[AntonioLao]

How do people stick together? Is it common interests? Is it monetary parity? Is it equal level of intellect? Is it social affinity? Or is it complementarities? On the other hand, for atoms and molecules, they stick together by the following types of chemical bonds: (1) covalent (polar or nonpolar), (2) ionic, and (3) metallic bonds. All these are dependent on the electromagnetic forces of charged particles, mainly, electrons and positive nuclei ions.
Covalent bonds are bonds by the sharing of electrons. When these are shared equally, the bonding is nonpolar, but polar when the sharing is not equal. Ionic bonds are transfers of electrons from one to the other atom. The third, metallic bonds are that which hold the atoms in a solid metal or alloy where these atoms are considered ionized, that is stripped of all valence electrons. These free electrons formed negatively charged cloud surrounding the positively charged lattice of nuclei. The bonding force is electrostatic. The free flow of electrons around the lattice promotes thermal and electrical conductivities for all metals. But in order to understand how they works requires a theory of energy bands. Two pioneers in this research area were Felix Bloch and Philip W. Anderson. Both were awarded the Nobel Prize for Physics. See http://en.wikipedia.org/wiki/Philip_Warren_Anderson and http://en.wikipedia.org/wiki/Felix_Bloch. Moreover, Bloch’s theorem could partially describe the thermal and conductive properties of all metals.
Energy bands are allowed quantum states for free electrons to occupy. Since these are closely packed all electrons in the sharply defined bands are practically in the same states. For all substances, there are two distinctive bands: the conduction band and the valence band. When these are separated by a wide forbidden energy gap the substance becomes an insulator. When they overlap and partially full, the substance is a conductor where electrons are free to move around and about which is a fundamental property of metals. When the forbidden gap is narrow then the substance is classified as semi-conductor.