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In case of copper, the electrons in the conduction band are very large. An electron in the conduction band experiences almost negligible nuclear attraction. In fact , an electron in the conduction band does not belong to any particular atom. But it moves randomly throughout the solid. Due to overlapping of valence and conduction bands, a slight potential difference across conductors causes the electrons in conduction band, called the free electrons, to constitute electric current. So, copper is a good conductor of electricity.