Sun in a Bottle by Charles Seife

and uranium, were very heavy. And these particles—these atoms—were fixed in their properties; it was impossible to transmute an atom of hydrogen, say, into an atom of lead.
    This picture explained the nature of matter extremely well. Within a century, atomic theory changed the subject of chemistry from a quasi-mystical hodgepodge of contradictory ideas into a real science. Physicists soon joined the chemists in their support of atomic theory; they began to provide evidence for the existence of tiny atomic particles. Theorists like Ludwig Boltzmann realized that you could explain the properties of gases simply by imagining matter as a collection of atoms madly bouncing around. Observers even saw the random motion of atoms indirectly: the jostling of water molecules makes a tiny pollen grain swim erratically about. (Albert Einstein helped explain this phenomenon—Brownian motion—in 1905.) Though a few stubborn holdouts absolutely refused to believe in atomic theory, 14 by the beginning of the twentieth century the scientific community was convinced. Matter was made of invisible atoms of various kinds: hydrogen atoms, oxygen atoms, carbon atoms, iron atoms, gold atoms, uranium atoms, and a few dozen others. But, as scientists were soon to find out, atoms are not quite as uncuttable as the ancient Greeks thought. Indeed, to figure out why different elements have different properties, physicists had to slice the atom into pieces.
    The first piece came off in 1898. The Cambridge physicist J. J. Thomson was studying a mysterious phenomenon known as cathode rays. He used electric and magnetic fields to deflect the rays and came to the correct conclusion that the rays were made up of negatively charged particles that had been stripped away from atoms. These very, very light particles came to be known as electrons.
    Since an atom is, on balance, neither positively nor negatively charged, the positive and negative charges in the atom must be equal and opposite; the charges in the atom have to cancel each other out. This means that for every electron in an atom, there has to be something else in the atom that carries the equivalent positive charge. About a decade after the discovery of the electron, the physicist Ernest Rutherford found out where that equal and opposite charge sits. It resides in tiny, but extremely solid, nucleus at the very center of the atom. This nucleus is quite heavy, thousands of times heavier than an electron, so the nucleus of an atom had to be made of stuff very different from electrons. Rutherford soon figured out what that positively charged stuff was: he realized that the positive charge is cloistered inside a heavy particle known as a proton.
    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