To explain the structure of the atom, the Danish physicist Niels Bohr developed in 1913 a hypothesis known as the Bohr theory of the atom. He assumed that electrons are arranged in definite shells, or quantum levels, at a considerable distance from the nucleus.
The arrangement of these electrons is called the electron configuration. The number of such electrons equals the atomic number of the atom; hydrogen has a single orbital electron, helium has 2, and uranium has 92.
The electron shells are built up in a regular fashion from a first shell to a total of seven shells, each of which has an upper limit to the number of electrons that it can accommodate. The first shell is complete with two electrons, the second can hold up to eight electrons, and successive shells hold still larger numbers.

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.
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. Beginning with the 19th element, the outermost electron starts a new shell before the previous shell is completely filled. A regularity is still maintained, however, as electrons fill successive shells in a repetitious 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 convenient to visualize the electrons moving about the nucleus of an atom much as if they were planets moving about the sun. This view is much more precise than that held by contemporary 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 resolved by attributing to the atom a cloudlike form, in which the electron's position is defined in terms of the probability of finding it at some distance from the nucleus. This rather fuzzy schematic conception of the atom may be reconciled with the solar-system model by noting that in the tiny space of the atom the electron, which makes many billions of orbits around the nucleus in a single second, is everywhere at once.
The cloud view thus gives a form to the atom that is not supplied by a solar-system model