Diatomic Molecules

Homonuclear and Heteronuclear

Chemical Bond
Covalent Chemical Bond. PASIEKA/Getty Images

There are hundreds of diatomic molecules. This list includes diatomic elements and diatomic chemical compounds.

Mononuclear Diatomic Molecules

Some of these molecules consist of one element or are diatomic elements. Diatomic elements are examples of homonuclear molecules, where all of the atoms in the molecule are the same. The chemical bonds between the atoms are covalent and nonpolar. The seven diatomic elements are:

Hydrogen (H2)
Nitrogen (N2)
Oxygen (O2)
Fluorine (F2)
Chlorine (Cl2)
Iodine (I2)
Bromine (Br2)

5 or 7 Diatomic Elements?

Some sources will say there are five diatomic elements, rather than seven. This is because only five elements form stable diatomic molecules at standard temperature and pressure: the gases hydrogen, nitrogen, oxygen, fluorine, and chlorine. Bromine and iodine form homonuclear diatomic molecules at slightly higher temperatures. It's possible that an eighth element forms a diatomic molecule. The status of astatine is unknown.

Heteronuclear Diatomic Molecules

Many other diatomic molecules consist of two elements. In fact, most elements form diatomic molecules, particularly at higher temperatures. Past a certain temperature, however, all molecules break into their constituent atoms. The noble gases do not form diatomic molecules. Diatomic molecules consisting of two different elements are called heteronuclear molecules.

Here are some heteronuclear diatomic molecules:

CO
NO
MgO
HCl
KBr
HF
SiO

Binary Compounds Are Not Always Considered Diatomic

There are many binary compounds consisting of a 1-to-1 ratio of two types of atoms, yet they are not always considered to be diatomic molecules. The reason is that these compounds are only gaseous diatomic molecules when they are evaporated.

When they cool to room temperature, the molecules form polymers. Examples of this type of compound include silicon oxide (SiO) and magnesium oxide (MgO).

Diatomic Molecule Geometry

All diatomic molecules have linear geometry. There isn't any other possible geometry because connecting a pair of objects necessarily produces a line. Linear geometry is the simplest arrangement of atoms in a molecule.

Other Diatomic Elements

It's possible for additional elements to form homonuclear diatomic molecules. These elements are diatomic when evaporated, yet polymerize when they are cooled. Elemental phosphorus can be heated to yield diphosphorus, P2. Sulfur vapor primarily consists of disulfur, S2. Lithium forms dilithium, Li2, in the gas phase (and no, you can't run a starship on it). Unusual diatomic elements include ditungsten (W2) and dimolybdenum (Mo2), which are joined via sextuple bonds as gasses.

Fun Fact About Diatomic Elements

Did you realize around 99 percent of the Earth's atmosphere consists of just two diatomic molecules? Nitrogen accounts for 78 percent of the atmosphere, while oxygen is 21 percent. The most abundant molecule in the universe is also a diatomic element. Hydrogen, H2, accounts for much of the mass of the universe, although it only accounts for one part per million concentration in Earth's atmosphere.

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Helmenstine, Anne Marie, Ph.D. "Diatomic Molecules." ThoughtCo, Feb. 8, 2018, thoughtco.com/what-are-diatomic-molecules-608496. Helmenstine, Anne Marie, Ph.D. (2018, February 8). Diatomic Molecules. Retrieved from https://www.thoughtco.com/what-are-diatomic-molecules-608496 Helmenstine, Anne Marie, Ph.D. "Diatomic Molecules." ThoughtCo. https://www.thoughtco.com/what-are-diatomic-molecules-608496 (accessed May 22, 2018).