We know that electrons move around nucleus of an atom in different orbits the electrons in the last orbit is called valence electron. in certain substances especially metals (i.e. copper, aluminium etc) the valence electron are so *weakly attached to their nuclei that they can be easily removed or detached.
Such electrons are called free electron it may be noted here that all valence electron in a metal or not a metal or not in a metal or not not free electrons. it has been found that one atom of metal can provided at the most one free electron free electron. Since a small piece of matel has billions of atoms, one canexpect a very large number of free electrons in metals.
For example, 1cm^3 of copper has about 8.5 × 10^22 free electrons at room temperature.
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