Sun in a Bottle - Charles Seife [21]
Once Chadwick discovered the neutron, though, the answer to the puzzle was obvious. Scientists already knew that the number of protons determined the chemical properties of an atom; hydrogen, deuterium, and tritium each had a single proton in the nucleus, so they were almost identical, chemically speaking. But neutrons can also sit in an atom’s nucleus. Because neutrons don’t have a charge (and don’t attract extra electrons), they don’t affect an atom’s chemical behavior; an extra neutron doesn’t turn hydrogen into a different element. But an extra neutron makes that hydrogen weigh more than before.
Ordinary hydrogen’s nucleus is simply one proton. It weighs as much as one proton, so it is known as hydrogen-1, or 1H. Deuterium’s nucleus, too, has one proton. But it also has a neutron that weighs roughly the same as the proton; the mass of the nucleus (hence, the mass of the atom) is doubled. Deuterium is thus known as hydrogen-2, 2H. Tritium has a single proton in its nucleus, but in addition it has two neutrons, making it three times as heavy as ordinary hydrogen. Tritium is therefore designated hydrogen-3, 3H. All these atoms are considered to be varieties, or isotopes, of hydrogen. In a chemical reaction, all three behave more or less the same way. But they have slightly different physical properties by virtue of their nuclei’s different weights.
Scientists were thrilled when they discovered the neutron because it gave them a complete model to explain an atom’s chemical behavior. Just figure out how many protons and neutrons are in a given atom and you can predict its properties extremely well.
Despite the spectacular success of atomic theory, scientists, in some sense, were astonished that atoms could exist at all. Nuclei are finicky things, and it is amazing that any of them are stable. By rights, they should fly apart instantly. They are filled with positively charged protons, and positively charged things repel one another. If the protons in a nucleus were to obey their electrical urges, they would flee each other’s presence, and the nucleus would explode in all different directions. But something forces the protons to stay put and in close proximity to one another. A very strong force—stronger than gravity, stronger than electromagnetism—glues nuclei together, trapping protons inside. In a great burst of creativity, scientists dubbed this strong force . . . the strong force. This force holds the secret to nuclear fusion.
The strong force is powerful enough to overcome the natural repulsion that protons have for other protons. However, it can do so only under a fairly narrow range of conditions. If there is the right balance of particles in the nucleus—the correct number of protons and neutrons—the strong force keeps the nucleus stable (or nearly so), preventing the nucleus from exploding. If there are too many neutrons or too few, the nucleus will be unstable. An unstable atom will destroy itself somehow, changing the balance of particles in its nucleus until the nucleus reaches a more stable state. A nucleus can break apart, spit a particle out, or swallow one to get closer to an ideal, stable balance of protons and neutrons.
For example, hydrogen (one proton) and deuterium (one proton and one neutron) are stable. Left to their own devices, they would not change at all. But add a second neutron to the mix, making tritium, and the atom has too many neutrons for comfort. It is no longer stable. Eventually, a tritium atom will, spontaneously, transmute one of its neutrons into a proton (and spit out an electron in the process). The substance left behind is no longer tritium; it has become helium-3, a stable if rare isotope of helium that has two protons