# What is the difference between an identical and a structural isomer?

$\text{Structural isomers}$ have identical chemical formulae, but different connectivity.
The given definition is best illustrated by an example: consider the isomers possible for $\text{butane}$, whose formula is ${C}_{4} {H}_{10}$.
Two structual isomers are possible: $\text{n-butane}$, ${H}_{3} C - C {H}_{2} C {H}_{2} C {H}_{3}$, and so-called $\text{isobutane}$, ${H}_{3} C - C H \left(C {H}_{3}\right) C {H}_{3}$. Clearly, these have the the same chemical formula, ${C}_{4} {H}_{10}$, but they have different connectivity, which results in different physical properties, viz. boiling point, $- 0.5$ ""^@C, $\text{n-butane}$, versus $- 11.7$ ""^@C, $\text{isobutane}$.
As expected the longer hydrocarbyl chain of $\text{n-butane}$ allows for greater $\text{van der Waals}$ interaction between the chains, and an increased boiling point with respect to $\text{isobutane}$.