Chemistry · Chemical Bonding · NEET
Bond order = (1/2)(bonding electrons - antibonding electrons). Bonding electrons hold the two atoms together. Antibonding electrons push them apart. When both numbers are equal, they fully cancel. Bond order = 0 means there is no net force holding the atoms, so the molecule cannot stay together. For He2, bond order = (1/2)(2 - 2) = 0, so He2 does not exist.
Each He atom has 2 electrons, so He2 has 4 electrons total. Fill the molecular orbitals in energy order: sigma-1s takes 2 electrons (bonding), then sigma*-1s takes 2 electrons (antibonding). So bonding electrons N_b = 2 and antibonding electrons N_a = 2. Bond order = (1/2)(2 - 2) = 0. This is why He2 has no bond.
Be2 has 8 electrons (4 from each Be atom). Filling order: sigma-1s (2), sigma*-1s (2), sigma-2s (2), sigma*-2s (2). Now N_b = 4 (both sigma bonding filled) and N_a = 4 (both sigma* antibonding filled). Bond order = (1/2)(4 - 4) = 0. Every bonding pair is cancelled by an antibonding pair, so Be2 does not exist as a stable molecule.
When two atomic orbitals combine, they make two molecular orbitals: a bonding one (lower energy, stabilises) and an antibonding one (higher energy, destabilises). One electron in the antibonding MO undoes the stabilising effect of one electron in the bonding MO. So if you have the same number in each type, the net stabilisation is zero and no bond forms.
He2+ has only 3 electrons: sigma-1s (2 bonding) and sigma*-1s (1 antibonding). Bond order = (1/2)(2 - 1) = 0.5. Because bond order is positive (not zero), He2+ can exist as a weak, unstable species. Removing one antibonding electron gives a small net bond. This shows the rule clearly: positive bond order = can exist, zero bond order = cannot exist.
H2 has only 2 electrons, both in the sigma-1s bonding orbital and none in the antibonding orbital. Bond order = (1/2)(2 - 0) = 1, so H2 has a strong single bond. He2 has 2 extra electrons that go into the sigma*-1s antibonding orbital, cancelling the bond. The extra antibonding electrons are the whole reason He2 fails.
Identify a molecule which does not exist.
Which of the following diatomic molecular species has only pi bonds according to Molecular Orbital Theory?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Both have a bond order of 0. For He2: (1/2)(2 - 2) = 0. For Be2: (1/2)(4 - 4) = 0. A zero bond order means no net bond forms, so neither molecule exists.
No. As a normal stable molecule, Be2 does not exist because its bond order is zero. (Very weak Be2 species detected at extremely low temperatures in labs are outside the NEET syllabus; for NEET, treat Be2 as non-existent.)
He2 and Be2 both have bond order 0 and do not exist. Also remember Ne2 has bond order 0. In contrast, He2+ has bond order 0.5 and can exist as a weak species.
Bond order = (1/2)(N_b - N_a), where N_b is the number of electrons in bonding molecular orbitals and N_a is the number in antibonding molecular orbitals. Higher bond order means a stronger, shorter bond; zero means no bond.
