Which of the following pictorial diagrams most correctly represents the * ( antibonding) molecular orbital between two atoms if the internuclear axis is taken to be in the z-direction ( z-axis) ?
- [figure]
- [figure]
- [figure]
- [figure]
Answer
(3) [figure]
Solution
∵ The internuclear axis is z, π-type MOs are formed by sideways overlap of px (or py) orbitals, which lie perpendicular to the axis. Lobes pointing along the axis, as in (4), belong to σ-type (end-on) overlap of pz orbitals.
In-phase combination (same shading on the same side of the axis) ⇒ π bonding MO: electron density above and below the axis, spread over both nuclei — this is shown in (1); (2) also shows the two p orbitals in phase.
Out-of-phase combination ⇒ π antibonding MO: the upper lobe of one atom has the same shading as the lower lobe of the other atom (like-shaded lobes lie diagonally opposite), the lobes bend away from each other and a nodal plane lies between the two nuclei.
∴ Option (3) represents the * MO.
Answer: (3)