Structural isomers

The molecular formula only counts the atoms. It does not show how they are joined. Carbon chains can be straight or branched, so the same atoms can be put together in more than one way.

Isomers are possible because of catenation. With 1, 2 or 3 carbon atoms joined by single bonds in a chain, there is only one way to join them. From 4 carbon atoms, a branch becomes possible.

A small example

Butane is C₄H₁₀. Draw the 4 carbons in a straight line: C–C–C–C. Now draw 3 carbons in a line with the 4th joined to the middle one. Fill each carbon up to 4 bonds with hydrogen. Both drawings use 4 C and 10 H. The formula is the same, the shape is not. So they are 2 structural isomers.

Where marks go

Question 21 of CBSE's 2026-27 Science sample paper shows four pairs of structures and asks which pair are isomers. The 2026-27 Science marking scheme gives the reason with the answer: the two compounds have the same molecular formula but a different structural formula.

So the working is to count the atoms in each structure first. Only a pair with equal counts can be isomers. It is 1 mark, with no part credit. See where the marks go in CBSE Science.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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