n-Butane and 2-methylpropane (isobutane) share the molecular formula C4H10 but differ in how their atoms are arranged. What is this relationship called?

1. Both molecules have exactly the same atoms in the same ratio, but the atoms are connected differently -- one chain is unbranched, the other branched. 2. The text states: "The compounds n-butane and 2-methylpropane are structural isomers (the term constitutional isomers is also commonly used). Constitutional isomers have the same molecular formula but different spatial arrangements of the atoms in their molecules." 3. Geometric isomers instead share the same connectivity but differ in spatial orientation around a rigid bond (like a C=C double bond), which is not the case here. 4. So n-butane and 2-methylpropane are structural (constitutional) isomers of each other. _Source: OpenStax Chemistry Ch 20 "Organic Chemistry", p.1103 §20.1 Hydrocarbons_