Straight chain molecules generally have higher density than their branched isomers due to a couple of reasons:
Packing Efficiency: Straight chain molecules have a linear arrangement of atoms, which allows them to pack closely together in a more efficient manner. The absence of branching leads to a more orderly and dense packing of molecules in the solid or liquid state. This close packing results in a higher overall density of the substance. In contrast, branched isomers have irregular and bulkier structures that hinder efficient packing, leading to lower density.
Increased Intermolecular Forces: Straight chain molecules often have a larger surface area available for intermolecular interactions compared to branched isomers. This increased surface area promotes stronger intermolecular forces, such as van der Waals forces (including London dispersion forces) and dipole-dipole interactions. These forces pull the molecules closer together, contributing to a higher density. In branched isomers, the branching disrupts the alignment and proximity of molecules, reducing the strength of intermolecular forces and, consequently, the density.
Overall, the linear structure of straight chain molecules allows for better packing and stronger intermolecular forces, leading to higher densities compared to the equivalent molecules with branched isomers. However, it's worth noting that there can be exceptions to this general trend, as factors such as molecular shape, molecular weight, and specific intermolecular interactions can also influence density.