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ChemistryMediumClass 11

Molecular Orbital Theory (MOT)

Chemical Bonding

45

JEE Qs

12%

Hard

75

min

Thoroughly memorize the two MO energy orders, practice drawing MO diagrams for various diatomic species (including ions), and consistently apply Hund's rule to correctly determine magnetic properties and bond orders.

šŸ§® Key Formulas

Bond Order (BO) = 1/2 * (Number of electrons in bonding MOs - Number of electrons in anti-bonding MOs)
MO Energy Order (for total electrons <= 14, e.g., B2, C2, N2): Ļƒ1s < Ļƒ*1s < Ļƒ2s < Ļƒ*2s < Ļ€2px = Ļ€2py < Ļƒ2pz < Ļ€*2px = Ļ€*2py < Ļƒ*2pz
MO Energy Order (for total electrons > 14, e.g., O2, F2, Ne2): Ļƒ1s < Ļƒ*1s < Ļƒ2s < Ļƒ*2s < Ļƒ2pz < Ļ€2px = Ļ€2py < Ļ€*2px = Ļ€*2py < Ļƒ*2pz

āœ… Key Points for JEE

  • 1Master the two distinct MO energy level sequences (for N2 and lighter vs. O2 and heavier) and identify the crossover point (s-p mixing). This is crucial for correctly filling MOs.
  • 2Be able to draw MO diagrams for homonuclear (H2 to Ne2) and simple heteronuclear (CO, NO, CN-) diatomic molecules, correctly placing electrons according to Aufbau, Pauli, and Hund's rules.
  • 3Calculate bond order accurately. Relate bond order directly to bond strength (higher BO = stronger bond), bond length (higher BO = shorter bond), and molecular stability (higher BO = more stable).
  • 4Determine magnetic properties (paramagnetic if unpaired electrons exist, diamagnetic if all electrons are paired) directly from the MO electron configuration.
  • 5Understand that MOT successfully explains phenomena like the paramagnetism of O2 and the existence of fractional bond orders in ions, which Valence Bond Theory cannot.

āš ļø Common Mistakes

  • āœ•Using the incorrect MO energy level sequence for a given molecule (e.g., using the O2 sequence for N2 or vice versa).
  • āœ•Errors in calculating the total number of electrons for charged species (anions or cations), leading to incorrect MO filling.
  • āœ•Incorrectly applying Hund's Rule or Pauli Exclusion Principle when filling degenerate molecular orbitals, especially Ļ€ and Ļ€* orbitals.
  • āœ•Confusing the relationship between bond order, bond strength, and bond length (e.g., thinking higher bond order means longer bond).
  • āœ•Forgetting to consider core electrons when constructing MO diagrams, although often only valence electrons are shown for simplification in diagrams, their count matters for total electrons.

šŸ“ Practice Questions

See all

Q75.Total number of non bonded electrons present in NO2āˆ’ ion based on Lewis theory is 2025 (29 Jan Shift 2) JEE Main Previous Year Paper

2025Ā·NumericalEasy

Q71.The number of molecules/ions that show linear geometry among the following is ________ SO2, BeCl2, CO2, Nāˆ’3 , NO2, F2O, XeF2, NO+2 , Iāˆ’3 , O3

2025Ā·NumericalMedium

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

2025Ā·MCQMedium

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 āˆ’O āˆ’H, H āˆ’N āˆ’H and H āˆ’C āˆ’H angles in the above molecules are 104.5āˆ˜, 107.5āˆ˜ and 109.5āˆ˜ , 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

2025Ā·MCQMedium

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 dx2āˆ’y2 D. 2 s and 2pz E. 2pz and 3dx2 āˆ’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

2025Ā·MCQMedium

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 āˆ’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 āˆ’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

2025Ā·Assertion ReasoningMedium

NCERT Chapters

  • Class 11 Chemistry Ch 4: Chemical Bonding and Molecular Structure