Oxygen is represented by the symbol O and has the atomic number 8. Its electronic configuration is 1s22s22p4. It belongs to group 16, i.e., the chalcogen group and the second period of the periodic table. At STP, oxygen atoms combine and form the O2 molecule. It is named dioxygen and is colourless as well as odourless. The bond between them is covalent in nature and the bond order is 2. It is paramagnetic in nature and has a linear shape with a bond angle of 180°.

There is another molecule of oxygen known as ozone, represented by O3. It is a highly reactive allotrope of oxygen.

Models to determine shapes of molecules

There are various models used to describe the shapes of molecules.

• The simplest model is the valence shell electron repulsion theory (VSEPR). It is based on the fact that electron groups, i.e., lone pairs, single bond, multiple bonds and single electrons, repel each other via coulombic forces. The repulsions between electron groups of the interior atoms of a molecule govern its geometry.

Order of repulsion is given by: l.p.-l.p. > l.p.-b.p. > b.p.-b.p.

A favourable geometry is the one with minimum repulsion.

For example, BeCl2 has 2 bond pairs of electrons. Its hybridisation is sp and is linear with 180° bond angle. However, in SnCl2 there are 2 bond pairs and 1 lone pair of electrons. Due to this, its hybridisation is sp² and it is V shaped with bond angle <120°.

• The next theory was advanced as compared to VSEPR and was called Valence Bond Theory (VBT).  When 2 atoms approach each other, their nuclei and electrons interact. If the energy of a system is reduced because of these interactions, a chemical bond is formed. 

The valence electrons of atoms in a molecule are present in the atomic orbitals s, p, d, f or hybridised orbitals. A chemical bond is formed when half filled valence shell orbitals of the 2 atoms overlap. The shape of molecule is determined by the geometry of overlapping orbitals.

There is a general formula used to find the hybridisation of a molecule, which can be used to find its geometry:

 H=1/2 ( V+M-C+A)

Where, H= number of orbitals involved in hybridisation,

                  V= valence shell electrons,

                  M= number of monovalent atoms,

                  C= charge of cation, and

                  A= charge on anion.

Example: NH3 = ½( 5+3+0-0) = 8/2= 4; so, the hybridisation is sp3. These 4 orbitals are oriented to form a tetrahedral arrangement. 

O2 molecule: it has 2 unpaired electrons and only 2 electrons are required for completion of octet. The other oxygen atom can share its 2 electrons with it, so it has no unpaired electron. When there is no unpaired electron, the molecule is said to be diamagnetic. Diamagnetic materials are repelled in a magnetic field.

If there are unpaired electrons, the molecule is paramagnetic. Paramagnetic materials are attracted in a magnetic field.

According to VBT, O2 is said to be diamagnetic. However, it is found experimentally that O2 is attracted in an external magnetic field. VBT couldn’t explain this anomaly.

• The next theory is molecular orbital theory (MOT). This theory considers the entire molecule as a unit. Like an atomic orbital, there are molecular orbitals around the molecule’s nuclei. They are formed by the combination of atomic orbitals of comparable energies. When 2 atomic orbitals form molecular orbitals, one of the molecular orbitals has lower energy (bonding orbital) and the other has higher energy (antibonding orbital). They form because of constructive and destructive interference, respectively, of the overlapping atomic orbitals.

Now we can use MOT to find the magnetic nature of O2 molecules. The configuration can be written as: 

(σ1s² σ*1s²) (σ 2s² σ* 2s²) (σ 2pz ²)( π 2px² )(π2py²)( π*2px ¹)(π* 2py¹)

From the above configuration, it can be seen that there are 2 electrons in separate antibonding π* orbitals. Because of these unpaired electrons, dioxygen is paramagnetic in nature instead of being diamagnetic as expected from the previous theories.

We can also calculate bond order from the following relation: Bond order= ½ [number of electrons in bonding orbitals- number of electrons in antibonding orbitals]

So, bond order = ½ [ 8-4] =2 

This means a double bond exists between the 2 oxygen molecules.

Conclusion

The formula of the oxygen molecule is O2. This signifies that it has two O atoms combined by a double bond. It has linear geometry as shown by VSEPR and VBT theories. It has a bond angle of 180°. However, these theories incorrectly showed the magnetic nature of dioxygen to be diamagnetic as there were no unpaired electrons. The Molecular Orbital Theory was then applied, which has a concept of bonding and antibonding molecular orbitals. It was found that the electrons present in the two π* orbitals are unpaired. Due to the presence of these unpaired electrons, O2 was found to be paramagnetic in nature. This explains why liquid oxygen is attracted in an external magnetic field.

The bond order was also calculated to be equal to 2, signifying that it has a double bond between 2 oxygen atoms.

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