Chemistry · Chemical Bonding · NEET
Think of electrons as waves. When you bring two atoms close, their orbital waves mix. If the waves add together, electron density builds up BETWEEN the two nuclei. This is the bonding molecular orbital. That extra electron cloud between the nuclei acts like glue, so the orbital has LOW energy and makes the molecule stable. If the waves subtract (cancel), electron density is pushed AWAY from the middle, leaving almost no electrons between the nuclei. This is the antibonding orbital. It has HIGH energy and works against bonding. We mark antibonding orbitals with a star, like sigma-star or pi-star.
A node is a place where the chance of finding an electron is zero. In an antibonding orbital, the two waves cancel exactly at the midpoint between the nuclei, so a node forms right between the two atoms. With no electron glue in the middle, the two positive nuclei feel more repulsion. That is why the antibonding orbital sits HIGHER in energy than the original atomic orbitals, while the bonding orbital sits lower. NEET loves this fact: pi-star has a node between the nuclei.
A simple rule: the number of molecular orbitals equals the number of atomic orbitals you started with. Combine two 1s orbitals and you get exactly two MOs, one bonding (sigma-1s) and one antibonding (sigma-star-1s). The bonding one drops in energy by some amount; the antibonding one rises by a slightly larger amount. So MOs are always made in pairs, one low and one high.
It depends on HOW the atomic orbitals overlap. Head-on (end-to-end) overlap along the bond axis gives a sigma molecular orbital, which is symmetrical around the bond axis. Sidewise (parallel) overlap of p orbitals gives a pi molecular orbital, which has more electron density above and below the axis and is NOT symmetrical around it. This exact point was tested in NEET 2023.
Fill the MOs with electrons like a ladder, lowest first. H2 has 2 electrons: both go into the bonding sigma-1s, so it is stable. He2 has 4 electrons: 2 fill bonding sigma-1s and 2 fill antibonding sigma-star-1s. The bonding effect is cancelled by the antibonding effect, so bond order is zero and He2 does not exist. This is a classic NEET trap.
For NEET, yes, learn two orders. For light molecules up to N2 (14 electrons or fewer), the two pi-2p orbitals are BELOW sigma-2pz. For O2, F2 and heavier, sigma-2pz is BELOW the pi-2p orbitals. Getting this order right is exactly what NEET 2023 asked. It also decides magnetism, which the next concept covers.
Which one of the following statements is incorrect related to Molecular Orbital Theory?
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.
Electrons in a molecule do not belong to one atom. Atomic orbitals combine to form new orbitals called molecular orbitals that spread over the whole molecule. Combining gives one lower-energy bonding orbital and one higher-energy antibonding orbital.
A bonding orbital has extra electron density between the nuclei and lower energy. An antibonding orbital has a node (zero electron density) between the nuclei, higher energy, and is written with a star symbol.
They weaken the bond. Every electron in a bonding orbital helps hold the atoms together, and every electron in an antibonding orbital cancels one bonding electron. If both are equal, bond order is zero and the molecule cannot exist.
MOT explains things VBT cannot, like why O2 is paramagnetic and why He2 does not exist. NEET asks direct questions on MO energy order, bond order and magnetism almost every year in Chemical Bonding.
Next you learn to calculate bond order using the formula 1/2 (Nb - Na) and to predict whether a molecule is paramagnetic or diamagnetic from unpaired electrons. That is the next concept in this chapter.
