Any crystal is composed of a positively charged nucleus surrounded by negatively charged electrons. According to Bohr’s and Pauli’s principle, a certain number of electrons orbit the nucleus in some specific orbitals. So, the inner electrons are held more tightly to the nucleus because they possess unlike charges, and only the outermost electrons, called the valence electrons, are set free to move when the crystal is a part of any solid. In this condition, an atom can have positively charged ion cores and negatively charged valence electrons. When these atoms are closed together to form a solid, the orbitals of different atoms overlap. The overlapping is so extensive that the argument of every valence electron being associated with its atom does not hold, and thus, in a solid, these electrons move freely and are termed free electrons.