Why Are Homonuclear Diatomic Molecules Important in Chemistry?
Diatomic molecules are the ones, which are composed of only two atoms, either of the same or different chemical elements. The prefix 'di-' is the Greek origin, which means "two". If a diatomic molecule contains two atoms of similar elements, such as oxygen (O2) or hydrogen (H2), it is referred to as homonuclear. On the other side, if a diatomic molecule contains two different atoms, such as a nitric oxide (NO) or carbon monoxide (CO), it is referred to as heteronuclear. The bond present in a homonuclear diatomic molecule is non-polar.
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Occurrence
Mostly, hundreds of diatomic molecules have been identified in the environment of the Earth, in the interstellar space, and in the laboratory. Around 99% of the atmosphere of the Earth is composed of two species of diatomic molecules, which are nitrogen (78%) and oxygen (21%). The natural abundance of hydrogen (H2) present in the atmosphere of the Earth is only of the order of parts per million, whereas H2 is the most abundant diatomic molecule present in the universe. Indeed, the interstellar medium is dominated by hydrogen atoms.
A Few Common Diatomic Molecules
Hydrogen Molecule (H2): Dihydrogen molecule belongs to the diatomic molecule's family, which contains two hydrogen atoms bonded to each other with a covalent bond. As per the atomic number of hydrogen, it has only 1 electron in its 1s orbital. The electronic configuration of the H2 molecule can be given as follows.
H2: (σ1s)2
Bond order = = = 1
Because of the absence of unpaired electrons present in the hydrogen molecule, it is described as diamagnetic in nature.
Lithium Molecule (Li2): The lithium molecule belongs to the diatomic molecule's family, which contains two lithium atoms that are bonded to each other with a covalent bond. The electronic configuration of the Li2 molecule can be given as follow.
Li2: (σ1s)2 (σ*1s)2 (σ2s)2
Bond order == 1
Therefore, the Li2 molecule is stable and diamagnetic because of the absence of unpaired electrons.
Carbon Molecule (C2): This molecule belongs to the diatomic molecule's family, which consists of two carbon atoms that are bonded to each other with a covalent bond. The electronic configuration of the Carbon molecule can be given as follows.
C2 :(σ1s)2 (σ*1s)2(σ2s)2 (σ *2s)2 (π2p2x= π 2p2y)
Bond order = = 2
Because of the absence of unpaired electrons, the C2 is diamagnetic in nature. Moreover, because of the presence of 4 electrons in the pi bonding orbitals, the double bond in C2 (bonding in homonuclear diatomic molecules) contains both pi bonds.
An Oxygen Molecule (O2): The oxygen molecule belongs to the diatomic molecule's family, which contains two oxygen atoms, which are bonded to each other with a covalent bond. The electronic configuration of the Oxygen molecule can be given as follows.
O2: (σ1s)2 (σ*1s)2 (σ2s)2 (σ *2s)2 (σ2pz)2 (π2p2x= π 2p2y) (π*2p1x= π*2p1y)
Bond order = = 2.
Because of the presence of 1 unpaired electron, the O2 molecule should be paramagnetic.
Helium Molecule (He2): As per helium atomic number, it contains 2 electrons in 1s orbital. The electronic configuration of this molecule as per the molecular orbital theory can be given as follows.
He2: (σ1s)2 (σ*1s)2
Bond order == 0.
Therefore, the He2 molecule is unstable and does not exist.
Diatomic Gas
A gas, having two atoms in its molecule (For example, H2 - Hydrogen molecule holds 2 H-atoms bound together using an electrostatic force field.
The electrons of 2 atoms overlap on each other, and this potential of overlapping plays a major role in its binding.
Gases are made up of only two atoms that can be either similar or various.
Oxygen, Hydrogen, Bromine, Nitrogen, Chlorine, Fluorine, and Iodine are the 7 common gases that exist as the diatomic molecules of a similar element. However, still, there are examples of diatomic molecules made up of non-identical atoms like hydrogen chloride, carbon monoxide, and nitric oxide.
About Gase's Diatomicity and Reason
We have many of the gaseous elements such as Oxygen, Hydrogen, Nitrogen, Fluorine, Chlorine, and more among the compounds Nitric oxide, Carbon monoxide, Hydrogen Fluoride, Chlorine monofluoride, Hydrogen Chloride, and so on.
Because it all comes down to the valency or the availability of electrons, forming chemical bonds. H contains only one bond and is monovalent. Thus the hydrogen molecule has only 1 bond.
Since oxygen can be given as O2, which we breathe, and O3 as Ozone. Ozone is not stable compared to dioxygen, and the equilibrium of the process given below lies to the right.
2O3⇌3O2
The molecule's stability is also a factor for O4, N3, N4, and so on are theoretically possible but are energetically unfavorable because of their diatomic counterparts. Whether such a type of molecule is feasible or not can be determined rigorously by the application of Molecular Orbital theory.
Did You Know?
An ideal gas can simply be described as a theoretical gas composed of many randomly-moving and non-interacting particles, which do not exist in nature. However, the real gases can behave the same as ideal gases under some specific conditions when the intermolecular forces become negligible.
FAQs on Homonuclear Diatomic Molecules Explained: Bond Order, Occurrence & More
1. What is a homonuclear diatomic molecule? Provide some common examples.
A homonuclear diatomic molecule is a molecule formed from two atoms of the same element. The prefix "homo-" means same, and "diatomic" means consisting of two atoms. These atoms are joined by a covalent bond, and because the electronegativity difference is zero, the bond is always nonpolar. Common examples that students learn in the CBSE syllabus include Hydrogen (H₂), Nitrogen (N₂), Oxygen (O₂), Fluorine (F₂), and Chlorine (Cl₂).
2. How is the bond order of a homonuclear diatomic molecule calculated using Molecular Orbital Theory?
The bond order is a key indicator of bond strength and is calculated using the Molecular Orbital Theory (MOT) formula: Bond Order = ½ [Nb - Na]. In this formula:
- Nb represents the total number of electrons in the lower-energy bonding molecular orbitals (like σ, π).
- Na represents the total number of electrons in the higher-energy antibonding molecular orbitals (like σ*, π*).
3. Why do elements like Nitrogen (N₂) form stable diatomic molecules while noble gases like Neon (Ne₂) do not?
This difference in stability is explained by their bond order, derived from Molecular Orbital Theory. For Nitrogen (N₂), the bond order is 3, indicating a strong and stable triple bond. This is because it has more electrons in bonding orbitals than in antibonding orbitals. In contrast, for Neon (Ne₂), the calculated bond order is zero. This means it has an equal number of electrons in bonding and antibonding orbitals, leading to no net chemical bond. Therefore, the Ne₂ molecule is unstable and does not exist under normal conditions.
4. What is the main difference between homonuclear and heteronuclear diatomic molecules?
The main difference lies in the identity of the constituent atoms.
- Homonuclear diatomic molecules are composed of two identical atoms, such as O₂ (Oxygen) or F₂ (Fluorine). Their bonds are perfectly nonpolar as there is no electronegativity difference.
- Heteronuclear diatomic molecules are composed of two different atoms, such as CO (Carbon Monoxide) or HCl (Hydrogen Chloride). Due to the difference in electronegativity between the two atoms, their bonds are polar.
5. How are bonding and antibonding molecular orbitals formed in these molecules?
Molecular orbitals are formed through a process called the Linear Combination of Atomic Orbitals (LCAO). When two atomic orbitals from the two atoms overlap:
- Constructive interference (in-phase overlap) leads to the formation of a lower-energy bonding molecular orbital (σ or π), which increases electron density between the nuclei and stabilises the molecule.
- Destructive interference (out-of-phase overlap) leads to the formation of a higher-energy antibonding molecular orbital (σ* or π*), which has a node between the nuclei and destabilises the molecule.
6. How does bond order relate to the stability and bond length of a homonuclear diatomic molecule?
There is a direct relationship between bond order, stability, and bond length. A higher bond order signifies more bonding electrons than antibonding ones, resulting in a stronger and more stable chemical bond. This stronger attraction pulls the atoms closer together, leading to a shorter bond length. For example, N₂ (bond order 3) is more stable and has a shorter bond length than O₂ (bond order 2), which in turn is more stable and shorter than F₂ (bond order 1).
7. Can a homonuclear diatomic molecule be paramagnetic? Explain with a key example.
Yes, a homonuclear diatomic molecule can be paramagnetic if it contains one or more unpaired electrons in its molecular orbitals. A classic example is the Oxygen molecule (O₂). According to Molecular Orbital Theory, the O₂ molecule has two unpaired electrons in its π* (pi-antibonding) orbitals. This presence of unpaired electrons is what makes oxygen paramagnetic, meaning it is weakly attracted to magnetic fields. This was a significant concept that Valence Bond Theory could not explain correctly.
