# Why can the formula C_5H_12 support isomerism?

Pentane has $5$ carbon atoms in a chain. It is ${\text{C"_5"H}}_{12}$. This molecular formula is also shared by a butane molecule with a methyl side chain, which can be on either $\text{C}$ atom number $2$ or $3$ in the chain i.e. 2-methylbutane or 3-methylbutane.
You can also have a propane molecule with two methyl side chains both on $\text{C}$ atom number $2$ in the chain i.e. 2,2-dimethylpropane.