Diatomic Molecules: Characteristics & Examples

Diatomic molecules are formed when two elements of similar nature or characteristics or of different natures get bonded in a chemical reaction. In the same way, when two different atoms react to form a bond, they form a heteronuclear diatomic molecule 

• If the diatomic molecule is formed of two similar nature elements’ atoms, they form a homonuclear diatomic molecule. 
• The bond between homonuclear molecules is non-polar.
• Noble gases like helium, argon and neon form monoatomic molecules.
• Bromine and iodine will form diatomic at elevated temperature.
• Hundreds of different diatomic molecules have been found in interstellar space and laboratory.
• O₂, H₂ and N₂ are the most common types of these molecules.

Key Terms: Diatomic Molecules, Heteronuclear Molecule, Homonuclear Molecule, Atoms, Elements, Chemical Reaction, Hydrogen, Oxygen, Nitrogen, Nobel Gases, Molecule, Carbon, Bond Order, Lithium

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What are Diatomic Molecules?

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Diatomic Molecules refers to molecules composed of two atoms of the same or different chemical elements. If these molecules are formed by two different atoms, then they are called heteronuclear molecules.

• In case diatomic are formed between two similar atoms, then they are called homonuclear molecules.
• The word is derived from Greek words where di means two.
• Astatine and tennessine are special cases for which diatomic molecules are not formed.
• All molecules are characterised by their bond length.
• These molecules are represented by two-point masses connected by a massless spring.

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Some Common Diatomic Molecules

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Some common types of diatomic molecules of both nature such as homonuclear as well as heteronuclear diatomic molecules are as follows:

Hydrogen Molecule (H₂)

This Hydrogen molecule belongs to the diatomic family as in this molecule two hydrogens make a bond and form a homonuclear diatomic molecule.

Lithium Molecule (Li₂)

This is also a diatomic family as in this two Lithium molecules form a diatomic molecule of Li₂ and also is a homonuclear diatomic molecule.

Carbon Molecule (C₂)

Carbon molecules also form diatomic molecules as two carbon molecules bond together to form homonuclear diatomic molecules.

Oxygen Molecule (O₂)

Oxygen reacts with other atoms/molecules of oxygen they form a diatom of oxygen (O₂) and it is a type of homonuclear diatomic molecule.

Carbon Monoxide (CO)

When carbon and oxygen react or form a bond they form a carbon monoxide molecule. This also is in the diatomic family but it is a kind of heteronuclear diatomic molecule

Hydrogen Chloride (HCl)

Here the hydrogen and chloride react to form a bond molecule of hydrogen chloride which is a type of heteronuclear diatomic molecule.

Nitric Oxide (NO)

Here the nitrogen and oxygen react to form a bond molecule of nitric oxide which is a type of heteronuclear diatomic molecule.

Diatomic Molecules 

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Characteristics of Diatomic Molecules

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Some important characteristic of diatomic molecules are as follows:

Heat Capacity

If we look at the diatomic molecules, they possess additional rotational motion, which allows them to store a large amount of thermal energy. It results in the rotational motion by which the kinetic energy gets converted.

• Hence, they store thermal energy.

Heteronuclear Molecule

There are various other diatomic molecules, which are chemical compounds of a combination of two different elements. Heteronuclear diatomic molecules are formed by the combination of many elements, which are mainly dependent on temperature and pressure.

Occurrence

The occurrence of diatomic molecules found in different environments, laboratories, etc. There are hundreds of molecular diatoms present. 

• If we take the earth's atmosphere, we would find that 99% of it is composed of two diatomic molecules.
• It include Nitrogen(78%) and Oxygen(21%).
• There is a greater amount of hydrogen diatomic molecules present in the outer space.

Molecular Geometry

Mainly, the diatomic molecules are linear in shape and are identified in single parameters. These types of molecules will not always be linear, i.e., single-bonded.

• There are some exceptions where you will find that the diatomic molecules can be triple bonded.
• It includes nitrogen, which can also be double-bound, with oxygen.

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Historical Significance

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Since the older times, diatomic molecules have played a vital role in the concept of the elements, atoms and molecules. However, John Dalton, in his original theory of atomic hypothesis, considered these to be monatomic.

• It created confusion about the atomic and molecular weights for almost a half-century.

At the beginning of 1805, Gay Lussac and Von Humboldt proved that water consists of two volumes of hydrogen and one volume of oxygen. Avogadro, in 1811, also reached the same conclusion, but these results were ignored up to 1860.

• At the end of 1860, Karlsruhe and Cannizzaro also proved the Avogadro number could be used in the formation of atomic weight.
• It can be used with the modern values in the form of a table.
• This has ultimately led to the formation of modern period law by Dmitri Mendeleev and Lothar Mayar.

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Excited Electronic State

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Diatomic molecules are generally in the state of the lowest order, which is conventionally called the X state. When these types of gases are targeted with energetic electrons, they get excited to their highest state of electronic energy.

• The thing is that these do not remain in the higher electronic state.
• They gradually start to lower their electronic state and reach back to their lowest or ground state. 
• In this process, they lose photons, which are called fluorescence.

In the quantum theory, the diatomic molecule's electronic state can be represented by the molecular term symbol as given below:

2S + 1 Λ(v)

Where

• S is given as the total electronic spin quantum number
•  Λ is given as the total electronic angular momentum quantum number along the internuclear axis
• v is given as the vibrational quantum number.

Λ takes on the values of 0, 1, 2, and so on, which are represented by the symbols of electronic states such as \(\Sigma\), \(\pi\),\(\bigtriangleup\)and so on.Read More: 

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Things to Remember

• When two elements of similar or different nature bonds through a chemical reaction, then they form a diatomic molecule.
• The modern period law was formed by Dmitri Mendeleev and Lothar Mayar.
• Diatomic molecules can be classified into homonuclear and heteronuclear based on the nature of the participating elements.
• Hydrogen, Carbon and Oxygen molecules are some of the common diatomic molecules.
• The vibrational energy level of the diatomic molecules  can be explained with harmonic oscillator approximation.