All practice pages/Interatomic theory

Isomers

Same atoms, different arrangement, different substance. Compare the pairs, then try the drills.

Isomers are compounds with the same chemical formula but different structures. Every atom is the same; what changes is which atom is bonded to which, or how the atoms sit in space. That one change can move a boiling point by a hundred degrees or turn a liquid into a gas. The book sorts isomers into two kinds and expects you to name them: structural isomers, in which the molecules differ in how the atoms are connected to each other, and spatial isomers, in which the relative orientations of the atoms in space differ.

Structural isomers

A structural isomer is one of two substances that have the same molecular formula but different physical and chemical properties, because their atoms are bonded differently. Every group below shares one formula. Count the atoms in each drawing, then compare the properties.

Not every formula has isomers. Two carbons and six hydrogens go together one way only: each carbon takes three hydrogens, and the two carbons bond to each other. That molecule is ethane, C₂H₆, and there is no second compound to find. When a question asks whether the same atoms can be arranged to make something different, the answer is sometimes simply no, so try to build the second structure before you decide one exists.

Quick drill

Is there a second arrangement?

Spatial isomers

A spatial isomer is one of two compounds in which the relative orientations of the atoms in space differ. These share not just the formula but the same bonds between the same atoms, and they are still different substances.

Carvone, C₁₀H₁₄O. The book's own example of a spatial pair. The two forms have the same atoms joined in the same order and differ only as a left hand differs from a right. S-(+)-carvone smells of caraway and R-(−)-carvone smells of spearmint, so your nose can tell them apart even though nothing about the bonding has changed.

Chiral or not?

Name the kind

Both categories in one drill. Ask first whether the two are isomers at all, then whether the difference is in the bonding or only in the arrangement in space.