compound is ionic. However, within the polyatomic phosphate ion, the atoms are held together by covalent bonds, so this compound contains both ionic and covalent bonds.
Answer c: The elements in \(\ce{N_2O_4}|\) are both nonmetals, rather than a metal and a nonmetal. Therefore, the atoms form covalent bonds.

Exercise \(\PageIndex{1}\)

Is each compound are formed from ionic bonds, covalent bonds, or both?

\(\ce{Ba(OH)_2}\)
\(\ce{F_2}\)
\(\ce{PCl_3}\)

Answer a:: both
Answer b:: covalent
Answer c:: covalent

Molecular Formulas

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 and acids are the prominent exceptions). 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.

CK12 Screenshot 7-1-1.png

Unlike ionic compounds, the subscripts for covalent compounds are never reduced. For example, a molecule of hydrogen peroxide contains two oxygen atoms and two hydrogen atoms. This means the formula is H₂O₂. it would be incorrect to reduce it to HO since this would imply that the molecule contains only one hydrogen atom and one oxygen atom.

Naming Covalent Compounds

Naming binary (two-element) covalent compounds is different than naming 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. (Note that the -ide suffix here does not indicate the presence of a monatomic anion.) 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.

chlorine trifluoride
phosphorus pentachloride
sulfur dioxide
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.

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

Exercise \(\PageIndex{2}\)

Write the molecular formula for each compound.

nitrogen dioxide
dioxygen difluoride
sulfur hexafluoride
selenium monoxide

Answer a:: a. NO₂
Answer b:: O₂F₂
Answer c:: SF₆
Answer d:: SeO

Example \(\PageIndex{3}\)

Write the name for each compound.

BrF₅
S₂F₂
CO

Solution

bromine pentafluoride
disulfur difluoride
carbon monoxide

Exercise \(\PageIndex{3}\)

Write the name for each compound.

CF₄
SeCl₂
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 some of these compounds, but we list them here explicitly:

H₂O: water
NH₃: ammonia
CH₄: methane
H₂O₂: hydrogen peroxide