Chemistry · Chemical Bonding · NEET
Follow 3 steps. (1) Count the total electrons in the molecule. (2) Fill them into the molecular orbitals in energy order, using Hund's rule (put one electron in each equal-energy orbital before pairing). (3) Look at the final picture: if every orbital that has electrons is FULL (2 electrons each), the molecule is diamagnetic. If even one orbital has a single lonely electron, the molecule is paramagnetic. That single unpaired electron is the whole test.
Yes. Paramagnetic means the molecule has at least one unpaired electron. Each unpaired electron acts like a tiny magnet, so the molecule gets attracted (pulled in) by an outside magnetic field. Diamagnetic means zero unpaired electrons — all are paired — so the molecule is weakly repelled (pushed out). This is why NEET loves O2: MO theory correctly predicts O2 has 2 unpaired electrons, so it is paramagnetic.
It is only about unpaired electrons, not about being a gas. N2 has 14 electrons; when you fill the MOs, every orbital ends up completely full, so 0 unpaired electrons → diamagnetic. O2 has 16 electrons; the last 2 electrons go into two equal-energy π* orbitals, and by Hund's rule they sit alone (one each) instead of pairing. So O2 has 2 unpaired electrons → paramagnetic. Same period, but the electron count changes the final picture.
Diamagnetic substances are weakly REPELLED (pushed away) by a magnet, because all electrons are paired and there is no net magnetic moment. Paramagnetic substances are ATTRACTED (pulled in), because unpaired electrons give a net magnetic moment. NEET often flips these two words in the option to trap you, so read carefully: paired → repelled (dia), unpaired → attracted (para).
After filling the MO diagram, just count how many orbitals have a single electron (one arrow instead of two). That count is the number of unpaired electrons. Example: O2 shows two π* orbitals with one electron each → 2 unpaired → paramagnetic. N2 shows no half-filled orbital → 0 unpaired → diamagnetic. If unpaired count = 0, it is diamagnetic; if it is 1 or more, it is paramagnetic.
Check the unpaired electrons after adding or removing electrons. O2 (16 e) has 2 unpaired → paramagnetic. O2+ (15 e, remove one from π*) has 1 unpaired → still paramagnetic. O2- (17 e) has 1 unpaired → paramagnetic. O2^2- peroxide (18 e) fills both π* fully → 0 unpaired → diamagnetic. A NEET trap says 'O2+ is diamagnetic' — that is wrong, O2+ is paramagnetic.
Which of the following is paramagnetic?
Which amongst the following is an incorrect statement?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Diamagnetic = all electrons paired, weakly repelled by a magnet. Paramagnetic = has unpaired electrons, attracted by a magnet.
O2 has 2 unpaired electrons, one in each of the two π*2p orbitals, which is why it is paramagnetic.
N2 is diamagnetic. All 14 of its electrons are paired, so there are 0 unpaired electrons.
NEET repeatedly asks 'which species is paramagnetic/diamagnetic' and puts trap options (like calling O2+ diamagnetic). MO theory gives the exact unpaired-electron count, so you answer with certainty and avoid the trap.
For simple homonuclear molecules you can, if you remember the results (O2 para, N2 dia). But the safe way is to fill the MO diagram and count unpaired electrons — that always works, including for ions like O2+ and O2^2-.
