Which of the following statements is not correct regarding bonding MOs?

Bonding MOs have low electron densities between the nuclei.

The correct answer is option 2. Bonding MOs have low electron densities between the nuclei.

Let us delve into each statement regarding bonding molecular orbitals (MOs) and explain their correctness in detail:

1. Bonding MOs possess less energy than the atomic orbitals from which they are formed.

Bonding molecular orbitals are formed when atomic orbitals overlap constructively. In this constructive overlap, the wavefunctions of the atomic orbitals add together, resulting in a bonding MO that has lower energy than the original atomic orbitals. This energy lowering is due to stabilization caused by the bonding interaction, where electrons are held more tightly between the nuclei of the bonded atoms. Therefore, this statement is correct. Bonding MOs indeed possess less energy than the atomic orbitals from which they are formed.

2. Bonding MOs have low electron densities between the nuclei.

This statement is incorrect. In reality, bonding molecular orbitals have high electron densities between the nuclei of the bonded atoms. This concentration of electron density between the nuclei is precisely what facilitates the bond formation. Electrons in bonding MOs are localized in the region between the nuclei, contributing to the attractive force that holds the atoms together. The high electron density between the nuclei is a hallmark of bonding MOs and distinguishes them from anti-bonding MOs, which have a nodal plane or region of low electron density between the nuclei.

3. Every electron in bonding MO contributes to the attraction between atoms.

This statement is generally true. Electrons in bonding MOs contribute to the stability of the bond by occupying the bonding molecular orbital and increasing the electron density between the nuclei. The more electrons there are in the bonding MO, the stronger the attraction between the nuclei of the bonded atoms. Therefore, this statement is correct.

4. They are formed when the lobes of the combining atomic orbitals overlap.

This statement is true. Bonding molecular orbitals are formed by the overlap of atomic orbitals from two or more atoms. When the lobes (regions of electron density) of these atomic orbitals overlap constructively, the resulting bonding MO has a region of increased electron density between the nuclei of the atoms involved. This constructive overlap leads to a bonding interaction that stabilizes the molecule. Therefore, this statement is correct.

In summary:

Statement (1) is correct because bonding MOs indeed have less energy than the atomic orbitals from which they are formed.

Statement (2) is incorrect because bonding MOs have high electron densities between the nuclei, not low.

Statement (3) is correct because every electron in a bonding MO contributes to the attractive force between atoms.

Statement (4) is correct because bonding MOs are formed through the overlap of atomic orbitals.

Thus, the statement that is not correct regarding bonding MOs is: (2) Bonding MOs have low electron densities between the nuclei.