We have seen that pure substances are composed of elements, i.e substances that cannot be decomposed by ordinary chemical means, as well as compounds - substances composed of two or more elements. Thus, substances can be decomposed into elements. The resulting compounds have much different properties than reactants. An example of this is sodium and chloride reacting to give salt. The formation of products is a function of energy.
Atoms combine together because the product is more stable than the reactants by themselves.
For atoms of the first three periods, the lowest energy state has 8 electrons in the outermost level of the atom. An atom with 8 electrons in the outermost level is said to have a filled shell. Because quantum mechanics requires it. Gases such as He, Ne, Ar have filled shells and are not very reactive. They do not form ions.