Models of the Atom

One of the most important developments in quantum physics was the combination of Rutherford's model of the atom with quantum concepts. The description and exploration of this quantum model of the atom will keep us busy for the rest of this chapter.

At the turn of the 20th century the existence of the electron was well established. However, the composition of atoms was not well understood.

One model of the atom was put forward by Thomson. It was the "plum pudding" model in which it was postulated that electrons (plums) were interspersed in a positive medium (pudding) that exactly canceled the charge of the electrons (see top part of the figure on the right). At about the same time, Rutherford showed that the alpha particle scattering data of Geiger and Marsden, two junior members of his research team, could be explained by assuming that most of the mass of an atom was concentrated in the center, the so-called nucleus of the atom. Rutherford further postulated that the electrons orbited the positively charged nucleus like a miniature solar system. Both of these postulates turned out to be essentially correct.

The problem with this model was that classical electrodynamics predicted that the electrons would radiate and fall into the nucleus in 10^-10 s. The Danish physicist Niels Bohr solved this problem by theorizing that the electrons must exist in quantized orbits, and that electrons could move from orbit to orbit in quantum leaps. Bohr's postulates successful explained most of the observed properties of atoms.

Before we go on, let us first get a feeling for the atomic length scales:

The typical diameter of an atom is 10^-10 m. This unit is often also called an Angstrom, Å: 1 Å = 10^-10 m
The typical diameter of a nucleus is between 10^-15 m and 10^-14 m.
Thus the nucleus turns out to be 10,000 to 100,000 times smaller in diameter than the atom!
Despite this huge difference in length scales it turns out that almost all of the mass of the atom is concentrated in the nucleus. In normal atoms, only 1 part in about 4000 of the atomic mass is in the electrons, the rest is concentrated inside the nucleus.