experiments in which alpha particles from a radioactive sample were shot through
very thin gold foil. Most of the time the particles passed through, but occasionally
one bounced back, indicating that the foil was mostly empty space, but that present
were particles which were small and, compared to the mass of the electron (which
was much too light to stop an alpha particle), massive. From these experiments
emerged our picture of the atom as consisting of a small, relatively massive positive
nucleus surrounded by electrons: the nuclear atom. Rutherford gave the name
proton(from Greekprotoz, primary or first) to the least massive of these nuclei
(the hydrogen nucleus).
There is another thread to the development of the concept of the atom as a
composite of subatomic particles. The enhanced effect of electrolytes (solutes that
provide electrically conducting solutions) on boiling and freezing points and on the
osmotic pressure of solutions led Arrhenius^14 in 1884 to propose that these sub-
stances exist in water as atoms or groups of atoms with an electric charge. Thus
sodium chloride in solution would not, as was generally held, exist as NaCl
molecules but rather as a positive sodium “atom” and a negative chlorine “atom”;
the presence of two particles instead of the expected one accounted for the
enhanced effects. The ability of atoms to lose or gain charge hinted at the existence
of some kind of subatomic structure, and although the theory was not warmly
received (Arrhenius was almost failed on his Ph.D. exam), the confirmation by
Thomson (ca. 1900) that the atom contains electrons made acceptable the concept
of charged atoms with chemical properties quite different from those of the neutral
ones. Arrhenius was awarded the Nobel Prize for his (albeit significantly modified)
Ph.D. work.
4.2.5 The Bohr Atom
The nuclear atom as formulated by Rutherford faced a serious problem: the
electrons orbit the nucleus like planets orbiting the Sun. An object engaged in
circular (or elliptical) motion experiences an acceleration because its direction is
changing and thus its velocity, which unlike speed is a vector, is also changing. An
electron in circular motion about a nucleus would experience an acceleration
toward the nucleus, and since from Maxwell’s equations of electromagnetism an
accelerated electric charge radiates away energy, the electron should lose energy by
spiralling in toward the nucleus, ending up there, with no kinetic and potential
energy; calculations show this should happen in a fraction of a second [ 8 ].
(^14) Svante Arrhenius, born near Uppsala, Sweden, 1859. Ph.D. University of Stockholm. Nobel
Prize in chemistry 1903. Professor Stockholm. Died Stockholm 1927.
94 4 Introduction to Quantum Mechanics in Computational Chemistry