The Concept of Diatomic Molecules Explained With Examples

Diatomic molecules
A covalent bond is a chemical bond that arises due to sharing of electrons between atoms. Diatomic molecules are usually formed by covalent bonding. This Buzzle post uses various examples of diatomic molecules to give you a better understanding of the concept.
Metals like tungsten and molybdenum may exist as diatomic molecules in their gaseous state by forming a sextuple bond. In this, all the six outermost electrons present in each tungsten or molybdenum atom take part in the atomic overlap to form six bonds.
Atomicity of a molecule is basically the number of atoms that make a molecule. Molecules can be monatomic, diatomic, or polyatomic depending on the number of atoms that make the molecule.

A monatomic element consists of elements that exist on their own in nature. They do not need to combine with other elements in order to be stable. These are usually inert gas elements like helium, neon, argon, etc. As the name suggests, a diatomic molecule is made of two atoms of the same or of different elements that are joined together by means of a chemical bond (details of which will be discussed below). In polyatomic molecules, three or more atoms of the same element or of different elements are joined by chemical bonds. Examples of triatomic molecules are O3, H2O, SO2, etc.

Depending on the type of atoms making the molecules, diatomic molecules can be classified into:
  • Homonuclear diatomic molecules
  • Heteronuclear diatomic molecules
Homonuclear Diatomic Molecules
Chlorine molecular structure
A homonuclear diatomic molecule is made of two atoms of the same element that are joined together by a chemical bond. These molecules usually exist in the gaseous phase. Some of the gases that exist as diatomic molecules at room temperature and at atmospheric pressure are H2, N2, O2, F2 and, Cl2. At elevated temperatures, bromine and iodine may be converted into their diatomic gases.
These molecules are non-polar in nature. This is because the shared pair of electrons between the atoms of the molecules experience an equal force of attraction from the two nuclei of the two atoms. Here, the molecule lies in only one plane, and usually the bond angle between the atoms in these molecules is 180°.
Examples
In Hydrogen: Hydrogen has one electron in its orbit. The atomic orbitals of two hydrogen atoms overlap so that their atomic orbital is complete, giving rise to a stable H2 molecule.

In Chlorine: Chlorine has seven electrons in its outermost orbit, and it needs one electron to complete its octet. The atomic orbitals of two chlorine atoms overlap so that their octet is complete, giving rise to a stable Cl2 molecule.

In Nitrogen: Nitrogen has five electrons in its outermost orbit, and it needs three electrons to complete its octet. It does so by forming three bonds with another nitrogen atom (three electrons are shared in the overlapping orbital).

In Oxygen: Oxygen has six electrons in its outermost orbit, and it needs two electrons to complete its octet. It does so by forming two bonds with another oxygen atom (two electrons are shared in the overlapping orbital).

Nitrogen and oxygen are the most abundant gases that are present in the atmosphere and responsible for supporting the existence of all life forms on earth. Hydrogen gas is, however, the most abundant gas present in the universe.
Other examples
Phosphorus on pyrolysis (heating to very high temperature) gives diatomic phosphorus. Phosphorus exists in a tetraatomic state. On heating, this tetraatomic molecule cracks to give diatomic molecules.

Lithium usually exists as dilithium in its vapor phase; approximately 1% of gaseous lithium is in the form of Li2.

Sulfur exists in the S8 state, and when heated to above 720°C, it breaks down to give S2.
Heteronuclear Diatomic Molecules
Monoxide molecular structure
A heteronuclear diatomic molecule is made of two atoms of different elements that are joined by a chemical bond. Some molecules that exist as diatomic molecules at room temperature are HCl, CO, CN*, NO*, etc.
These molecules are usually polar in nature. This is because the shared pair of electrons between the atoms of the molecule experience an unequal pull from the two nuclei of the atoms and are pulled to the more electronegative atom, giving rise to a δ- and δ+ charge. Again, these molecules too are planar in nature.
Examples
In HCl: This is a diatomic molecule in which the hydrogen atom and the chlorine atom are bonded by a covalent bond. Here, chlorine is more electronegative than hydrogen. Due to this, the electrons are pulled towards the chlorine atom, creating a dipole. The bond length of the H-Cl bond is 0.127 nm.

In CO: Carbon has 4 electrons in its outermost atomic orbital, whereas oxygen has six electrons in its outermost atomic orbital. In this diatomic molecule, the carbon atom and the oxygen atom are bonded by two covalent bonds and one dative or coordinate bond (oxygen provides a lone pair of electrons to form the dative bond) so that their octet is complete. The bond length of each C-O bond is 0.112 nm.

In NO*: Nitrogen has 5 electrons in its outermost atomic orbital, whereas oxygen has six electrons in its outermost atomic orbital. In this molecule, the nitrogen and oxygen atom are bonded by two covalent bonds. One of the electrons of the nitrogen that usually takes part in a chemical reaction remains unpaired. This forms a highly reactive chemical compound that is called a radical. Here, the bond length of the N-O bond is 0.115 nm.

In CN*: Carbon has four electrons in its outermost orbital, and nitrogen has 5 electrons. In this molecule, the nitrogen and carbon atoms are bonded by three covalent bonds. One of the bonding electrons of carbon remain unpaired to form a radical.
Certain molecules like NaCl, MgO, SiO, etc., seem to be heteronuclear diatomic molecules, but are not considered to be so. For a better understanding, let us consider NaCl. At room temperature, NaCl exists as a crystal lattice. In this, every sodium ion is surrounded by six other chloride ions and vice versa. Every sodium ion has an equal affinity for all the chloride ions that surround it, and the same applies for every chloride ion surrounded by sodium ions. These molecules, therefore, do not exist as diatomic molecules, but are written so for a simpler understanding.