Why Is O2 Paramagnetic? MO Theory Explained Simply
Chemistry · Chemical Bonding · NEET
O2 is paramagnetic because it has 2 unpaired electrons sitting in its two π*2p antibonding molecular orbitals. Lewis dot structures wrongly show all electrons paired, but Molecular Orbital (MO) Theory correctly predicts these 2 lone unpaired electrons, which is why oxygen is pulled toward a magnet. Memory hook: "Oxygen has 2 single friends in the π-star" (2 unpaired electrons in π*).
MO picture of O2: the top two electrons sit singly in the degenerate π*2px and π*2py antibonding orbitals (Hund's rule), giving 2 unpaired electrons and paramagnetism, while bond order stays 2 (double bond).
Your doubts, answered
Why is O2 paramagnetic and not diamagnetic?
A molecule is paramagnetic if it has one or more UNPAIRED electrons. O2 has 16 electrons. When you fill them into molecular orbitals, the last 2 electrons go into two equal-energy (degenerate) orbitals π*2px and π*2py. By Hund's rule, they go in singly, one in each, with parallel spins. So O2 has 2 unpaired electrons and is paramagnetic (attracted by a magnet). If all electrons were paired, it would be diamagnetic.
How many unpaired electrons does O2 have?
Exactly 2 unpaired electrons. Both sit in the antibonding π*2p orbitals (one in π*2px, one in π*2py). This is the single most tested fact about O2 in NEET. Remember: O2 → 2 unpaired e⁻ → paramagnetic.
Why does the Lewis dot structure of O2 fail to explain paramagnetism?
In the Lewis structure O=O, we draw a double bond and lone pairs, and every electron looks paired. So Lewis theory wrongly predicts O2 is diamagnetic. This was a famous failure. MO Theory succeeds because it places the top 2 electrons into two separate degenerate π* orbitals, showing them as unpaired. This is a classic NCERT point: MOT explains what Lewis structure cannot.
Which orbitals hold the unpaired electrons in O2?
The two antibonding π*2p orbitals — written π*2px and π*2py. These are degenerate (same energy). The MO order for O2 is: σ1s < σ*1s < σ2s < σ*2s < σ2pz < (π2px = π2py) < (π*2px = π*2py) < σ*2pz. Note: in O2 the σ2pz is BELOW the π2p (opposite of N2).
What is the full MO electron configuration of O2?
O2 has 16 electrons: σ1s² σ*1s² σ2s² σ*2s² σ2pz² (π2px² = π2py²) (π*2px¹ = π*2py¹). The last two π* electrons are unpaired. Bond order = ½(Nb − Na) = ½(10 − 6) = 2, so O2 has a double bond AND is paramagnetic at the same time.
Is O2+ or O2- also paramagnetic?
Yes, but check the count. O2+ (15 e⁻) has 1 unpaired electron → paramagnetic. O2- superoxide (17 e⁻) has 1 unpaired electron → paramagnetic. O2²- peroxide (18 e⁻) has 0 unpaired electrons → diamagnetic. NEET loves the trap that O2+ is diamagnetic — it is NOT, it is paramagnetic.
⚠️ The NEET trap ✗ O2 has a double bond and a normal Lewis structure, so all its electrons are paired and it must be diamagnetic. And O2+ loses an electron, so it becomes diamagnetic. ✓ O2 is PARAMAGNETIC: its top 2 electrons occupy two degenerate π*2p orbitals singly (Hund's rule), giving 2 unpaired electrons. O2+ still has 1 unpaired electron in π*2p, so O2+ is ALSO paramagnetic, not diamagnetic. 🧠 Count unpaired electrons in the π* box, do NOT trust the Lewis picture. O2 → 2 unpaired, O2+ → 1 unpaired: both paramagnetic.
Real NEET questions
NEET 2019 (Odisha)
Which of the following is paramagnetic?
A · N2
B · H2
C · Li2
D · O2 ✓
Solution: By molecular orbital theory, O2 has two unpaired electrons in its degenerate π*2p antibonding orbitals, making it paramagnetic. N2, H2 and Li2 have all electrons paired, so they are diamagnetic. Answer: (D) O2.
NEET 2022 / NEET 2019 (Odisha)
Which amongst the following is an incorrect statement?
A · The bond orders of O2+, O2, O2- and O2^2- are 2.5, 2, 1.5 and 1 respectively
B · C2 molecule has four electrons in its two degenerate π molecular orbitals
C · H2+ ion has one electron
D · O2+ ion is diamagnetic ✓
Solution: O2+ has configuration ...(π*2p)¹, i.e. ONE unpaired electron, so it is paramagnetic, not diamagnetic. Statement (D) is therefore incorrect (the wrong statement asked for). The other three statements are all correct. Answer: (D).
Solved Chemical Bonding NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
O2 is paramagnetic. It has 2 unpaired electrons in its π*2p antibonding molecular orbitals, so it is attracted by a magnetic field.
How many unpaired electrons are there in O2?
Two unpaired electrons, one each in the degenerate π*2px and π*2py orbitals, following Hund's rule.
What is the bond order of O2?
Bond order of O2 = ½(Nb − Na) = ½(10 − 6) = 2. So O2 has a double bond and is still paramagnetic.
Why can't Lewis structure explain O2's paramagnetism?
The Lewis structure O=O shows all electrons paired, wrongly predicting diamagnetism. Only MO Theory shows the 2 unpaired electrons in the π* orbitals, correctly predicting paramagnetism.
Is O2+ diamagnetic?
No. O2+ has 15 electrons with 1 unpaired electron in π*2p, so O2+ is paramagnetic. A common NEET trap says it is diamagnetic — that is wrong.
