Chemistry · Chemical Bonding · NEET
NCERT lists seven main points. (1) Electrons in a molecule sit in molecular orbitals, just like electrons in an atom sit in atomic orbitals. (2) Atomic orbitals of similar energy and proper symmetry combine to form molecular orbitals. (3) An electron in an atomic orbital feels one nucleus (monocentric), but in a molecular orbital it feels two or more nuclei (polycentric). (4) The number of molecular orbitals formed equals the number of atomic orbitals combined; two atomic orbitals give two molecular orbitals, one bonding and one antibonding. (5) The bonding MO has lower energy (more stable) than the antibonding MO. (6) A molecular orbital gives the electron probability around a group of nuclei. (7) Molecular orbitals are filled using the Aufbau principle, Pauli exclusion principle, and Hund's rule. Learn these seven for NEET.
Molecular orbital (MO) theory was developed by F. Hund and R. S. Mulliken in 1932. NEET sometimes asks this direct fact, so remember: Hund and Mulliken, 1932.
An atomic orbital belongs to a single atom, so an electron in it is controlled by only one nucleus — it is monocentric. A molecular orbital belongs to the whole molecule, so an electron in it is controlled by two or more nuclei — it is polycentric. Both describe where an electron is likely to be found, but a molecular orbital spreads over several atoms.
Exactly two. This is a strict rule: the number of molecular orbitals formed is always equal to the number of atomic orbitals that combine. When two atomic orbitals combine, one bonding molecular orbital (lower energy) and one antibonding molecular orbital (higher energy) are formed. Orbitals are never lost or created extra.
Two conditions from the salient features: the combining atomic orbitals must have comparable (similar) energies, and they must have proper symmetry about the bond axis. If energies are very different or symmetry does not match, they cannot combine effectively to form a molecular orbital.
Yes. The seventh salient feature says molecular orbitals are filled following the Aufbau principle (lowest energy first), Pauli's exclusion principle (max two electrons, opposite spins), and Hund's rule (fill degenerate MOs singly before pairing). So the filling rules you learned for atoms apply directly to molecules.
When atomic wave functions add up, electron density increases between the two nuclei, which lowers the energy — this gives the bonding MO, which is more stable. When wave functions subtract, a node forms between the nuclei and energy rises — this gives the antibonding MO, which is less stable. Lower energy always means more stability.
Which one of the following statements is incorrect related to Molecular Orbital Theory?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Molecular orbital theory of homonuclear diatomic molecules is part of the Chemical Bonding and Molecular Structure chapter. NEET regularly asks about MOT features, bond order and paramagnetism, so these salient features are important base knowledge.
LCAO stands for Linear Combination of Atomic Orbitals. It says a molecular orbital is made by adding or subtracting the wave functions of atomic orbitals (psi_MO = psi_A plus or minus psi_B). Adding gives the bonding MO; subtracting gives the antibonding MO.
Yes, and this is its biggest win. MOT correctly predicts that O2 is paramagnetic because it has two unpaired electrons in antibonding pi* orbitals, something valence bond theory could not explain.
Group them: (1) electrons live in MOs, (2) orbitals of similar energy and symmetry combine, (3) MOs are polycentric, (4) orbitals in equal orbitals out, (5) bonding is lower energy than antibonding, (6) MO gives electron probability, (7) fill by Aufbau, Pauli, Hund.
