Chemistry · Chemical Bonding · NEET
LCAO means Linear Combination of Atomic Orbitals. Each atomic orbital is described by a wave function (ψ), which is like a wave that shows where the electron is. In LCAO we combine two of these atomic waves by simple addition and subtraction. From two atomic orbitals we always get two molecular orbitals: one lower in energy (bonding) and one higher in energy (antibonding). It is called 'linear' because we just add or subtract the waves, nothing more complicated.
The Schrödinger wave equation can be solved exactly only for a system with one electron (like the hydrogen atom). A molecule has two or more nuclei and usually many electrons, so the equation cannot be solved directly. LCAO is an approximate method that gets around this problem. We already know the atomic orbital wave functions, so we just combine them to estimate the molecular orbital wave functions. This is why NCERT calls it an 'approximate method'.
ψA is the wave function of the 1s orbital on atom A, and ψB is the wave function of the 1s orbital on atom B. When you ADD them (ψMO = ψA + ψB), the two electron waves are in the same phase, so they reinforce each other (constructive interference). This builds up electron density between the two nuclei and forms the bonding molecular orbital (σ). When you SUBTRACT them (ψMO = ψA − ψB), the waves are in opposite phase and cancel out between the nuclei (destructive interference). This creates a node between the nuclei and forms the antibonding molecular orbital (σ*).
In the bonding orbital, electron density piles up between the two nuclei. The negative electrons sit between the positive nuclei and pull them together, so nuclear repulsion is low and energy is low (more stable). In the antibonding orbital there is a nodal plane between the nuclei where electron density is zero. The nuclei now repel each other strongly, so the energy is high (less stable). Key NCERT point: the bonding orbital energy is lowered below the atomic orbitals, and the antibonding orbital energy is raised above them, but the total energy of the two molecular orbitals stays the same as the two starting atomic orbitals.
The number of molecular orbitals formed is always equal to the number of atomic orbitals combined. If you combine 2 atomic orbitals, you get exactly 2 molecular orbitals (one bonding + one antibonding). If you combine 8 atomic orbitals (like 2s and 2p of two second-row atoms), you get 8 molecular orbitals. Orbitals are never lost; they are just rearranged into new energy levels.
No, be careful here. Orbital overlap is part of Valence Bond (VB) theory, where two half-filled atomic orbitals overlap and the shared electron pair stays between the two atoms. LCAO belongs to Molecular Orbital (MO) theory, where the atomic wave functions combine mathematically and the resulting molecular orbitals belong to the whole molecule, not to one bond. NEET can mix these two ideas in trap options, so remember: LCAO = MO theory, overlap language of a shared pair = VB theory.
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.
LCAO stands for Linear Combination of Atomic Orbitals. It is the method used in Molecular Orbital theory to build molecular orbitals by adding and subtracting atomic orbital wave functions.
Molecular Orbital theory was developed by F. Hund and R. S. Mulliken in 1932. LCAO is the approximate method used within this theory to form molecular orbitals.
Constructive interference happens when the two electron waves are in the same phase and add up (ψA + ψB), building electron density between the nuclei to form the bonding orbital. Destructive interference happens when the waves are in opposite phase and cancel (ψA − ψB), creating a node and forming the antibonding orbital.
No. The bonding orbital drops below the atomic orbital energy and the antibonding orbital rises by the same amount above it. The total energy of the two molecular orbitals equals the total energy of the two original atomic orbitals.
LCAO is the foundation for MO diagrams, bond order, and magnetic behaviour (paramagnetic O2). NEET regularly tests the σ vs π symmetry, node position, and the reinforce/cancel wording that come directly from the LCAO idea, as in NEET 2023 Phase 2.
