Electrons in Shells

Bohr introduced the idea, and others refined it, that the electron orbits lie in shells, each of which has a certain maximum capacity. The innermost shell, where electrons have the smallest possible orbits, can hold only two electrons. In the next shell, with somewhat larger orbits, up to eight electrons can be accommodated. The third will hold 18, the fourth 32. These numbers were derived from a study of the different possible shapes of the orbits, circular and elliptical, according to Bohr’s “quantum” theory.

The extent to which these shells are filled depends on the number of electrons in any given atom, that is, its atomic number. Thus, in helium, with two electrons, the innermost shell is filled. The elements from lithium to neon, Nos. 3 to 10, have successively one to eight electrons in the second shell. The next elements, sodium, with 11 electrons, has a single electron in the third shell, and so on.

The electrons in the outer shell control the atom’s interaction with other atoms; so the chemical behavior of an element depends on how many electrons occupy the outer shell. Now we can see why lithium and sodium are in the same family. They each have a single electron in the outer shell. This is also true of the other alkali metals, potassium, rubidium and cesium. In the halogen family, fluorine, chlorine, bromine and iodine each have seven electrons in the outer shell.

It turns out that in each of the inert gases—neon, argon, krypton and xenon—there are eight electrons in the outer shell. Eight electrons form a very stable arrangement. We might say that such atoms are well satisfied with themselves, and smugly resist all offers to give or take electrons. By contrast, the loose electron in sodium or potassium is easily lost. Such metals react vigorously with almost any substance, even air or water. At the other end of a period, fluorine or chlorine will try to take an electron from another element, to attain the stable number of eight. So these elements, too, are chemically active, but for the opposite reason.

The activity of sodium metal makes it quite dangerous to handle, and elementary chlorine gas is very poisonous. But move a single electron from sodium to chlorine and see what a difference it makes. Chlorine now has its deficiency satisfied, with a full shell of eight like the inert gas argon. And sodium has left a similar shell of eight, like neon. So in the compound sodium chloride (common table salt) both elements are quite innocuous, even safe to eat.

Further pioneering research opened up the atom for a more detailed look at what it is like inside. First, J. J. Thomson showed that negatively charged electrons could be separated from atoms of all kinds. Ernest Rutherford showed that the positive charge of the atom was concentrated in a very small volume, called the nucleus. Niels Bohr conceived of an atom as being like the solar system, with numerous electrons in various orbits around the nucleus at the center. The positive charges came in multiples of a unit charge. The hydrogen atom had just one unit of charge; it was called a proton. Different elements had atoms each with a certain number of protons, and the protons in the nucleus were neutralized by an equal number of electrons in orbit.

A remarkable discovery by Henry Moseley made it possible to tell just how many protons and electrons are in each kind of atom. He measured the energy of X rays emitted by various elements when the innermost electrons are disturbed. Moseley found that this energy increases in a mathematically regular fashion from one element to the next in the order of Mendeleyev’s table. Where there was a gap, the energy jump was twice as much. He proposed putting a serial number on each element, starting with hydrogen as number 1, helium as number 2, and so on. This atomic number is the number of protons in the nucleus, as well as the number of electrons, in each kind of atom.