A Lewis Structure of O2 and a molecular orbital diagram.

A Lewis Structure of O2 and a molecular orbital diagram. Compare the bond order and state which one justifies the paramagnetism observed in liquid oxygen.

The Correct Answer and Explanation is:

Correct Answer:

• Lewis Structure of O₂:
  :Ö=Ö:
  (Each oxygen atom has two lone pairs and shares a double bond)
• Molecular Orbital (MO) Diagram of O₂: markdownCopyEditσ(1s) σ*(1s) σ(2s) σ*(2s) σ(2pz) π(2px) = π(2py) π*(2px) = π*(2py) ← two unpaired electrons here σ*(2pz)
• Bond Order from MO Theory:
  Bond order = (number of bonding electrons – number of antibonding electrons) / 2
  = (10 – 6) / 2 = 2

Explanation

Oxygen (O₂) has a total of 12 valence electrons (6 from each oxygen atom). In the Lewis structure, each oxygen shares two electrons, forming a double bond (O=O), with each atom also holding two lone pairs. This representation suggests a bond order of 2.

However, the Lewis structure fails to predict the magnetic behavior of oxygen. When we cool oxygen into a liquid, it shows paramagnetism, meaning it is attracted by a magnetic field. This property implies the presence of unpaired electrons, which is not shown in the Lewis structure.

To understand this, we turn to Molecular Orbital (MO) theory. The MO diagram for O₂, based on the combination of atomic orbitals from each oxygen atom, shows the filling of electrons into different molecular orbitals, ordered by energy. The important levels for bonding are:

• Bonding MOs: σ(2s), σ(2pz), π(2px), π(2py)
• Antibonding MOs: σ*(2s), π*(2px), π*(2py), σ*(2pz)

O₂ has 16 total electrons. When distributed into the MO diagram, the two π* antibonding orbitals each receive one electron. This results in two unpaired electrons, which are responsible for the paramagnetic behavior of liquid oxygen.

The MO bond order is calculated using the formula:
Bond order = (bonding electrons – antibonding electrons)/2 = (10 – 6)/2 = 2

This matches the bond order predicted by the Lewis structure. However, only MO theory explains the presence of unpaired electrons and thus the paramagnetism. Therefore, the molecular orbital diagram provides the more accurate description of the electronic structure of O₂.