MOLECULAR FORCES

A naïve application of Coulomb’s law would say that complete atoms do not interact. A complete atom has as many electrons outside as protons in the nucleus, and thus zero total charge. Thus by Coulomb’s law, which says that the electric force between two objects is proportional to the product of the charges on them, one would predict that there is no electric force between two complete atoms. Tell that to the two hydrogen atoms that bind to form hydrogen molecules, oxygen atoms that bind to form to O₂ molecules we breathe, or the hydrogen and oxygen atoms that combine to form the water molecules we drink. These are all complete atoms that have combined together through electric forces to form molecular structures.
The reason that neutral atoms attract electrically to form molecules is the fact that the negative charge in an atom is contained in the electrons which are moving about the nucleus, and their motion can be affected by the presence of other atoms. When trying to understand molecular forces, the planetary picture of an atom, with electrons in orbits like the planets moving around the sun, is not a particularly useful or accurate model. A more useful picture, which has its origin both in quantum mechanics and Newtonian mechanics is to picture the electrons as forming a cloud of negative charge surrounding the nucleus. You can imagine the electrons as moving around so fast that, as far as neighboring atoms are concerned, the electrons in an atom simply fill up a region around the nucleus with negative electric charge. When doing accurate calculations with quantum mechanics, one finds that the electron clouds have definite shapes, shapes which the chemists call orbitals.
One does not need quantum mechanics in order to get a rough understanding of the origin of molecular forces. Simple arguments about the behavior of the electron clouds gives a fairly good picture of what the chemists call covalent bonding. We will illustrate this with a discussion of the hydrogen molecule.