WHY it matters: Lewis/VBT draws O₂ with a double bond and all electrons paired → predicts
diamagnetic → WRONG. MOT correctly predicts paramagnetism. This is the textbook "victory"
of MOT over VBT, so it is a guaranteed exam favourite.
Step 1 — Count the electrons.
Each O atom has 8 electrons, so O₂ has 8+8=16 electrons total.
Why this step? MOT fills molecular orbitals just like the Aufbau principle fills atomic orbitals — we need the total electron count first.
Step 2 — Know the MO energy order.
For O₂ and F₂ (i.e. molecules with total electrons >14, where s–p mixing is small), the order is:
Why this order? For O₂/F₂ the σ2pz sits below the two π-bonding orbitals. (Compare: for B₂, C₂, N₂ with ≤14 electrons, strong s–p mixing pushes σ2pzabove the π's — a common trap.)
Step 3 — Fill 16 electrons using Aufbau + Hund's rule.
MO
electrons
running total
σ1s
2
2
σ1s∗
2
4
σ2s
2
6
σ2s∗
2
8
σ2pz
2
10
π2px,π2py
4
14
π2px∗,π2py∗
2
16
σ2pz∗
0
16
Step 4 — The crucial last two electrons.
The final 2 electrons go into the degenerate (equal energy) pair π2px∗ and π2py∗.
By Hund's rule, they occupy separate orbitals with parallel spins before pairing.
Why this step? Hund's rule (maximise total spin) forces one electron in each π* orbital → 2 unpaired electrons → paramagnetic. This is the whole answer.
Why this matters: MOT gives a double bond (B.O. = 2) just like Lewis — so it doesn't contradict
the bond strength — AND it additionally reveals the 2 unpaired electrons Lewis missed. Best of both.
Imagine 16 kids getting onto a set of buses (the molecular orbitals), filling the front seats first. Right at the end, two kids are left and there are two empty double-seat buses of the exact same "coolness." Kids prefer to sit alone rather than squeeze next to someone, so each takes one bus. Those two "loners" are lonely spinning electrons — and a spinning lone electron acts like a tiny magnet. That's why oxygen sticks to a magnet, even though the simple dot-picture says it shouldn't!
It has 2 unpaired electrons, one in each degenerate π₂p orbital (Hund's rule).
How many total electrons in O₂ and how many unpaired? ::: 16 total; 2 unpaired.
Which MOs hold the last 2 electrons of O₂? ::: The degenerate pair π₂pₓ and π₂p_y (one electron each).
Bond order of O₂ (show it)? ::: (N_b − N_a)/2 = (10 − 6)/2 = 2.
Spin-only magnetic moment of O₂? ::: √(n(n+2)) = √8 ≈ 2.83 BM for n = 2.
Why does Lewis structure fail for O₂ magnetism? ::: It pairs all electrons and can't show single occupancy of degenerate π orbitals.
For O₂/F₂, where is σ2p_z relative to the π orbitals?
Below the π bonding orbitals (weak s–p mixing).
Is O₂²⁻ (peroxide) para- or diamagnetic and its bond order?
Diamagnetic (n=0); bond order 1.
Is O₂⁻ (superoxide) magnetic; bond order?
Paramagnetic (n=1, μ≈1.73 BM); bond order 1.5.
What decides paramagnetism — bond order or unpaired electrons?
Dekho, agar tum O₂ ka simple Lewis structure banao — O=O — to saare electrons pair ho jaate hain,
matlab molecule diamagnetic hona chahiye. Lekin experiment me liquid oxygen magnet se chipak jaata hai,
yaani wo actually paramagnetic hai. Yahi cheez Lewis/VBT explain nahi kar paata, aur MOT (Molecular
Orbital Theory) bilkul sahi predict karta hai — isliye ye topic exams me favourite hai.
MOT ka funda simple hai: O₂ me total 16 electrons hain, aur inhe hum molecular orbitals me Aufbau order
se bharte hain, bilkul waise jaise atoms me bharte the. Last ke 2 electrons π* (pi-star antibonding)
ke do degenerate (equal energy) orbitals me jaate hain. Hund's rule kehta hai ki degenerate orbitals
me pehle ek-ek electron alag-alag baithega parallel spin ke saath, tabhi pair banega. Isliye O₂ me 2
unpaired electrons ban jaate hain — aur ek akela spinning electron chhote magnet ki tarah behave karta
hai. Bas isiliye O₂ paramagnetic hai.
Bond order bhi nikaalna easy hai: (Nb−Na)/2=(10−6)/2=2, matlab double bond — jo Lewis ke saath
match karta hai. To MOT double advantage deta hai: bond order bhi sahi aur magnetism bhi sahi. Yaad rakhna:
magnetism sirf unpaired electrons pe depend karta hai, bond order pe nahi. O₂⁻ (superoxide) me 1 unpaired
(paramagnetic, B.O. 1.5) aur O₂²⁻ (peroxide) me 0 unpaired (diamagnetic, B.O. 1) — yahi pattern trick
questions me poochha jaata hai.
