Sun in a Bottle - Charles Seife [18]
Atomic theory, in its most primitive form, goes back to the ancient Greeks. In the fifth century BCE, the philosopher Democritus held that all matter was created out of little indivisible particles. These particles, far too tiny to see, were considered to be uncuttable. Democritus’s idea was just one of a huge number of competing theories about the universe. Some philosophers argued that everything was made of fire; others thought that objects were made from a mixture of earth, air, fire, and water. Some argued that matter was infinitely divisible; others, like Democritus, argued that there was a limit to how finely you could slice an object. Though we now know that Democritus’s idea was closest to the truth, for millennia it had no special status.
More than two thousand years later, a steady march of experimentation and observation led scientists to the conclusion that Democritus was essentially correct: matter is made up of tiny atoms. Chemists had led the way; the work of chemists such as the Briton John Dalton, the Italian Amedeo Avogadro, and the Russian Dmitri Mendeleev began to produce a picture in which all matter consisted of a collection of invisible “elemental” particles. Water, for example, was made up of two particles of hydrogen and one of oxygen; alcohol had two of carbon, six of hydrogen, and one of oxygen.
There was only a handful of known elements, and they each had different properties. For example, the atoms of some elements, such as hydrogen, oxygen, and carbon, were very light. Other elements’ atoms, like those of mercury, lead, 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