Physical Chemistry , 1st ed.

Although the nuclear atom fit the dramatic results of the experiment, there

was a major problem: According to Maxwell’s electromagnetic theory, such an

atom shouldn’t be stable. (The equations of electrodynamics summarized by

James Clerk Maxwell in the 1860s were another major advance in under-

standing nature.) Any time a charged particle is accelerated, any time it changes

its speed or direction(since acceleration is a vector quantity), it should radiate

energy. If an electron is attracted to a proton (and it was known then that op-

posite charges attract), it should accelerate toward the proton, and as it moves

it should radiate energy. Eventually all of the energy of the particles should be

radiated, they would have no energy, so the particles should collapse together

and electrically neutralize each other. But they didn’t.

If Maxwell’s theory of electromagnetism, which worked so well with macro-

scopic bodies, also worked for atoms and subatomic particles, then electrons

and protons—matter as we know it—shouldn’t even exist! They would con-

stantly be radiating energy, losing energy, and eventually collapsing together.

But these investigators did not doubt the fact that matter was stable. The cur-

rent theories of electromagnetism and classical mechanics simply did not ex-

plain the existence of atoms. Their very composition as separated charged par-

ticles flew in the face of the accepted understanding of the universe. (The

eventual discovery of the uncharged neutron, announced by Chadwick in

1932, did not figure into this problem, since the neutron is electrically neutral.)

252 CHAPTER 9 Pre-Quantum Mechanics

Observation

port to watch

falling droplets

Oil

sprayed

in

Charged metal plates

Closed chamber

Oil

droplets

X rays

Figure 9.8 A representation of the Millikan oil drop experiment, in which the exact charge on

the electron was determined. Using that information along with the charge-to-mass ratio (deter-

mined from experiments using magnets), the mass of the electron was determined to be much

smaller than that of an atom. Dalton’s atomic theory was not destroyed, just revised. An under-

standing of the behavior of the electron was the central focus of modern quantum mechanics.

Figure 9.9 (a) A schematic of Rutherford and Marsden’s experimental apparatus with platinum

foil. (b) The nuclear model of the atom, based on the experiments. Three paths of alpha particles

through the atom show how the alpha particles are influenced by a massive and heavily charged

nucleus. Although some details of the model have been modified, the general idea remains intact:

a massive nucleus with lighter electrons moving around it.

Electrons

Nucleus

Most of the

 particles

Some of the

A few of the  particles

 particles



Lead shield

Alpha particle

source

Film /ZnS

scintillation

screen

Pt foil











(a) (b)