Pentane, $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_3$, is monochlorinated under free-radical conditions. How many different monochloro structural isomers are possible?

Count distinct hydrogen environments: the two terminal $\mathrm{CH}{}_3$ groups are equivalent (C1), the two $\mathrm{CH}{}_2$ next to them are equivalent (C2), and the middle $\mathrm{CH}{}_2$ is unique (C3). So 3 products: 1-, 2- and 3-chloropentane.