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Tetravalence of Carbon and Hybridisation in Organic Molecules

Chemistry · General Principles Of Organic Chemistry · NEET

Carbon is tetravalent: it always forms exactly four covalent bonds because its four valence electrons (2s and three 2p, after one 2s electron is promoted to the empty 2p) mix into four equal hybrid orbitals. These hybrids can be sp3 (4 sigma bonds, tetrahedral, 109.5 degrees), sp2 (3 sigma bonds, trigonal planar, 120 degrees) or sp (2 sigma bonds, linear, 180 degrees). Memory hook: count the sigma bonds around a carbon plus its lone pairs, and that number tells you the hybridisation.
Hybridisation of Carbon: Shape and Bond AngleCsp3tetrahedral 109.5Csp2trigonal planar 120Csplinear 180
Carbon uses sp3, sp2 or sp hybrid orbitals. Count the sigma bonds (plus lone pairs) around a carbon: 4 means sp3 (tetrahedral, 109.5 degrees), 3 means sp2 (trigonal planar, 120 degrees), 2 means sp (linear, 180 degrees).

Your doubts, answered

Why does carbon form 4 bonds when its ground state has only 2 unpaired electrons?

Carbon ground state is 1s2 2s2 2p2, so it looks like it has only two unpaired electrons in the 2p orbitals. But the small energy needed to promote one 2s electron to the empty 2p orbital is more than paid back by forming two extra bonds. After promotion carbon has four unpaired electrons (2s1 2p3), and these mix (hybridise) into four equal orbitals. So carbon always shows a valence of four.

What is the fastest way to find the hybridisation of a carbon atom?

Count the number of sigma bonds around that carbon and add any lone pairs on it. A single bond is one sigma. A double bond is one sigma plus one pi, so it still counts as ONE sigma for hybridisation. A triple bond is one sigma plus two pi, again ONE sigma. If the total (sigma + lone pairs) is 4 the carbon is sp3, if 3 it is sp2, if 2 it is sp. Pi bonds are never counted for hybridisation.

Is a double bond made of two sigma bonds?

No. A double bond is ONE sigma bond and ONE pi bond, not two sigmas. A triple bond is ONE sigma and TWO pi. This matters because only sigma bonds (plus lone pairs) decide hybridisation and shape. A carbon in C=C has 3 sigma bonds so it is sp2 and trigonal planar, even though it has a double bond.

Why is an sp carbon more electronegative than an sp3 carbon?

An sp hybrid orbital has 50% s-character, sp2 has about 33%, and sp3 has 25%. More s-character means the electrons stay closer to the nucleus. So an sp carbon holds its bonding electrons more tightly, making it more electronegative and forming shorter, stronger bonds than sp2 or sp3 carbon. Order of electronegativity: sp > sp2 > sp3.

How does hybridisation change bond length and bond strength?

More s-character pulls the orbital closer to the nucleus, so the bond gets shorter and stronger. That is why a C-H bond on an sp carbon (ethyne) is shorter and stronger than a C-H on an sp3 carbon (ethane). Order of bond length: sp3 > sp2 > sp; order of bond strength is the reverse: sp > sp2 > sp3.

⚠️ The NEET trap
✗ A carbon with a C=C double bond is counted as sp3 because it has a double bond and looks bonded to many things.
✓ A C=C carbon has 3 sigma bonds and 1 pi bond, so it is sp2 (trigonal planar, 120 degrees). Only sigma bonds and lone pairs decide hybridisation; pi bonds are ignored.
🧠 NTA loves lining up structures like CH2=CH-C#CH and asking the left-to-right hybridisation order. Count sigma bonds only, never pi.

Real NEET questions

2018

In which of the following molecules is the order of hybridisation sp2, sp2, sp, sp for the carbon atoms from left to right?

A · CH2=CH-CH=CH2
B · CH2=CH-C#CH ✓
C · CH#C-C#CH
D · CH3-CH=CH-CH3
Solution: Count sigma bonds (plus lone pairs) on each carbon. In CH2=CH-C#CH the carbons from left to right are: =CH2 has 3 sigma so sp2; =CH- has 3 sigma so sp2; the first triple-bond carbon has 2 sigma so sp; the terminal #CH carbon has 2 sigma so sp. That gives sp2, sp2, sp, sp. Option A is all sp2, C is all sp, D is sp3, sp2, sp2, sp3. Hence B.

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Frequently asked

What does tetravalence of carbon mean?

It means carbon always forms four covalent bonds. Its four hybrid orbitals each share one electron with another atom, so carbon can link to four hydrogens (as in CH4) or a mix of carbon and other atoms.

Which hybridisation gives which shape and bond angle?

sp3 gives a tetrahedral shape with 109.5 degrees (like CH4), sp2 gives trigonal planar with 120 degrees (like C2H4), and sp gives linear with 180 degrees (like C2H2).

Do lone pairs count when finding hybridisation of carbon?

Yes. The rule is sigma bonds plus lone pairs. Most carbons have no lone pairs, so you usually just count sigma bonds, but for carbanions or carbenes the lone pair must be added.

Why is this concept important for NEET?

Almost every organic chapter builds on hybridisation. NEET regularly asks the hybridisation order of carbons in a chain, shapes, bond angles, and comparing bond length or strength, so this one skill unlocks many quick marks.

Related concepts:
Shapes of Carbon Compounds (sp, sp2, sp3 Geometry)Sigma and Pi Bonds: Formation and PropertiesHow to Count Sigma and Pi Bonds in a MoleculeCatenation: Why Carbon Forms Long ChainsCoplanarity of Atoms in Organic Molecules
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)General Principles Of Organic Chemistry PYQs