Q.Using molecular orbital theory write electronic configuration, calculate bond order and magnetic property of O2 molecule.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Sigma Pi Bond Counting
Sigma Pi Bond Counting: From Intuition to Precision
Imagine you're building a molecular model with sticks and balls. Every single bond you see — a single line between two atoms — is made of one sigma bond. That's the backbone. A double bond? That's one sigma plus one pi bond. A triple bond? One sigma plus two pi bonds.
This is the core idea: sigma bonds are the first bond formed between any two atoms; any additional bonds are pi bonds.
Why sigma comes first
When two atoms approach each other, their orbitals overlap end-to-end along the line joining the nuclei. That head-on overlap creates a sigma bond — strong, cylindrically symmetric, and free to rotate. If the atoms need to share more electrons (to satisfy octets, for example), they can't form another sigma bond because the orbitals are already used up in that direction. Instead, they use sideways overlap of p-orbitals above and below the internuclear axis. That sideways overlap is a pi bond — weaker, and it locks the molecule into a plane (no free rotation).
So the rule is simple: between any two bonded atoms, exactly one bond is sigma; the rest are pi.
The precise counting method
For any molecule, you can count sigma and pi bonds systematically:
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Count sigma bonds: Every single bond is one sigma. Every double bond contributes one sigma (and one pi). Every triple bond contributes one sigma (and two pi). Also, every bond to hydrogen is sigma.
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Count pi bonds: For each multiple bond, subtract 1 from the bond order. That remainder is the number of pi bonds.
For a bond of order n between two atoms:
- Sigma bonds = 1
- Pi bonds = n−1
So:
- Single bond (n=1): 1 sigma, 0 pi
- Double bond (n=2): 1 sigma, 1 pi
- Triple bond (n=3): 1 sigma, 2 pi
A worked example: ethene (CX2HX4)
Draw the structure: each carbon is double-bonded to the other, and each carbon has two single bonds to hydrogen.
- The C=C double bond: 1 sigma + 1 pi
- Each C–H single bond: 1 sigma (4 such bonds)
- Total: 5 sigma bonds, 1 pi bond
Check: The molecule has 5 sigma bonds holding the skeleton together, and 1 pi bond in the double bond region.
A trickier case: benzene (CX6HX6)
Benzene has six C–C bonds that are all equivalent — each is 1.5 bonds (resonance hybrid). But for counting purposes, treat each ring bond as a single bond (sigma) plus a delocalised pi system.
- 6 C–H bonds: all sigma
- 6 C–C ring bonds: each is sigma
- The pi system: 3 pi bonds (delocalised over the ring)
Total: 12 sigma bonds, 3 pi bonds. …
O2 has 16 electrons; filling MOs in order of energy gives two unpaired electrons in the antibonding π* orbitals, correctly predicting its well-known paramagnetism (a success of MOT over VBT). …
[!TLDR]
Configuration: σ1s² σ1s² σ2s² σ2s² σ2pz² π2px²=π2py² π2px¹=π2py¹. Bond order = (Nb-Na)/2 = (10-6)/2 = 2. Magnetic property: paramagnetic, due to two unpaired electrons in the degenerate π*2p orbitals.
Method …
Showing the 12 most recent of 28 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.How many σ and π bonds are present in the following molecule? N≡C-CH-C≡N(a) σ = 5, π = 4(b) σ = 6, π = 3(c) σ = 4, π = 2(d) σ = 3, π = 5
›Reveal solutionSolution
Count bonds by type: every single bond contributes 1 sigma bond; every double bond contributes 1 sigma + 1 pi; every triple bond contributes 1 sigma + 2 pi. Total them across the whole structure.
Note on this question: the stem's structure notation was partially lost in transcription from the source scan (a subscript or a double-bond mark did not survive digitisation), so the exact connectivity has some uncertainty. Working through the bonding systematically for a chain built from the pieces shown (a terminal C#N triple bond, a central CH carbon, a C-C/C=C linkage, and a second terminal C#N triple bond):
- Each C#N (nitrile) group: 1 sigma + 2 pi bonds (triple bond).
- With two such nitrile-type end groups: 2 sigma + 4 pi from the two triple bonds alone. …
- CBSE 2026Set ANNUAL1 markQ.In propyne, ______ sigma (σ) and ______ pi (π) bonds are present.
›Reveal solutionSolution
Propyne (CH3-C≡CH) contains 6 sigma bonds and 2 pi bonds in total.
Propyne's structure is CH3-C≡C-H. Count bond by bond: the terminal CH3 group has 3 C-H sigma bonds; the C(methyl)-C(alkyne) bond is 1 sigma bond; the C≡C triple bond consists of 1 sigma bond plus 2 pi bonds; and the terminal ≡C-H bond is 1 more sigma bond.
…
- CBSE 2026Set ANNUAL1 markQ.Fill in the blank: The number of sigma (σ) bonds in CH4 is ______.
›Reveal solutionSolution
CH4 has 4 C-H single bonds, and every single bond is exclusively a sigma bond, so CH4 contains 4 sigma bonds and 0 pi bonds.
In methane (CH4), the carbon atom is sp3 hybridized and forms four equivalent single covalent bonds with four hydrogen atoms by head-on (axial) overlap of orbitals. Every single bond, by definition, consists of exactly one sigma bond (formed by direct/head-on orbital overlap) and no pi bonds (which require sideways overlap of unhybridized …
- CBSE 2026Set ANNUAL1 markMCQQ.Acetylene molecule is(a) Tetrahedral(b) Trigonal planar(c) Linear(d) None of these
›Reveal solutionSolution
Acetylene (C2H2) is linear.
In acetylene, H−C≡C−H, each carbon is sp hybridised (two sigma bonds, no lone pairs). The two sp orbitals point 180° apart, so all four at …
- CBSE 2026Set ANNUAL1 markMCQQ.A compound containing −C≡C− group is an example compound(a) Ethane(b) Ethylene(c) Ethyne(d) Propene
›Reveal solutionSolution
Ethyne (HC≡CH) contains the −C≡C− group.
A carbon-carbon triple bond (−C≡C−) is characteristic of alkynes. Among the options, ethyne (acetylene), HC≡CH, has the triple bond. Ethane (C−C s …
- CBSE 2025Set ANNUAL1 markMCQQ.Number of sigma bonds in P4O10 is(a) 6(b) 7(c) 17(d) 16
›Reveal solutionSolution
P4O10 contains 16 sigma bonds (and 4 pi bonds).
P4O10 has a cage-like structure built on a tetrahedron of 4 P atoms: each of the 6 tetrahedron edges is bridged by an oxygen atom (P-O-P linkage), and each P additionally carries one terminal P=O bond pointing outward.
- 6 bridging oxygens x 2 (P-O) sigma bonds each = 12 sigma bonds. …
- CBSE 2025Set ANNUAL1 markMCQQ.How many bonds are present in the given structure?(a) 14σ, 8π(b) 18σ, 8π(c) 9σ, 4π(d) 14σ, 2π
›Reveal solutionSolution
The figure's exact substituent/position is not fully legible in the scan, so the sigma/pi count below is a best-effort estimate using the described structure, and it should be checked against the original printed diagram.
The source figure shows a benzene ring connected, via a two-carbon chain containing a C=C double bond, to a halogen substituent (read as iodine, though not fully certain from the scan) — i.e. a skeleton similar to a styryl halide, C6H5-CH=CH-X.
Counting bonds for this structure:
- Aromatic ring: 6 C-C sigma bonds + 3 C-C pi bonds (delocalised, conventionally counted as 3 discrete pi bonds) + 5 C-H sigma bonds (since one ring position is substituted by the chain).
- Ring-to-chain bond: 1 sigma bond (ring carbon to the first chain carbon).
- The C=C in the chain: 1 sigma + 1 pi bond, plus 1 C-H sigma bond on the first chain carbon.
- The chain carbon bonded to the halogen: 1 C-H sigma bond and 1 C-X sigma bond.
Total sigma bonds = 6 (ring C-C) + 5 (ring C-H) + 1 (ring-chain) + 1 (C=C sigma) + 1 (C-H) + 1 (C-H) + 1 (C-X) = 16.
Total pi bonds = 3 (ring) + 1 (chain C=C) = 4.
…
- CBSE 2025Set ANNUAL1 markMCQQ.In the compound CH2=CH-CH2-CH2-C≡CH, the C2-C3 bond is(a) sp - sp2(b) sp3 - sp3(c) sp - sp3(d) sp2 - sp3
›Reveal solutionSolution
In CH2=CH-CH2-CH2-C≡CH, the C2-C3 bond connects an sp2 carbon to an sp3 carbon.
Number the chain: C1(=CH2) = C2(H)-C3H2-C4H2-C5(≡)C6H.
- C1 and C2 are part of the C1=C2 double bond, so both are sp2 hybridised.
- C3 is bonded only by single bonds (to C2 and C4, plus 2 H atoms), so it is sp3 hybridised. …
- CBSE 2025Set ANNUAL1 markMCQQ.The numbers of sigma and pi bonds in a molecule of benzene are(a) 6 sigma and 6 pi(b) 12 sigma and 6 pi(c) 12 sigma and 12 pi(d) 12 sigma and 3 pi
›Reveal solutionSolution
Every single bond (whether part of a double bond or standalone) is one sigma bond; a double bond adds one extra pi bond on top of its sigma bond. Counting benzene's bonds this way gives 12 sigma + 3 pi.
Benzene, C6H6, is drawn as a hexagonal ring of 6 carbon atoms with alternating single and double bonds (Kekule structure), each carbon also bonded to one hydrogen.
Count the sigma bonds:
- 6 C-C bonds around the ring (every C-C bond, single or double, contributes exactly one sigma bond) -> 6 sigma bonds
- 6 C-H bonds (one per carbon) -> 6 sigma bonds
- Total sigma bonds = 6 + 6 = 12
Count the pi bonds:
- In the Kekule structure, 3 of the 6 C-C bonds are drawn as double bonds; each double bond consists of 1 sigma + 1 pi bond, so the 'extra' bond beyond the sigma framework contributes 1 pi bond each
- Total pi bonds = 3 …
- CBSE 2025Set ANNUAL1 markQ.Fill in the blank: Total number of sigma (σ) and pi (π) bonds in C2H2 is ___________.
›Reveal solutionSolution
Counting bond-by-bond in H-C≡C-H: every single bond is one sigma bond, and the C-C triple bond is 1 sigma + 2 pi bonds.
Structure of acetylene (ethyne): H−C≡C−H
Bond-by-bond count:
- Each C-H bond is a single bond = 1 sigma bond each ⇒ 2 sigma bonds from the two C-H bonds. …
- CBSE 2025Set ANN1 markQ.How many sigma bonds are present in the following molecule ? HC(triple bond)C-CH=CH-CH3
›Reveal solutionSolution
Every bond (single, or the sigma part of a multiple bond) counts once: 4 C-C sigma + 6 C-H sigma = 10 sigma bonds.
Molecule: HC(triple bond)C-CH=CH-CH3, i.e. C1(H)(triple)C2-C3(H)=C4(H)-C5(H3).
Carbon-carbon sigma bonds (each multiple bond has exactly ONE sigma bond):
- C1-C2 (in the triple bond): 1 sigma
- C2-C3 (single): 1 sigma
- C3-C4 (in the double bond): 1 sigma
- C4-C5 (single): 1 sigma => 4 C-C sigma bonds. Carbon-hydrogen sigma bonds: …
- CBSE 2024Set ANNUAL1 markMCQQ.The number of sigma and pi bonds in a molecule of benzene is(a) 6 sigma and 6 pi(b) 12 sigma and 6 pi(c) 12 sigma and 12 pi(d) 12 sigma and 3 pi
›Reveal solutionSolution
Benzene's ring skeleton and C-H bonds are all sigma bonds (12 total), while the alternating double bonds contribute 3 pi bonds in the classical Kekule structure.
Benzene, C6H6, has a hexagonal ring of 6 carbon atoms each bonded to one hydrogen.
Counting sigma bonds:
- 6 C-C sigma bonds around the ring.
- 6 C-H sigma bonds (one per carbon). Total sigma bonds = 6 + 6 = 12. …
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