What are structural isomers?

Structural isomers are defined as molecules that share the same molecular formula but have different connectivity between their atoms, leading to variations in their structure and consequently different properties. The essence of structural isomers lies in how atoms are arranged, which can result in differences in physical properties like boiling points, melting points, and reactivity.

For instance, the two structural isomers of butanol (1-butanol and 2-butanol) have different arrangements of the carbon atoms, leading to distinct characteristics although they share the same empirical formula (C4H10O). The diversity in bonding arrangements is what distinguishes structural isomers from other types of isomers, such as stereoisomers, which maintain the same connectivity but differ in spatial arrangement.

This understanding aligns specifically with the characteristics outlined in the correct answer, as it captures the essence of how the different bonding arrangements lead to variations in properties across structural isomers.