Kimmy Riexinger, age 12, of Toronto, Ontario, Canada, for her question:

How does radium differ from uranium?

Uranium and radium are radioactive substances. Their unstable atoms are steadily breaking into smaller atoms, giving off nuclear radiation as they do so. Heavy uranium atoms start an orderly breakdown into smaller and smaller atoms. At one stage, the process produces a number of radium atoms.

Radioactive uranium is the ancestor of a number of radioactive descendants. The last of the family tree is a form of lead and this lead isotope is non-radioactive. It takes 4 1/2 billion years for a certain number of uranium atoms to become these stable atoms that end the long chain of radioactive decay. In the meantime, the uranium atoms become different isotopes and other atoms, each one smaller than its parent. The breakdowns occur at a fixed date and radium atoms are formed at one stage of the radioactive decay from uranium to lead.

The core of an atom is the tight fisted assorted particles in its nucleus. The nature of an atom depends upon the number of positively charged protons in its nucleus. A big, massive atom of uranium has 92 protons and its atomic number is 92. The assortment of other nuclear particles may vary, but if these atoms each have 92 protons, they are all isotopes of the element uranium.

In the process of radioactive decay, uranium loses its atomic particles stage by stage at a steady, orderly rate. A beta particle is a fast moving negatively charged electron particle. When one of these is lost, the uranium becomes another isotope of uranium. Alpha particles are heavier fragments, each composed of two neutrons and two protons. With the loss of two protons, the original uranium becomes another element.

Each breakdown occurs after a set period of time. Each stage releases a certain particle and a certain quota of nuclear energy, or radiation. After about half a dozen of these changes, the original uranium has lost four of its protons. It also has lost more than a dozen units of its original mass, or weight. The smaller, lighter atoms are ~ ' radium. It took almost a million years for a certain number of atoms in the original uranium to decay into an even smaller number of radium atoms.

The radium stage of breakdown, however, is still unstable. In fact, a gram of radium is three million times more radioactive than a gram of its uranium ancestor. In every supply of precious radium, it is estimated that one in 100 billion atoms break apart per second. Nothing can change the schedule or the timetable of the process of radioactivity. Radium is used to treat serious diseases and a hospital keeps a small quantity of the precious material. After 1660 years, the doctors can be certain that exactly half of their radium supply will decay. During the next 1660 years, exactly half of what remains will decay.

In a radioactive substance, a fixed number of atoms breaks apart every second. As they do so, they release atomic radiation of alpha, beta and gamma rays. The alpha rays of rather massive particles speed fast enough to pierce several sheets of paper. Beta rays of streaming electrons speed fast enough to pierce an inch of wood. Gamma rays are waves of electromagnetic energy, somewhat like Xrays. They travel at the speed of light and their piercing destruction must be shielded with sheets of lead.