Revision/Fast Recall

1. TITLE

MOT Energy Levels: The 2s−2p Mixing Switch — Complete revision in 3 minutes

2. MOT ENERGY LEVELS — QUICK FACTS

  • MOT (Molecular Orbital Theory): Atomic orbitals combine linearly (LCAO) to form MOs.
  • Mixing Switch: Occurs at 14 electrons ($N_2$).
  • $\le$ 14 Electrons ($Li_2$ to $N_2$): 2s-2p mixing occurs. $\sigma 2p_z$ energy rises above $\pi 2p$.
  • > 14 Electrons ($O_2, F_2$): No significant mixing. $\sigma 2p_z$ is lower than $\pi 2p$.
  • $O_2$ Exception: MOT explains $O_2$ is paramagnetic (unpaired $e^-$ in $\pi^*$).
  • $C_2$ Unique Bond: The double bond in $C_2$ consists of two $\pi$ bonds (no $\sigma$).
  • Stability: $N_b > N_a$ means the molecule is stable.

3. FORMULA SHEET

FORMULAS:

→ Bond Order (B.O.):

$$B.O. = \frac{1}{2} (N_b - N_a)$$


Use to determine stability and bond type (1=single, 2=double, 3=triple).

 

→ Stability Order:

$$\text{Bond Order} \propto \text{Bond Enthalpy} \propto \frac{1}{\text{Bond Length}}$$


Use when comparing strength of bonds in isoelectronic species.

 

4. MUST-REMEMBER VALUES/CONSTANTS

KEY VALUES:

  • $N_2$ Bond Order = 3 (Triple Bond)
  • $O_2$ Bond Order = 2 (Double Bond)
  • $He_2$ Bond Order = 0 (Does not exist)
  • $N_2$ Bond Enthalpy = $946 \text{ kJ mol}^{-1}$ (Very High)

5. COMPARISON SUMMARY: ORDER OF ENERGY LEVELS

Feature Up to 14 $e^-$ ($B_2, C_2, N_2$) More than 14 $e^-$ ($O_2, F_2$)
Mixing Yes (2s-2p mixing) No
Energy Order $\pi 2p < \sigma 2p_z$ $\sigma 2p_z < \pi 2p$
Diagram $\pi$ set is lower $\sigma$ set is lower
Source    

6. COMMON MISTAKES — QUICK LIST

DON'T:
* Assume the "normal" order ($\sigma < \pi$) applies to Nitrogen ($N_2$).
* Forget that $C_2$ has a bond order of 2 but zero sigma bonds.
* Think $O_2$ is diamagnetic because it has even electrons (16). It is paramagnetic.

DO:
* Fill degenerate $\pi$ orbitals singly first (Hund’s Rule) before pairing.
* Remember that increasing Bond Order always decreases Bond Length.

7. MNEMONICS & MEMORY TRICKS

REMEMBER:

  • "Up to 14, Pi is Low": For molecules with $\le 14$ electrons ($N_2$), the $\pi$ orbitals are lower energy than $\sigma$.
  • "After 14, Sigma is Low": For $O_2$ and $F_2$, the $\sigma$ orbital drops below $\pi$ (normal expected order).
  • "2-1-2-1" vs "1-2-2-1":
    • $N_2$ (Mixed): $\pi(2) \to \sigma(1) \to \pi^(2) \to \sigma^(1)$
    • $O_2$ (Unmixed): $\sigma(1) \to \pi(2) \to \pi^(2) \to \sigma^(1)$

8. HIGH-YIELD NEET POINTS

NEET LOVES TO ASK:
* Paramagnetism of $O_2$: MOT is the only theory that successfully explains why $O_2$ is paramagnetic (unpaired electrons in antibonding $\pi^$ orbitals),.
* $C_2$ Bonding: $C_2$ exists in vapour phase and has a double bond made of two pi bonds.
* Isoelectronic Species: $F_2$ and $O_2^{2-}$ have the same bond order (1).
* Ionization Stability*: Removing an electron from $N_2$ (Bonding orbital) decreases stability. Removing an electron from $O_2$ (Antibonding orbital) increases Bond Order (2 $\to$ 2.5),.

9. ONE MCQ — SELF-TEST

Q: Which of the following molecules has the energy order $\sigma 2p_z < \pi 2p_{x,y}$?
A) $N_2$
B) $C_2$
C) $O_2$
D) $B_2$

Answer: C) $O_2$
Explanation: $O_2$ has >14 electrons, so the 2s-2p mixing is not effective, making $\sigma 2p_z$ lower in energy than $\pi 2p$ orbitals.

10. NCERT PAGE REFERENCES

Revise from:
- Page 128-129: Energy Level Diagrams & Mixing,
- Page 130 (Fig 4.21): Table of MO occupancy ($B_2$ to $Ne_2$)
- Page 129: Bond Order & Stability Rules

11. CTA

Detailed explanation → Chemical Bonding & Molecular Structure Guide
Quick test → Mock exams