Molecules, Structure, and Sigma vs Pi Bonds

Pick up any molecule and ask one question: which atom is joined to which, and by how many bonds? Answer that, and you can count its sigma and pi bonds in seconds. That count shows up again and again in JEE. Let's earn it.

First, what a molecule even is

Here's a fact that trips people up. There is no such thing as a molecule for an ionic compound. Only covalent compounds form molecules. NaCl is a giant lattice, not a tidy little unit. So when we talk molecules, we're talking covalent — atoms sharing electrons, locked into a definite shape.

So what is a molecule made of? Atoms. And often you'll want to know how many atoms of each kind are in there.

Molecular formula vs structural formula

The molecular formula gives you the head count. Take $CH_4$ — one carbon, four hydrogens. Done. It tells you what's in the box.

But it hides one thing: the connections. Which atom is bonded to which? The molecular formula stays silent.

For that you need the structural formula. Every molecule has a structure, and the structure comes from its covalent bonds. Two molecules can even share a molecular formula and still be different compounds because the connections differ. The structure is the real story.

Sigma and pi bonds

When two atoms join, they can share one bond or several. We sort those bonds into two kinds: sigma (σ) and pi (π).

The rule is simple, and it's the whole lesson:

• The first bond between any two atoms is always the sigma bond.
• Any extra bond between those same two atoms is a pi bond.

So count the lines:

• A single bond is $1\sigma$.
• A double bond is $1\sigma + 1\pi$.
• A triple bond is $1\sigma + 2\pi$.

Two atoms only ever share one sigma bond. Everything past the first line is pi.

This connects straight to hybridisation. The highest bond on a carbon sets its hybrid state — triple gives sp, double gives sp², single-only gives sp³. If that's fuzzy, read Carbon and Its Hybridisation States. Here we stay on the bond count.

Counting in a real molecule: benzene

Benzene is the classic exam question. How many sigma and pi bonds does it have?

Walk it slowly:

• 6 C–C bonds around the ring → 6 sigma.
• 6 C–H bonds, one per carbon → 6 sigma.
• That's 12 sigma so far.
• The ring also holds three double bonds. Each double bond adds one extra line → 3 pi.

So benzene has 12 sigma and 3 pi bonds. Count the lines, never guess.

Your turn. Count the sigma and pi bonds in ethyne, $H{-}C{\equiv}C{-}H$.

Check: two C–H single bonds give 2 sigma. The triple bond gives $1\sigma + 2\pi$. Total: 3 sigma and 2 pi.

The lines to remember

• Only covalent compounds form molecules; ionic compounds form lattices.
• Molecular formula = how many atoms. Structural formula = how they connect.
• First bond between two atoms is sigma; every extra bond is pi.
• single $= 1\sigma$ · double $= 1\sigma{+}1\pi$ · triple $= 1\sigma{+}2\pi$.

Get this counting reflex solid. From here, the structure tells you shape, polarity, and reactivity — and those are where the marks live.