What Is an Isomer? Same Formula, Different Molecule

Here is a fact that quietly breaks a rule you learned early on. Two bottles on a shelf: one holds a gas that boils at −0.5 °C, the other a gas that boils at −11.7 °C. They are different compounds with different properties. And both have the molecular formula C₄H₁₀.

The short answer: isomers are compounds with the same molecular formula but different arrangements of atoms. Because chemical behaviour depends on how atoms are connected and positioned in space — not merely on how many of each there are — isomers are genuinely different substances with different physical and chemical properties.

What a molecular formula leaves out

A molecular formula is an inventory: C₄H₁₀ says four carbons and ten hydrogens. It says nothing about which carbon is joined to which. Once a molecule has four or more carbons, there is usually more than one legal way to wire them up.

For C₄H₁₀ there are exactly two:

• Butane — a straight chain, CH₃–CH₂–CH₂–CH₃. Boils at −0.5 °C.
• 2-methylpropane — a three-carbon chain with a methyl branch, sometimes called isobutane. Boils at −11.7 °C.

Both are alkanes, both burn to CO₂ and water, but they are not the same substance. The branched one is more compact, so it presents less surface area for neighbouring molecules to touch, dispersion forces have less to work on, and it boils lower. That is the general rule: branching lowers the boiling point.

The count grows fast. C₅H₁₂ has 3 structural isomers. C₆H₁₄ has 5. Decane's formula, C₁₀H₂₂, gives 75. By C₂₀H₄₂ there are over 366,000 possibilities. This combinatorial explosion is precisely why organic chemistry contains so many compounds — and why a systematic naming system became unavoidable.

When the functional group changes too

Isomers don't have to belong to the same family. C₂H₆O has two isomers, and they are not remotely alike:

	Ethanol	Dimethyl ether
Structure	CH₃–CH₂–OH	CH₃–O–CH₃
Functional group	Alcohol	Ether
Boiling point	78 °C	−24 °C
Solubility in water	Fully miscible	Soluble (about 71 g/L at 20 °C)
State at room temp	Liquid	Gas

Same nine atoms. One is the alcohol in wine; the other is a gas used as an aerosol propellant and refrigerant. The gap of more than 100 °C in boiling point comes down to one thing: ethanol's hydrogen is attached to oxygen, so ethanol molecules hydrogen-bond to each other. Dimethyl ether's oxygen has no hydrogen of its own to donate, so it can't bond to its neighbours the same way.

The two big branches

Every isomer falls into one of two categories, and the distinction is about connectivity:

• Structural isomers (also called constitutional isomers) have their atoms joined together in a different order. Butane and 2-methylpropane are structural isomers, as are ethanol and dimethyl ether.
• Stereoisomers have exactly the same connectivity but a different arrangement in three-dimensional space. Cis- and trans-but-2-ene have identical bonding — the difference is which side of the double bond each methyl group sits on.

A quick test: write out which atom is bonded to which for both molecules. If those lists differ, they're structural isomers. If the lists are identical and the molecules still aren't the same, they're stereoisomers. We unpack both branches in the next post.

Three things isomers are not

This is where marks get lost, so be precise.

• Isotopes are atoms of one element with different neutron counts — carbon-12 and carbon-13. That's about the nucleus of a single atom, not the arrangement of a molecule.
• Allotropes are different structural forms of a single element: diamond, graphite and graphene are all pure carbon. Isomers are different compounds with the same formula.
• Resonance structures are multiple drawings of one molecule whose real structure is a blend of them. Benzene's two Kekulé drawings are resonance forms of a single compound; they are not two different molecules you could ever separate. Isomers, by contrast, can be isolated, bottled and sold separately.

Worked examples

1. How many structural isomers does C₃H₈ have? One. Three carbons in a row is the only option — a branch would need a fourth carbon.

2. Are CH₃CH₂CH₂OH and CH₃CH(OH)CH₃ isomers? Yes, both are C₃H₈O. Propan-1-ol has the –OH on an end carbon, propan-2-ol on the middle one. This is position isomerism.

3. Are CH₃COOH and HCOOCH₃ isomers? Yes, both C₂H₄O₂. One is ethanoic acid (vinegar), the other methyl methanoate (an ester). Same formula, different functional group entirely.

4. Are C₂H₄ and C₄H₈ isomers? No. They have different molecular formulas, so the question doesn't arise. C₄H₈ is simply twice as many atoms.

Common mistakes to avoid

• Drawing the same molecule twice. CH₃–CH₂–CH₂–CH₃ written left to right and right to left is one compound, not two. Bending a chain on paper doesn't create an isomer either — single bonds rotate freely, so a zigzag and a straight line are the same molecule.
• Confusing isomers with isotopes. The words look alike and mean entirely different things. Isotopes: same element, different neutrons. Isomers: same formula, different structure.
• Assuming isomers behave alike because the formula matches. Ethanol and dimethyl ether should end that assumption for good.

FAQ

What is the difference between an isomer and an isotope?
An isotope is a version of an element with a different number of neutrons in its nucleus. An isomer is a compound with the same molecular formula as another but a different structural arrangement. One is about atoms, the other about molecules.

Do isomers have the same molar mass?
Yes. Identical formula means identical atom count, so identical molar mass. Butane and 2-methylpropane are both 58.12 g/mol — which is why mass alone can never distinguish them.

Why do isomers have different boiling points?
Because shape controls how much surface molecules present to each other. Branched molecules are more compact, so they make less contact, their dispersion forces are weaker, and they boil lower than their straight-chain isomers.

How many isomers does C₆H₁₄ have?
Five: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane.

The takeaway

An isomer is what you get when the same collection of atoms is put together a different way. Molecular formula tells you the parts list; the structure tells you what it actually is. Because reactions happen at particular bonds in particular places, two isomers can differ in boiling point, solubility, smell, reactivity and biological effect — which is why organic chemists draw structures, not formulas.

Come from [What Is a Hydrocarbon?] and compare with What Is an Isotope? Atoms, Mass, and Examples and Empirical vs Molecular Formula: The Difference. Next in this series: [Structural vs Stereoisomers] and [What Is IUPAC Nomenclature?].

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