A: The two strongest forces in nature — the electromagnetic force that makes atoms stick together to form molecules and the strong force that holds together the nucleus — battle within a tiny arena: the nucleus.
The reason a radium watch glows is occasionally the stronger of the two forces loses the battle, which causes a radium atom to decay and release energy. When the atom decays, one of the particle it ejects (an alpha particle) hits a phosphor molecule in the surrounding paint that the manufacturer used to paint the watch dial's numbers. Then the phosphor glows a faint blue-green light. The radium, however, does not glow; only the phosphor does, physicist Erik Ramberg of Fermilab says.
The strong force and the electromagnetic force always duke it out for every element. A typical battlefront is an iron atom, with its 26 like-charged protons and 26 neutrally-charged neutrons. The electromagnetic force tells each proton to push the other protons out of the nucleus: like charges repel. But the strong force says no. Touching protons stay together. The strong force can withstand enormous electrostatic forces, as long as the protons touch.
Ah, that's the rub. As we move up in atomic mass — from iron (with 26) to lead (with 82) to radium (with 88 protons) — the battle swings to favor the electromagnetic force. Indeed, all elements with atomic mass greater than 82 (lead) are radioactive*. More protons mean the protons push harder on each other to escape the nucleus, but the strong force still can hold them together, as long as they touch. Classical theory says the protons in a nucleus will always touch, so an atom can never decay. Classical theory is wrong. Quantum mechanics sorts things out.
The problem is the dual nature of things. An electron, for example, is both a particle and a wave. You would think a particle would know where it is in the world, but it's also a smeared-out wave with a vague position. Heisenberg's uncertainty principle says we can never know both the exact position of a particle and its momentum. The smaller an object, the fuzzier its position is. So there's a tiny chance that some protons don't touch and, therefore, the strong force no longer can win against the electromagnetic force. Protons thereby escape by 'tunneling' under the energy barrier erected by the strong force. The atom decays.
A radium atom (with 88 protons) decays into a radon atom (with 86 protons) and emits an alpha particle (which is helium nucleus with 2 protons) and energy. With a half-life of 1600 years, the radium atom doesn't do this often, but when it does, and there's some handy phosphor atoms around for the alpha particle to hit, the phosphor in the paint glows, and the radium-watch numbers glow in the dark.
That's the main picture but I must mention the importance of neutrons. The nucleus consists of positively charged protons and neutrally-charged neutrons. Neutrons help stabilize the nucleus, because the strong force binds a proton and a neutron together tighter than it can two protons.
Take iron, for example, and its isotopes. Isotopes of a given element have the same number of protons but differ in the number of neutrons. So, Iron 52 (ordinary iron) has 26 protons and 26 neutrons: 26 + 26 = 52. Iron 53, Iron 54 and Iron 55 all have 26 protons, but Iron 53 has 27 neutrons, Iron 54 has 28 neutrons, etc.
Moreover, Iron 52, Iron 53, Iron 54 and Iron 55 are all radioactive. But, bingo! Iron 56, with its 30 neutrons, finally has enough neutrons to allow the strong force to win. "Iron 56 has an extremely strong nucleus (born in the fiery core of supernovae)," Ramberg says. Iron 57 and Iron 58 are also stable.
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* Even bismuth, the next element 'up' in the Periodic Table of Elements, with 83 protons, "is very, very slightly radioactive, with a half-life a billion times longer than the age of the universe."
The difference between the electromagnetic and the strong force, WonderQuest
Radium watch makes a Geiger Counter go nuts --- hear it! Alan's Vintage Watches and YouTube.com
X-ray images of glowing radium watches, Alan's Vintage Watches
How everything works: making physics out of the ordinary, Louis Bloomfield
Nuclear forces, HyperPhysics
Periodic Table of Elements, Webelements, University of Sheffield
Radioactive decays, Stanford Linear Accelerator Center
Radium, chemistry with Mr. Olsen
The discovery of radium, LateralScience.co.uk
(Answered April 16, 2007)