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Did you know that diatomic molecules make up the majority of the Earth’s atmosphere? Diatomic molecules are made up of two atoms. In this lesson, we will discuss diatomic molecules and go over some examples.
What Is a Diatomic Molecule?
In Greek, the prefix ‘di-‘ means ‘two.’ Knowing that, it isn’t hard to guess that diatomic molecules are molecules composed of only two atoms. What you may not realize is that diatomic molecules are all around us. The Earth’s atmosphere is mainly composed of the diatomic molecules oxygen (O2) – about 21% and nitrogen (N2) – about 78%.
The subscript 2 of oxygen and nitrogen indicates the number of oxygen atoms and nitrogen atoms. So the oxygen molecules present in the atmosphere have two oxygen atoms and the nitrogen molecules have two nitrogen atoms.
There are also other diatomic molecules that we encounter every day. Carbon monoxide (or CO) is produced from combustion fumes of cars and trucks and is made of one carbon atom and one oxygen atom. Hydrochloric acid (HCl) is present in the gastric acid produced by our stomach and is a common acid that we use in the chemistry lab. It is made of one hydrogen atom and one chlorine atom.
Diatomic molecules are either homonuclear or heteronuclear. Homonuclear diatomic molecules are composed of two atoms of the same element, like in the case of oxygen and nitrogen shown in the illustration here. Heteronuclear diatomic molecules are composed of two atoms of different elements, like in the case of hydrochloric acid and carbon monoxide.
The two atoms in a diatomic molecule are connected in a straight line. The molecular geometry, which is what describes the shape of a diatomic molecule, is representative of linear geometry. Shown here are the structures, which show how atoms are bonded in the molecule and lone pairs of electrons, of the diatomic molecules O2, N2, HCl, and CO. The red dots represent the electrons and the lines represent the bonds. We can see here that the diatomic molecules exhibit linear molecular geometry.
Homonuclear Diatomic Molecules
There are seven elements that occur naturally as homonuclear diatomic molecules. All of these molecules are in the gaseous state. Five of these elements – hydrogen (H2), nitrogen (N2), oxygen (O2), fluorine (F2), and chlorine (Cl2) – occur as diatomic elements at room temperature, which is 25 degrees Celsius.
At slightly higher temperatures, iodine (I2) and bromine (Br2) exist as homonuclear diatomic molecules.
A useful mnemonic device to remember the seven homonuclear diatomic molecules that exist is: Have No Fear Of Ice Cold Beer.
Mnemonic Device for Homonuclear Diatomic Molecules
Heteronuclear Diatomic Molecules
Heteronuclear diatomic molecules consist of two atoms of two different elements. There is an abundance of heteronuclear molecules. Two types of bonding can occur in a heteronuclear diatomic molecule: ionic and covalent.
Ionic bonding occurs when a metal bonds with a nonmetal. Covalent bonding occurs when two nonmetals bond together. How do we know if an element is a metal or a nonmetal? We refer to the periodic table:
All elements, except for hydrogen, at the left side of the stairs are classified as metals and all elements at the right side of the stairs are classified as nonmetals. The lanthanides and actinides are also classified as metals.
The following are examples of heteronuclear diatomic molecules with ionic bonding.
 
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