Electrical resistance in metals is caused by the temperature of the metal or impurities in the metal. Both increases in temperature and higher levels of impurities impede the flow of electricity through the metal material. The shape and size of the particular metal also influences its level of resistance to electrical flow.
Metals are good conductors of electricity. Electrons in atoms hold atoms together in a crystal structure. When electrons move up to the surface of a material, they move freely to conduct electricity when a charge is applied to the substance. In metal, electrons move readily from the valence band, where they usually reside, to the conduction band, where they are free to move through the material. This allows electrons to move freely carrying an electrical charge. Because of this, metals are commonly used to make wires and other devices to carry electrical current.
On the other hand, metals are not perfect conductors. Impurities in metals cause electrical resistance by keeping the electrons from moving smoothly. Increases in temperature likewise slow down the flow of electrons by causing electrical resistance. Wires that are thinner or longer also provide more resistance to electricity than thicker or shorter wires. Thin wires do not have enough electrons to carry electricity smoothly, and long wires have more distance to allow interference from electrons hitting impurities.