Chemistry · Chemical Bonding · NEET
A sigma (σ) bond forms by head-on overlap of orbitals along the internuclear axis (the straight line joining the two nuclei). A pi (π) bond forms by side-on (sideways) overlap of two parallel p-orbitals, giving electron clouds above and below the axis. So the difference is the DIRECTION of overlap: σ overlaps end-to-end, π overlaps side-to-side. This matters for NEET because it decides bond strength, rotation, and how you count bonds.
Use one rule: the FIRST bond between any two atoms is always a sigma bond; every extra bond is a pi bond. So a single bond = 1σ, a double bond = 1σ + 1π, a triple bond = 1σ + 2π. To count all sigma bonds, just count the TOTAL number of bond lines connecting atoms (treat each single, and the first line of each double/triple, as one σ). To count pi bonds, count how many double and triple bonds there are: each double adds 1 π, each triple adds 2 π.
A single bond is always a sigma bond. A pi bond can never exist alone — it only forms in ADDITION to a sigma bond when atoms make a double or triple bond. That is why the first bond between two atoms is always sigma.
A sigma bond is stronger. NCERT says bond strength depends on the extent of overlap. In a sigma bond the orbitals overlap head-on to a LARGE extent, so it is strong. In a pi bond the p-orbitals overlap sideways to a SMALLER extent, so it is weaker. This is why the pi bond usually breaks first in a chemical reaction (like when a double bond reacts).
Between two normal atoms, no — you always make the sigma bond first, then add pi bonds. But there is one special NEET exception from Molecular Orbital Theory: the C2 molecule has a bond order of 2 made of TWO pi bonds and NO sigma bond. This appears in NEET (2024 matching question), so remember C2 as the odd one out.
A sigma bond is symmetric around the axis, so the two atoms can spin freely — free rotation is allowed. A pi bond is formed by sideways p-orbital overlap above and below the plane; to rotate you would have to break this side-on overlap. So a double bond (which has a pi bond) is rigid and cannot rotate freely. This is why alkenes show cis-trans (geometrical) isomerism but alkanes do not.
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.
The first bond between two atoms is always sigma; the extra bonds are pi. Single = 1σ, double = 1σ+1π, triple = 1σ+2π. Count total bond lines for sigma; count double and triple bonds for pi.
Because the sigma bond is formed by head-on overlap, which is a large amount of overlap. A pi bond uses sideways overlap, which is a smaller amount. More overlap means a stronger, more stable bond, so sigma is stronger.
No. A double bond has 1 sigma bond and 1 pi bond. The first bond is always the sigma; the second is the pi. Only the sigma part gives a straight head-on overlap.
The pi bond breaks first because it is weaker (smaller overlap). This is why double bonds (which contain a pi bond) react more easily, for example in addition reactions of alkenes.
Yes. According to Molecular Orbital Theory, C2 has a bond order of 2 formed by two pi bonds and no sigma bond. It is the classic NEET exception where pi bonds exist without a sigma bond.
