The electrons in the outer most orbit of an atom experiences least attraction force. So the outermost atom can easily be detached from the parent atom. Let’s explain the details with band theory. When a number of atoms are brought together, the electrons of one atom experience forces of other atoms. This effect is most pronounced in outer most orbits. Due to this force, the energy levels, which were sharply defined in an isolated atom, are now broadened into energy bands. Due to this phenomenon generally two bands result, namely valance band and conduction band. In metals, the atoms are so tightly packed that electron of one atom experience sufficiently significant force of other closed atoms. The result, the valance band and conduction band in metals come very closer to each other and may even overlap. Consequently, by receiving very small amount of energy from external heat or electrical energy source, the electrons readily ascend to higher levels in the metal. Such electrons are known as free electrons. These free electrons are responsible for current that flows through a metal. When external electric source is connected to a piece of metal, these free electrons start flowing towards higher potential terminal of the source, causing current to flow in the metal. In metal, density of free electrons in conduction band is much higher than other materials, hence metal is referred as very good electrical conductor. In other words electrical conductivity of metal is very good.