Chemical Bonding is the structural foundation of all Chemistry. If your bonding concepts are fragile, Coordination Compounds, Organic reaction mechanisms, and p-block chemistry will become a frustrating memory exercise. Master the underlying geometric and molecular orbital principles to solve any NEET bonding question in under 45 seconds.
1. The Universal Steric Number Shortcut for Hybridization
Instead of drawing full Lewis structures during the exam, use the steric number formula:
Steric Number (Z) Formula:
Z = (1/2) * [ V + M - C + A ]
- V: Valence electrons of the central atom (Group 15 = 5, Group 16 = 6, Group 17 = 7, Group 18 = 8)
- M: Number of surrounding monovalent atoms (H, F, Cl, Br, I; ignore divalent atoms like O, S)
- C: Positive charge on cation
- A: Negative charge on anion
| Steric No. (Z) | Hybridization | Electron Geometry | Bond Pairs (bp) / Lone Pairs (lp) | Molecular Shape | NEET Example |
|---|---|---|---|---|---|
| 2 | sp | Linear | 2 bp, 0 lp | Linear (180°) | BeCl2, CO2 |
| 3 | sp2 | Trigonal planar | 3 bp, 0 lp 2 bp, 1 lp |
Trigonal planar (120°) Bent / V-shaped |
BF3 SO2, NO2^- |
| 4 | sp3 | Tetrahedral | 4 bp, 0 lp 3 bp, 1 lp 2 bp, 2 lp |
Tetrahedral (109.5°) Trigonal pyramidal Bent / Angular |
CH4, NH4^+ NH3, H3O^+ H2O, OF2 |
| 5 | sp3d | Trigonal bipyramidal | 5 bp, 0 lp 4 bp, 1 lp 3 bp, 2 lp 2 bp, 3 lp |
Trigonal bipyramidal See-Saw T-shaped Linear |
PCl5 SF4 ClF3, BrF3 XeF2, I3^- |
| 6 | sp3d2 | Octahedral | 6 bp, 0 lp 5 bp, 1 lp 4 bp, 2 lp |
Octahedral (90°) Square pyramidal Square planar |
SF6 BrF5, IF5 XeF4 |
2. Molecular Orbital Theory (MOT) Simplified
NEET frequently asks for bond order, bond length, and paramagnetism of homonuclear diatomic molecules and their ions (e.g., O2, O2^+, O2^-, O2^2-).
The 14-Electron Rule
- Total electrons ≤ 14 (Li2, Be2, B2, C2, N2, CO, CN^-): Significant 2s-2p mixing occurs. Order of orbital energy:
sigma1s < sigma*1s < sigma2s < sigma*2s < (pi2px = pi2py) < sigma2pz < (pi*2px = pi*2py) < sigma*2pz. - Total electrons > 14 (O2, F2, Ne2): No 2s-2p mixing. Order of orbital energy:
sigma1s < sigma*1s < sigma2s < sigma*2s < sigma2pz < (pi2px = pi2py) < (pi*2px = pi*2py) < sigma*2pz.
The Rapid Bond Order Staircase
Memorise the 14-electron peak: At 14 electrons (N2), Bond Order = 3.0. For every electron added or removed from 14, decrease the bond order by 0.5:
- 10 electrons (B2): Bond Order = 1.0 (Paramagnetic, 2 unpaired in pi2p)
- 12 electrons (C2): Bond Order = 2.0 (Diamagnetic, all 4 electrons in pi2p - note: double bond consists entirely of pi bonds!)
- 14 electrons (N2): Bond Order = 3.0 (Diamagnetic)
- 15 electrons (O2^+): Bond Order = 2.5 (Paramagnetic)
- 16 electrons (O2): Bond Order = 2.0 (Paramagnetic, 2 unpaired in pi*2p)
- 17 electrons (O2^-): Bond Order = 1.5 (Paramagnetic, superoxide)
- 18 electrons (O2^2-): Bond Order = 1.0 (Diamagnetic, peroxide)
Crucial NEET Relationship: Bond Order ∝ Bond Dissociation Energy ∝ 1 / (Bond Length).