Molecular Orbital Theory — σ, π bonds, bond order
Chemical Bonding
15
JEE Qs
8%
Hard
75
min
Master the MO energy level diagrams and electron filling rules for homonuclear diatomic molecules (H₂ to Ne₂), as these are frequently tested for bond order and magnetic properties.
🧮 Key Formulas
✅ Key Points for JEE
- 1Draw molecular orbital (MO) energy level diagrams for diatomic molecules (H₂ to Ne₂). Remember the s-p mixing effect: for molecules with total electrons ≤ 14 (like N₂), π2p orbitals are lower in energy than σ2p orbitals. For > 14 electrons (like O₂, F₂), σ2p is lower than π2p.
- 2Correctly fill electrons in MOs according to Aufbau principle, Hund's rule, and Pauli exclusion principle. Count bonding (Nb) and anti-bonding (Na) electrons accurately for bond order calculation.
- 3Bond order directly correlates with bond strength and inversely with bond length. A higher bond order means a stronger, shorter bond and greater stability.
- 4Determine magnetic properties (paramagnetic if unpaired electrons exist in MOs, diamagnetic if all electrons are paired). O₂ is paramagnetic, which VBT fails to explain but MOT successfully predicts.
⚠️ Common Mistakes
- ✕Incorrectly applying the MO energy level order, especially confusing the s-p mixing rule for N₂ vs. O₂/F₂ type molecules.
- ✕Miscounting total electrons for ions (e.g., O₂⁻, N₂⁺) or incorrectly assigning them to bonding/anti-bonding MOs.
- ✕Confusing atomic orbital (AO) shapes/overlap with molecular orbital shapes/energies, or incorrectly predicting sigma vs pi bond formation.
📝 Practice Questions
See allQ75.Total number of non bonded electrons present in NO2− ion based on Lewis theory is 2025 (29 Jan Shift 2) JEE Main Previous Year Paper
Q71.The number of molecules/ions that show linear geometry among the following is ________ SO2, BeCl2, CO2, N−3 , NO2, F2O, XeF2, NO+2 , I−3 , O3
Q69.Arrange the following compounds in increasing order of their dipole moment : HBr, H2 S, NF3 and CHCl3 (1) H2 S < HBr < NF3 < CHCl3 (2) NF3 < HBr < H2 S < CHCl3 (3) HBr < H2 S < NF3 < CHCl3 (4) CHCl3 < NF3 < HBr < H2 S
Q64.Which of the following statement is true with respect to H2O, NH3 and CH4 ? A. The central atoms of all the molecules are sp3 hybridized. B. The H −O −H, H −N −H and H −C −H angles in the above molecules are 104.5∘, 107.5∘ and 109.5∘ , respectively. C. The increasing order of dipole moment is CH4 < NH3 < H2O. D. Both H2O and NH3 are Lewis acids and CH4 is a Lewis base. E. A solution of NH3 in H2O is basic. In this solution NH3 and H2O act as Lowry-Bronsted acid and base respectively. Choose the correct answer from the options given below: (1) A, B and C Only (2) A, D and E Only (3) C, D and E Only (4) A, B, C and E Only
Q70.Which of the following linear combination of atomic orbitals will lead to formation of molecular orbitals in homonuclear diatomic molecules [internuclear axis in z -direction] ? A. 2pz and 2px B. 2 s and 2px C. 3 dxy 2025 (24 Jan Shift 1) JEE Main Previous Year Paper and 3 dx2−y2 D. 2 s and 2pz E. 2pz and 3dx2 −y2 Choose the correct answer from the options given below: (1) A and B Only (2) D Only (3) E Only (4) C and D Only
Q53.Given below are two statements : Statement (I): Experimentally determined oxygen-oxygen bond lengths in the O3 are found to be same and the bond length is greater than that of a O = O (double bond) but less than 2025 (24 Jan Shift 2) JEE Main Previous Year Paper that of a single (O −O) bond. Statement (II) : The strong lone pair-lone pair repulsion between oxygen atoms is solely responsible for the fact that the bond length in ozone is smaller than that of a double bond (O = O) but more than that of a single bond (O −O). In the light of the above statements, choose the correct answer from the options given below : (1) Both Statement I and Statement II are false (2) Statement I is false but Statement II is true (3) Statement I is true but Statement II is false (4) Both Statement I and Statement II are true
NCERT Chapters
- Class 11 Chemistry Ch 4: Chemical Bonding and Molecular Structure