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Every double bond is one sigma and one pi bond, and every triple bond is one sigma and two pi. Most students can recite that. Fewer can say why the pi bond is the weaker one, or count sigma and pi bonds quickly in a molecule like acrylonitrile. This post covers the overlap picture, uses NCERT's bond enthalpy table to put a number on the pi bond, and shows where the simple rule needs care.
What does the syllabus ask?
The JEE (Main) 2026 syllabus lists "sigma and pi-bonds" together with bond order, bond length and bond energy in its bonding unit. The JEE (Advanced) 2026 brochure lists "Orbital overlap and covalent bond" and, in organic chemistry, "σ and π-bonds". The NCERT treatment is in sections 4.5.3 to 4.5.5 of the Class 11 chapter Chemical Bonding and Molecular Structure.
How do sigma and pi bonds form?
NCERT describes a sigma bond as one formed by "head-on overlap of bonding orbitals along the internuclear axis". Three combinations can do this: s with s, s with p, and p with p when the p orbitals point at each other.
A pi bond forms when two p orbitals lie parallel to each other and perpendicular to the internuclear axis and overlap sideways. The result is two lobes of electron density, one above and one below the plane of the bonded atoms. An s orbital is spherical, so it can never overlap sideways in this way. Every pi bond in your syllabus comes from p orbitals.
NCERT's Figure 4.9 also shows overlaps that cancel. When the signs of the two lobes do not match, or the orbitals point in different directions, the overlap is zero and no bond forms.
Why is a pi bond weaker than a sigma bond?
NCERT's answer in section 4.5.5 is short: bond strength depends on the extent of overlap. Head-on overlap is large. Sideways overlap is smaller. So a sigma bond is stronger than a pi bond between the same two atoms. NCERT also notes that in a multiple bond, the pi bonds form in addition to a sigma bond. You never get a pi bond on its own.
Worked example 1: estimating the pi bond from bond enthalpies
Table 5.3 in the NCERT Thermodynamics chapter gives mean bond enthalpies at 298 K: C–C 347, C=C 611 and C≡C 837 kJ mol⁻¹.
The double bond is one sigma plus one pi, so the pi part is about 611 − 347 = 264 kJ mol⁻¹.
The triple bond adds a second pi bond: about 837 − 611 = 226 kJ mol⁻¹.
Both are smaller than the 347 kJ mol⁻¹ of the C–C sigma bond.
Notice too that 611 is less than twice 347 (694). A double bond is stronger than a single bond, but not twice as strong. These are mean values averaged over many compounds, so treat the subtraction as an estimate, not an exact pi bond energy.
Bond length tells the same story from the other side. NCERT's Table 4.3 gives C–C 154 pm, C=C 133 pm and C≡C 120 pm. Each extra pi bond pulls the atoms closer, even though it adds less strength than the sigma bond did.
Worked example 2: counting sigma and pi bonds
Take acrylonitrile, CH₂=CH–C≡N.
Single bonds are one sigma each: three C–H and one C–C give 4 sigma.
The C=C is one sigma and one pi.
The C≡N is one sigma and two pi.
Total: 4 + 1 + 1 = 6 sigma and 0 + 1 + 2 = 3 pi.
Quick checks to practise: ethyne, HC≡CH, has 3 sigma and 2 pi. Carbon dioxide, O=C=O, has 2 sigma and 2 pi. Benzene in its Kekulé form has 6 C–C and 6 C–H sigma bonds (12 in all) and 3 pi bonds.
Where does the simple rule need care?
Run the same subtraction for nitrogen using NCERT's table: N–N 159, N=N 418, N≡N 946 kJ mol⁻¹. Now 418 − 159 = 259, which is larger than the single bond itself. The NCERT statement compares overlap for the same pair of atoms, and the N–N single bond in that table is unusually weak, so bond enthalpy differences do not line up with the simple rule here. Carbon is the clean case. Use carbon when you explain the rule, and do not assume "pi is weaker" from every enthalpy table.
Common mistakes and how to avoid them
Counting a double bond as two sigma bonds. It is one sigma and one pi.
Letting s orbitals form pi bonds. Only p orbitals (and, later, d orbitals) can overlap sideways.
Thinking the pi bond forms first. The sigma bond sets the internuclear axis; the pi bond forms alongside it.
Saying a double bond is weaker than a single bond. The pi part is weaker, but the double bond as a whole (611) is stronger than the single bond (347).
A practice routine
Redraw NCERT Figure 4.9 from memory, labelling positive, negative and zero overlaps.
Count sigma and pi bonds in ten molecules: ethene, ethyne, HCN, CO₂, acetone, acrylonitrile, benzene, N₂, H₂SO₄ and buta-1,3-diene.
Redo the bond enthalpy subtraction for C=O (741 kJ mol⁻¹ in NCERT's table) against C–O (351).
Link it to hybridisation: our guide to sp, sp² and sp³ hybridisation shows which orbitals are left over to make the pi bonds, and bond enthalpy calculations uses the same table.
Sideways overlap is hard to picture from a flat diagram. Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read, which can help when a textbook figure is not enough.
Pick any three organic molecules from your textbook today and count their sigma and pi bonds before you read the answers.
