Bohr's ideas were developed further to explain the chemical properties of the elements. It was realized that the electrons' orbits are grouped together into "shells", each of which has an upper limit to the number of electrons that it can accommodate. The first shell is complete when it contains two electrons, the second can hold up to eight, and successive shells hold still larger numbers. The seventh shell is not filled in any naturally occurring atom. The "last" electrons, those which are outermost or added last to the atom's structure, determine the chemical behavior of the atom.

   The inert, or noble, gases (helium, neon, argon, krypton, xenon, and radon) all have completely filled outer shells. They do not enter into chemical combinations in nature, although the three heaviest inert gases (krypton, xenon, and radon) have formed chemical compounds in the laboratory. On the other hand, the outermost shells of such elements as lithium, sodium, and potassium contain only one electron. These elements combine readily with other elements (transferring their outermost electrons to them) to form a great many chemical compounds. Correspondingly, such elements as fluorine, chlorine, and bromine lack only one electron to acquire a filled outermost shell. They too combine readily with other elements, receiving electrons from them.

   Atomic shells do not necessarily fill up with electrons in consecutive order. The electrons of the first 18 elements in the periodic table are added in a regular manner, each shell being filled to a designated limit before a new shell is started. From the 19th element onwards, the outermost shell is started before the previous shell is completely filled. A regularity is still maintained, however, as electrons fill successive shells in a repetitive back-and-forth pattern. The result is the regular repetition of chemical properties for atoms of increasing atomic weight that corresponds to the arrangement of the elements in the periodic table.

   It is still convenient to visualize the electrons as Rutherford and Bohr did originally, as moving about the nucleus of an atom much as if they were planets moving about the Sun. This view is much simpler than that held by present-day physicists, however. It is now known that it is impossible to pinpoint the precise position of an electron in the atom's space without disturbing its predicted location at some future time. This uncertainty is expressed by attributing to the atom a cloud-like form, in which the electron's position is defined in terms of the probability of finding it at some distance from the nucleus. The "probability-cloud" view of the atom has superseded the solar-system model.

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