By 1900, it was clear that atoms are not indivisible but, instead, are constructed of charged particles. Atomic sizes were known to be $\approx {10}^{-10}\text{m,}$ but the electrons common to all atoms are much smaller and much less massive than the smallest atom. How do they “fit” into the larger atom? What is the positive charge of the atom? Where are the charges located inside the atoms?

Thomson proposed the first model of an atom. Because the electrons are very small and light compared to the whole atom, it seemed reasonable to think that the positively charged part would take up most of the space. Thomson suggested that the atom consists of a spherical “cloud” of positive charge, roughly ${10}^{-10}\text{m}$ in diameter, in which the smaller negative electrons are embedded. The positive charge exactly balances the negative, so the atom as a whole has no net charge. This model of the atom has often been called the “plum-pudding model” or the “raisin-cake model” for reasons that should be clear from FIGURE 37.15.

FIGURE 37.15 Thomson’s raisin-cake model of the atom.

Thomson was never able to make any predictions that would enable his model to be tested, and the Thomson atom did not stand the test of time. His model is of interest today primarily to remind us that our current models of the atom are by no means obvious. Science has many sidesteps and dead ends as it progresses.

One of Thomson’s students was a New Zealander named Ernest Rutherford. While Rutherford and Thomson were studying the ionizing effects of x rays, in 1896, the French physicist Antoine Henri Becquerel announced the discovery that some new form of “rays” were emitted by crystals of uranium. These rays, like x rays, could expose film, pass through objects, and ionize the air. Yet they were emitted continuously from the uranium without having to “do” anything to it. This was the discovery of radioactivity, a topic we’ll study in Chapter 42.

With x rays only a year old and cathode rays not yet completely understood, it was not known whether all these various kinds of rays were truly different or merely variations of a single type. Rutherford immediately began a study of these new rays. He quickly discovered that at least two different rays are emitted by a uranium crystal. The first, which he called alpha rays, were easily absorbed by a piece of paper. The second, beta rays, could penetrate through at least 0.1 inch of metal.

Thomson soon found that beta rays are high-speed electrons emitted by the uranium crystal. Rutherford then showed that alpha rays are positively charged particles. By 1906 he had measured their charge-to-mass ratio to be

where $m_{\text{H}}$ is the mass of a hydrogen atom. This value could indicate either a singly ionized hydrogen molecule ${{\text{H}}_2}^{+}(q=e\text{,}m=2m_{\text{H}})$ or a doubly ionized helium atom ${\text{He}}^{++}$ $(q=2e\text{,}m=4m_{\text{H}})$.

In an ingenious experiment, Rutherford sealed a sample of radium—an emitter of alpha radiation—into a glass tube. Alpha rays could not penetrate the glass, so the particles were contained within the tube. Several days later, Rutherford used electrodes in the tube to create a discharge and observed the spectrum of the emitted light. He found the characteristic wavelengths of helium. Alpha rays (or alpha particles, as we now call them) consist of doubly ionized helium atoms (bare helium nuclei) emitted at high speed $(\approx 3\times {10}^7\text{m}\text{/}\text{s})$ from the sample.