Why do different materials have different electrical resistivities?

Different materials exhibit varying electrical resistivities primarily due to their atomic bonding and the resulting availability of free electrons to carry electric current.12

In materials held together by metallic bonds, electrons are not bound to specific atoms and can move freely when an electrical force is applied, resulting in low resistivity and high conductivity. However, even in these conductors, resistance occurs because moving electrons collide with ions, transferring some energy as heat. In contrast, materials like plastics and ceramics act as insulators because their electrons are locked into ionic or covalent bonds, leaving no free charges available to facilitate current flow.2

Semiconductors possess intermediate resistivity values between those of conductors and insulators. Furthermore, resistivity is a fundamental property that is influenced by temperature; for instance, the resistivity of metallic conductors typically increases with temperature, whereas the resistivity of semiconductors generally decreases.1