Sun in a Bottle - Charles Seife [19]
For every electron zipping around in the outer regions of the atom, a proton had to be sitting in the nucleus. Since positively charged objects attract negatively charged ones, the nucleus attracts the electrons through electrical forces, in roughly the same way that the sun attracts its planets with gravitational forces. Rutherford took this analogy fairly literally; he imagined the atom to be like a miniature solar system. At the center is a heavy, dense, positively charged nucleus. Quite a distance away, lighter, quick-moving, negatively charged electrons are in “orbit” around it.15 In between, there is empty space—lots of it.
When physicists discovered the proton and electron, they sparked a revolution in the scientific understanding of matter. Two subatomic particles suddenly explained the properties of the elements. No longer were atoms of different elements considered to be fundamentally different objects; an atom of gold need not be thought of as a different sort of creature compared with an atom of lead. Gold and lead were essentially the same kind of object: bundles of protons surrounded by bundles of electrons. Gold has properties different from those of lead—and they both have properties different from the other elements—because they have different numbers of protons in their nuclei (and, hence, different numbers of electrons). A hydrogen atom has one proton per nucleus, helium has two. Oxygen has eight; gold, forty-seven; lead, eighty-two; uranium, ninety-two. In each case, the number of protons in a nucleus—known as an atom’s atomic number—determines how the atom behaves chemically. It tells you which atoms it will react with and which it won’t; it tells you whether a collection of atoms is likely to be a gas or a metal, whether it will burn in oxygen or explode in water or refuse to react with anything at all. This theory was a tremendous success for science. The uncuttable atom had been dissected into its component parts. But one piece was still missing.
The discovery of the electron had come from Thomson’s investigations into cathode rays. Cathode rays come from a fairly simple piece of laboratory equipment: put a couple of pieces of metal in a vacuum tube, hook them to a battery, and radiation streams from one end to the other.
The concept of radiation was a new phenomenon at the turn of the twentieth century. Scientists knew little about it, but they were beginning to detect it everywhere. Marie Curie’s radium emitted a substance—particles or rays or something as yet unknown—that carried energy; something was fogging a photographic plate. That was one kind of radiation. The German scientist William Roentgen discovered another kind in 1895. When he sent electrical current through an evacuated tube, he noticed it would generate invisible rays that could make fluorescent screens glow. Like the rays coming from radioactive elements like radium and uranium, Roentgen’s x-rays could expose a photographic plate. X-ray radiation, too, carries energy. (It turned out that x-rays are beams of light so energetic that they pass right through flesh.) Then there were the mysterious rays coming from Thomson’s cathode. By the turn of the century, scientists across the world were finding all sorts of rays in strange places. The scientific world was going radiation crazy.
We now know that these “radiations” are not all the same thing. Some, like x-rays, are varieties of light. (Gamma rays, too, are light beams even more energetic, and more penetrating, than x-rays.) These high-energy light rays penetrate matter relatively easily.