Definition properties bond order and examples of homonuclear diatomic molecules
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 in Chemistry
1. What are homonuclear diatomic molecules?
A homonuclear diatomic molecule is a molecule made of two atoms of the same element bonded together. These molecules contain identical atoms and are represented by the general formula X2.
Common examples include:
- H2 (hydrogen)
- N2 (nitrogen)
- O2 (oxygen)
- F2, Cl2, Br2, I2 (halogens)
2. Which elements exist as homonuclear diatomic molecules?
The elements that naturally exist as homonuclear diatomic molecules are hydrogen, nitrogen, oxygen, and the halogens. They are commonly remembered as:
- H2
- N2
- O2
- F2
- Cl2
- Br2
- I2
3. Why do some elements form homonuclear diatomic molecules?
Some elements form homonuclear diatomic molecules because bonding allows them to achieve a stable electron configuration, often satisfying the octet rule (or duet rule for hydrogen).
For example:
- Each H atom has 1 electron; in H2, they share electrons to achieve a stable duet.
- Each O atom has 6 valence electrons; in O2, they share two pairs to complete their octets.
- In N2, three shared pairs form a triple bond for maximum stability.
4. What is the bond order of homonuclear diatomic molecules?
The bond order of a homonuclear diatomic molecule is the number of shared electron pairs between the two atoms. It can be determined from Lewis structures or molecular orbital (MO) theory.
Examples:
- H2: bond order = 1 (single bond)
- O2: bond order = 2 (double bond)
- N2: bond order = 3 (triple bond)
Bond order = (bonding electrons − antibonding electrons) / 2.
5. Are homonuclear diatomic molecules polar or nonpolar?
Homonuclear diatomic molecules are nonpolar molecules because both atoms have identical electronegativity.
Since the atoms are the same:
- The electronegativity difference is zero.
- Electrons are shared equally.
- No permanent dipole moment is formed.
6. What is the molecular orbital diagram for homonuclear diatomic molecules?
The molecular orbital (MO) diagram for homonuclear diatomic molecules shows how atomic orbitals combine to form bonding and antibonding orbitals.
Key features:
- Atomic orbitals combine to form σ (sigma) and π (pi) molecular orbitals.
- Electrons fill orbitals according to the Aufbau principle, Pauli exclusion principle, and Hund’s rule.
- Bond order is calculated as (bonding − antibonding)/2.
7. Why is O2 paramagnetic?
Oxygen gas (O2) is paramagnetic because it has two unpaired electrons in its antibonding π* molecular orbitals.
According to molecular orbital theory:
- O2 has 12 valence electrons.
- The last two electrons occupy separate π* orbitals.
- These unpaired electrons cause attraction to a magnetic field.
8. What is the difference between homonuclear and heteronuclear diatomic molecules?
The main difference is that homonuclear diatomic molecules contain two identical atoms, while heteronuclear diatomic molecules contain two different atoms.
Examples:
- Homonuclear: H2, N2, Cl2
- Heteronuclear: CO, NO, HCl
9. What type of bond is present in homonuclear diatomic molecules?
Homonuclear diatomic molecules contain nonpolar covalent bonds formed by equal sharing of electrons between identical atoms.
Depending on the number of shared electron pairs:
- Single bond: H2, Cl2
- Double bond: O2
- Triple bond: N2
10. Can you give examples of reactions involving homonuclear diatomic molecules?
Yes, homonuclear diatomic molecules commonly participate in combustion, synthesis, and redox reactions.
Examples of balanced chemical reactions:
- 2H2(g) + O2(g) → 2H2O(l)
- N2(g) + 3H2(g) → 2NH3(g) (Haber process)
- H2(g) + Cl2(g) → 2HCl(g)
