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Bonding in Some Homonuclear Diatomic Molecules

Bonding in Some Homonuclear Diatomic Molecules

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Introduction to Bonding in Some Homonuclear Diatomic Molecules

Homonuclear Diatomic Molecules are those molecules which are made up of single nucleus and consist of only two atoms. In this article, we will be discussing about some Homonuclear Diatomic Molecules of first and second rows of periodic table, about their bond order, magnetic behavior etc. The list of Homonuclear Diatomic Molecules is as follows:

Hydrogen molecule (H2)

Combination of two hydrogen atoms results in formation of H2. Each hydrogen atom possesses one electron in 1s-orbital and, therefore, each hydrogen molecule has two electrons in it. Both these electrons are to be fixed in the molecular orbital having lowest energy. These two electrons should have opposite spins (Pauli’s Exclusion Principle). The molecular orbital energy level diagram for H2 molecule is shown in Fig number 1

Molecular Orbital Diagram for H2 molecule                 Fig. No. 1 Molecular Orbital Diagram for H2 molecule

The molecular orbital electronic configuration of Homonuclear Diatomic Molecule H2 is:

Therefore, there is a covalent bond between the two hydrogen atoms. The bond dissociation energy of hydrogen molecule is 438 kJ mol-1 and bond length equal to 74 pm. Moreover, it is diamagnetic as there is no unpaired electron in hydrogen molecule.

Hydrogen molecule ion (H2+)

Combination of hydrogen atom containing one electron and hydrogen ion having no electron results in the formation of H2+. Therefore, this ion consists of only one electron. The molecular orbital diagram is given in figure below.

Molecular Orbital Energy Level Diagram for H2+ ionFig. No. 2 Molecular Orbital Energy Level Diagram for H2+ ion 

The electronic configuration of the molecule is

The molecule is considered stable the bond order has positive value. As H2+ consist of unpaired electrons therefore it is considered as a paramagnetic molecule. Experimental data confirms the stability of the ion as well as its paramagnetic character.

The bond length of H2+ ion is larger when compared with H2 molecule (bond length H2+ = 104 pm; H2 = 74 pm) but H2+ has less bond dissociation enthalpy than H2 molecule (bond dissociation energy H2+ = 269 kJ mol-1, H2 = 458 kJ mol-1) Therefore in H2+ ion is weaker than H2 molecule.

Hypothetical Helium Molecule (He2)

Helium atom consist of two electrons in 1s-orbital, which means four electrons in He2 molecule. These electrons will fit in σ 1s and σ *1s MOs as shown in figure number 3. The electronic configuration for helium molecule may be written as:

Molecular Orbital Energy Diagram for He2 molecule (hypothetical)

 Fig. No. 3 Molecular OrbitalEnergy Diagram for He2 molecule (hypothetical)

There is no net bonding as it has zero bond order and therefore He2 molecule cannot exist. There is no experimental evidence for the existence of this molecule.

Helium Molecule Ion (He2+)

This molecule contains three electrons, two from one helium atom and one from the other (He+ ion). The molecular orbital electronic configuration of the molecule is

The value of bond order states that He2+ is stable. Its bond dissociation energy is 242 kJ mol-1. Therefore, the molecule is paramagnetic and the same has been experimentally observed also.

Lithium Molecule (Li2)

The electronic configuration of lithium is 1s22s1 that is, lithium molecule has six electrons. The electronic configuration of Li2 molecule may be written as:

Li2:  (σ 1s) 2 (σ *1s) 2 (σ 2s) 2

The above configuration may also be written as:

KK (σ 2s) 2

where KK represents fully filled inner K shells in two atoms that is, (σ 1s) 2 (σ *1s) 2
From the above electronic configuration of Li2 molecule we conclude that there are 4 electrons in bonding MOs and 2 electrons in antibonding MOs.
Therefore, its bond order is:
Bond Order = 4 – 2/2 = 1
Thus, there is one Li-Li sigma bond. The bond energy of the molecule is about 110 kJ mol-1 which is quite low. The bond length of the molecule is 265 pm. It is diamagnetic as it has no unpaired electrons and diamagnetic Li2 molecules are known to Exist in the Vapor Phase.

Comparison of Li-Li and H-H bonds.

The comparison of Li-Li and H-H bonds reveals that the sigma bond in Li2 molecule is weaker and much longer than σ bond in H2 molecule. This may be attributed to the following reasons:

  • The 2s orbital of lithium is bigger in size than the 1s orbital. Therefore, the overlapping of 2s-2s atomic orbitals is less effective than the overlapping of 1s-1s orbitals
  • In lithium molecule, the outer electrons in (σ2s) molecular orbital are shielded by the inner 1s electrons Therefore, the attraction between the nuclei and the electrons in (σ2s) molecular orbital is less

Moreover, the inner 1s2 electrons of two lithium atoms cause repulsion between the atoms Therefore, they do not allow them to come closer.

Beryllium Molecule (Be2)

Beryllium (Z = 4) has electronic configuration 1s22s2. In the formation of a diatomic molecule, two outer electrons of each atom that is, four in all, are to be filled in molecular orbitals. Two of these go into the bonding (σ2s) orbital while the other two have to go into the anti-bonding (σ*2s) orbital.

The molecular orbital electronic configuration is:

The zero-bond order suggests that Be2 molecule does not exist.

Boron Molecule (B2)

The electronic configuration of boron is 1s22s22p1. The outermost shell of each atom contains 3 electrons. When two B atoms combine to form molecule B2, there are 6 electrons which need to be accommodated in the molecular orbitals of B2. The molecular orbital electronic configuration is:

The molecule has only one bond. The electrons which contribute towards bonding are the π2px and π2py MOs. This indicates that the molecule is formed by a weak π -bond. The bond dissociation energy of B2 molecule has been found to be 290 kJ mol-1 and bond length equal to 159 pm.
Since each π2px and π2py MO contains single electron, the molecule B2 is paramagnetic.

Carbon Molecule (C2)

The electronic configuration of carbon is 1s22s22p2. The valence shell of each atom contains 4 electrons. At the time of formation of C2 molecule, there are 8 electrons which are required to be filled in the outer molecular orbitals of C2. The molecular orbital electronic configuration is:

Thus, the C2 molecule has two bonds. The bond dissociation energy of C2 molecule is 620 kJ mol-1 and bond length equal to 131 pm.

Further, it is diamagnetic as C2 molecule does not have any, unpaired electron. The diamagnetic C2 molecule has been detected in vapor phase.

C2 molecule consists of both the pi bonds because of the presence of 4 electrons in two pi MOs. While in other molecules, a double bond is made up of pi bond and sigma bond.

Note: The Bond Order of a Homonuclear Diatomic Molecule can be decreased by removing electron from a Bonding Molecular Orbital or by adding electrons to AntiBonding Molecular Orbital.

Nitrogen Molecule (N2)

The electronic configuration of nitrogen atom is 1s2 2s1 2px1 2y1 2pz1 and N2 molecule has 14 electrons. The Molecular orbital diagram is shown in Figure number 4

The molecular orbital electronic configuration of the molecule is:

Thus, nitrogen molecule has three bonds, one σ and two π-bonds. This is in accordance with very high bond dissociation energy (945 kJ mol-1) and small bond length (110 pm) of the molecule. The molecule has no unpaired electrons and therefore it is diamagnetic.

Comparison of N2 and N2+ ion
N2+ ion is formed when one electron is removed from N2 molecule. This electron will be lost from 2pz M.O. Hence the electronic configuration of N2+ ion will be

Molecular orbital energy level diagram for N2 molecule.Fig. No. 4 Molecular orbital energy level diagram for N2 molecule. 

Since bond order of N2+ ion (2.5) is less than the bond order of N2 (3) molecule, therefore, the bond strength of N2+ will be less than that of N2 molecule and bond length of N2+ will be more than that of N2 molecule.

Oxygen Molecule (O2)

Oxygen atom has electronic configuration of is 1s2 2s1 2px2 2y1 2pz1 Therefore, oxygen molecule has 16 electrons. In the formation of molecular orbitals, the electrons in the inner shells are expressed as KK denoting (σ1s) 2(σ*1s) 2. The remaining 12 electrons are filled in molecular orbitals as shown in figure number 5

The electronic configuration of the molecule is

Thus, oxygen molecule has two bonds one σ and one π Further, in accordance with Hund’s rule, the last two electrons in π*2px and π*2py orbitals will remain unpaired. Therefore, the molecule has paramagnetic character due to the presence of two unpaired electrons. These facts are in accordance with experimental observations. The bond dissociation energy in O2 molecule is 498 kJ mol-1 and bond length is 121 pm.

Comparison of O2, O2+, O2- and O22- species

Oxygen Molecule Ion (O2+)

This ion is formed when one electron is removed from O2 molecule as:

It is clear from the figure number 5, that the electron will be lost from anti-bonding MO that is, π*2py and electronic configuration of O2+ is:

Molecular orbital energy level diagram for O2 molecule

Fig. No. 5 Molecular orbital energy level diagram for O2 molecule

As the bond order of O2+ ion is greater than the bond order of O2 molecule, therefore, the bond strength of O2+ will be more than that of O2 molecule and bond length of O2+ will be less than that of O2 molecule.

Superoxide Ion (O2-)

This ion is formed when one electron is added as

According to figure 5, the electron will be added to either of the antibonding MO (s) π*2px or π*2py. The electronic configuration of O2- is

Since the bond order of O2- ion is less than O2 molecule, therefore, the bond in O2- will be weaker than in O2- and bond length of O2 will be larger than that of O2 molecule.

Peroxide ion (O22-)

This ion is formed when two electrons are added to O2 molecule.

The additional electrons enter the two half-filled π-anti-bonding MOs. The electronic configuration of O22- is

Since bond order of O22- ion is less than that of O2 molecule, therefore, the bond O22- will be weaker than in O2 and bond length of O22- will be larger than that of O2 molecule.

The species O2, O2+, O2- and O22- can be arranged as:

Bond dissociation energy: O2+ > O2 > O2- > O22-

Bond length: O22- > O2- > O2   > O2+

Fluorine Molecule (F2)

The electronic configuration of fluorine atom is 1s2 2s1 2p5 and therefore, there are 14 electrons in the valence shell of F2 molecule. The molecular orbital electronic configuration of the molecule is:

Thus, there is one σ-bond in the molecule. Since all the electrons in the molecular orbitals are paired, it is diamagnetic. The single bond in the molecule has a bond dissociation energy of 159 kJ mol-1 and bond length equal to 143 pm.

Hypothetical Neon Molecule

The electronic configuration of neon atom is 1s2 2s1 2p6 and the molecular orbital electronic configuration of neon molecule is

Thus, Ne2 molecule does not exist.

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