Electrical conductivity in metals is a result of the movement of electrically charged particles.
The atoms of metal elements are characterized by the presence of valence electrons - electrons in the outer shell of an atom that are free to move about. It is these 'free electrons' that allow metals to conduct an electric current.
Because valence electrons are free to move they can travel through the lattice that forms the physical structure of a metal. Under an electric field, free electrons move through the metal much like billiard balls knocking against each other, passing an electric charge as they move.
The transfer of energy is strongest when there is little resistance. On a billiard table, this occurs when a ball strikes against another single ball, passing most of its energy onto the next ball. If a single ball strikes multiple other balls, each of those will carry only a fraction of the energy.
By the same token, the most effective conductors of electricity are metals that have a single valence electron that is free to move and causes a strong repelling reaction in other electrons. This is the case in the most conductive metals, such as silver and copper, who each have a single valence electron that moves with little resistance and causes a strong repelling reaction.
Semi-conductor metals (or metalloids) have a higher number of valence electrons (usually four or more) so, although they can conduct electricity, they are inefficient at the task. However, when heated or doped with other elements semiconductors like silicon and germanium can become extremely efficient conductors of electricity.
Conduction in metals must follow Ohm's Law, which states that the current is directly proportional to the electric field applied to the metal. The key variable in applying Ohm's Law is a metal's resistivity.
Resistivity is the opposite of electrical conductivity, evaluating how strongly a metal opposes the flow of electric current. This is commonly measured across the opposite faces of a one-meter cube of material and described as an ohm meter (Ω⋅m). Resistivity is often represented by the Greek letter rho (ρ).
Electrical conductivity, on the other hand, is commonly measured by siemens per metre (S⋅m−1) and represented by the Greek letter sigma (σ). One siemens is equal to the reciprocal of one ohm.
Conductivity & Resistivity in Metals
*Note: The resistivity of semiconductors (metalloids) is heavily dependent on the presence of impurities in the material.
Chart Source Data:
Eddy Current Technology Inc.
Wikipedia: Electrical Conductivity