Electricity and magnetism/Electrostatics/The cohesion of matter is electrostatic

A tiny grain of salt File:A very little grain of salt.gif Each line represents a bond between two ions in a NaCl crystal (sodium chloride is common salt). The blue and green colors represent the two ion species. It is a centered cubic crystal. Each Na+ ion is surrounded by 8 close neighbors Cl-. Likewise, each Cl- ion is surrounded by 8 close neighbors Na+. Two nearby ions always have opposite charges, because opposite charges attract each other while charges of the same sign repel each other. The force that explains the cohesion of solids, atoms and molecules is the Coulomb force. The cohesion of liquids is also caused in part by this electrostatic force, but because the atoms or molecules are in motion, electrodynamic forces can also come into play. A hydrogen atom consists of one proton, zero, one or two neutrons, and one electron. A proton is a positive charge. An electron is a negative charge, equal but opposite. A neutron is neutral. The electron is linked to the proton by the Coulomb electric force. More generally, an atom consists of a nucleus surrounded by electrons. The charge of the nucleus is positive, equal but opposite to the total charge of the electrons. 640px When the hydrogen atom is in its ground state, the lowest energy state, the presence of the electron around a proton resembles a spherical cloud: 480px The proton is at the center. The cloud around represents the presence of the electron. If the atom is excited, the presence of the electron can have various forms: 480px 480px A molecule is an assembly of atoms. Electrons are like glue that binds nuclei together. Two positive charges repel each other, but two positive charges separated by a negative charge can attract each other. 700px An ion is an atom or molecule that has lost or gained one or more electrons. Since atoms and molecules are always electrically neutral, ions are always charged. An atom or molecule that has gained one or more electrons is a negative ion. An atom or molecule that has lost one or more electrons is a positive ion. A solid is an assembly of atoms, molecules or ions, linked to each other by electrostatic force. To break an ionic crystal, one must separate ions attracted by electric force: File:Sodium-chloride-crystal-breaking.svg The attractive forces between charges of opposite signs are greater than the repulsive forces between charges of the same sign. This difference makes all the materials cohesive. The mass of the nuclei is approximately 5000 times greater than the mass of the electrons accompanying them, because the mass of a proton or neutron is approximately 2000 times that of an electron, and an atom always has the same number of protons and electrons, and a number of neutrons approximately equal to or a little greater than the number of protons (except ordinary hydrogen, which has no neutrons). Since almost all mass is carried by nuclei (made of protons and neutrons), it is more natural to say that electrons are a glue that binds nuclei than to say that nuclei are a glue that binds electrons. Two point charges of opposite signs should attract each other until they come together. We can calculate that the energy it could provide by falling on each other in this way is infinite. But matter doesn't usually collapse on itself, and it never releases an infinite amount of energy. Bodies always have a lower energy state, which is their ground state. In this state, they cannot give up energy, because they have no lower energy state to go to. Atoms, molecules and ions are in their ground state or in excited states which have higher energy. The hotter a gas is, the more excited its atoms or molecules are. A solid is in an excited state unless its absolute temperature is zero Kelvin. To explain why an electron from a hydrogen atom does not fall on the proton, or more generally why materials do not collapse on themselves, Coulomb's law is not enough, we need quantum physics.