bei48482_FM

Classical physics fails to provide a meaningful analysis of atomic structure because

it approaches nature in terms of “pure” particles and “pure” waves. In reality particles

and waves have many properties in common, though the smallness of Planck’s con-

stant makes the wave-particle duality imperceptible in the macroworld. The usefulness

of classical physics decreases as the scale of the phenomena under study decreases, and

we must allow for the particle behavior of waves and the wave behavior of particles to

understand the atom. In the rest of this chapter we shall see how the Bohr atomic

model, which combines classical and modern notions, accomplishes part of the latter

task. Not until we consider the atom from the point of view of quantum mechanics,

which makes no compromise with the intuitive notions we pick up in our daily lives,

will we find a really successful theory of the atom.

4.3 ATOMIC SPECTRA

Each element has a characteristic line spectrum

Atomic stability is not the only thing that a successful theory of the atom must account

for. The existence of spectral lines is another important aspect of the atom that finds

no explanation in classical physics.

We saw in Chap. 2 that condensed matter (solids and liquids) at all temperatures

emits em radiation in which all wavelengths are present, though with different

intensities. The observed features of this radiation were explained by Planck without

reference to exactly how it was produced by the radiating material or to the nature of

the material. From this it follows that we are witnessing the collective behavior of a

great many interacting atoms rather than the characteristic behavior of the atoms of a

particular element.

At the other extreme, the atoms or molecules in a rarefied gas are so far apart on

the average that they only interact during occasional collisions. Under these circum-

stances we would expect any emitted radiation to be characteristic of the particular

atoms or molecules present, which turns out to be the case.

When an atomic gas or vapor at somewhat less than atmospheric pressure is suitably

“excited,” usually by passing an electric current through it, the emitted radiation has a

spectrum which contains certain specific wavelengths only. An idealized arrangement for

observing such atomic spectra is shown in Fig. 4.7; actual spectrometers use diffraction

Atomic Structure 127

Figure 4.7An idealized spectrometer.

Rarefied gas or vapor

excited by electric

discharge

Slit

Prism

Screen

bei48482_ch04.qxd 1/14/02 12:20 AM Page 127