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.
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.
- NaCl - Sodium Chloride
- MgO - Magnesium Oxide
- KBr - Potassium Bromide
- CaO - Calcium Oxide
Let us take NaCl, for example, sodium (Na) is on the far left side of the periodic table, indicating it is a metal, and chlorine (Cl) is a nonmetal, so NaCl is a heteronuclear diatomic molecule with an ionic bond.
The following are examples of heteronuclear diatomic molecules with covalent bonding.
- HCl - Hydrochloric Acid
- CO - Carbon Monoxide
- ClF - Chlorine Monofluoride
- HBr - Hydrobromic Acid
Let's look at HBr, for example. Hydrogen is on the left side of the stairs that divide metals and nonmetals in the periodic table, but hydrogen is a nonmetal. Bromine (Br) is also a nonmetal, so HBr is a heteronuclear diatomic molecule with a covalent bond.
Diatomic molecules consist of two atoms that are either from the same element or from different elements. If the diatomic molecule consists of atoms of the same element, then this is classified as a homonuclear diatomic molecule. If the diatomic molecule consists of atoms from two different elements, then it is a heteronuclear diatomic molecule.
There are seven elements that naturally occur as homonuclear diatomic molecules in their gaseous states: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. Bromine and iodine need slightly higher temperatures than room temperature to occur as gaseous diatomic molecules.
Heteronuclear diatomic molecules can either have an ionic bond or a covalent bond. An ionic bond occurs in a heteronuclear diatomic molecule when a metal is bonded to a nonmetal. A covalent bond occurs when two nonmetals are bonded together.
Diatomic Molecules Key Terms
- Diatomic Molecules: Two atoms from the same or different elements
- Homonuclear Diatomic Molecules: Two atoms from the same element
- Heteronuclear Diatomic Molecules: Two different elements' atoms
- Ionic Bond: Occurs when metal bonds with nonmetal
- Covalent Bond: Occurs when two nonmetals bond together
After this lesson is finished, students should be able to:
- Define the types of molecules in a diatomic molecule
- Describe the bonding nature of metals and nonmetals
- Recall the seven natural homonuclear diatomic molecules
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Diatomic molecules are molecules that contain only two atoms bonded to one another. The atoms can be the same or different which results in homo- or hetero- nuclear diatomic molecules. The bonding of diatomic molecules is based on whether the atoms bonded together are metals or nonmetals.
1. What is the difference between an ionic and covalent bond?
2. What are the seven elements that form a homo-nuclear diatomic molecule?
3. Is water a diatomic molecule?
4. Is nitrogen oxide considered a diatomic molecule? If so is it a homo-nuclear diatomic molecule or hetero-nuclear diatomic molecule?
5. What is the molecular geometry of a diatomic molecule?
6. Is HF a diatomic molecule? Why or why not?
1. An ionic bond involves the bonding of a metal and a nonmetal atom while a covalent bond is a bond between two nonmetals.
2. Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine form homo-nuclear diatomic molecules.
3. No, water contains three atoms which include two hydrogens and one oxygen atom.
4. Yes, it is. Since the elements are different, it is considered a hetero-nuclear diatomic molecule.
5. Diatomic molecules are linear.
6. Yes, it is, there are two different atoms so it is a hetero-nuclear diatomic molecule.
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