Chemistry · Chemical Bonding · NEET
Use this fixed rule. A single bond = 1 sigma, 0 pi. A double bond = 1 sigma, 1 pi. A triple bond = 1 sigma, 2 pi. First draw the structure, then count every LINE between atoms as one sigma bond. After that, add 1 pi for each double bond and 2 pi for each triple bond. This works for every molecule NEET asks.
Yes. The very first bond formed between any two atoms is ALWAYS a sigma bond, because it comes from head-on (end-to-end) overlap of orbitals, which is the strongest overlap. Any extra bond (making it double or triple) forms by sidewise overlap and is a pi bond. So one atom pair can never have a pi bond without a sigma bond first. NCERT states this clearly in section 4.5.4.
Every C-H, N-H, O-H bond is a single bond, so each one is 1 sigma bond and 0 pi bond. Students forget to count these. For example in CH3-CH=CH-C#CH (pent-2-en-4-yne), there are 4 C-H bonds and 1 more terminal C-H plus the chain, all counted as sigma. Always count H bonds too, or your sigma total will be wrong.
Benzene (C6H6) has 6 C-C sigma (ring) + 6 C-H sigma = 12 sigma bonds and 3 pi bonds. Pyridine (C5H5N) has 6 ring sigma bonds (5 C and 1 N in the ring) + 5 C-H sigma = 11 sigma bonds and 3 pi bonds, plus 1 lone pair on nitrogen. NEET 2023 asked exactly this pyridine count.
A triple bond (like C#C in ethyne or in a nitrile C#N) has 1 sigma bond and 2 pi bonds. Never write 3 pi bonds. Think of it as one strong head-on sigma bond plus two sidewise pi bonds. So ethyne (HC#CH) has 3 sigma (2 C-H + 1 C-C) and 2 pi bonds total.
No. Lone pairs and unpaired electrons are NOT bonds, so they add zero to your sigma or pi count. Only shared pairs between two atoms count. NEET sometimes asks for sigma, pi AND lone pairs together (like pyridine: 11 sigma, 3 pi, 1 lone pair), so read the question carefully and count each thing separately.
The number of sigma (σ) and pi (π) bonds in pent-2-en-4-yne is
The number of σ bonds, π bonds and lone pairs of electrons in pyridine (C5H5N), respectively, are:
Match List-I (Molecule) with List-II (bonds between the two carbon atoms): A. ethane; B. ethene; C. C2; D. ethyne — I. one σ and two π; II. two π; III. one σ; IV. one σ and one π
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Normally the first bond is sigma, but the C2 molecule is a special MO theory case. Its bond order of 2 comes entirely from two pi bonds and has NO sigma bond. NEET tests this as a trick, so remember C2 = two pi bonds, zero sigma. This applies only to gaseous C2, not to normal carbon compounds.
A C=O double bond has 1 sigma bond and 1 pi bond, just like any double bond. So in a molecule like CO2 (O=C=O) there are 2 sigma bonds and 2 pi bonds total. The rule is the same for C=O, C=C or C=N double bonds.
A sigma bond forms by head-on (axial) overlap of orbitals, which overlaps a large amount, so it is strong. A pi bond forms by sidewise (lateral) overlap, which overlaps a smaller amount, so it is weaker. This is why the extra bonds in double and triple bonds break more easily in reactions.
Yes. In any ring, each side of the ring is one sigma bond. Benzene's ring has 6 C-C sigma bonds, and pyridine's ring has 6 sigma bonds. Then add all the C-H (and other) sigma bonds outside the ring. Never forget the ring bonds.
