Kevin Flaherty, age 14, of Asheville, N.C., for his question:

WHAT EXACTLY ARE ISOTOPES?

Isotopes belong in the realm of invisibly small atoms. We know of 100 or so different chemical elements, and the atoms in each element are alike. In stage language, a stand in is someone who can take over the role of a regular player. In the

language of science, an isotope is a sort of stand in for a regular atom. However, it weighs more than the original, and its role may be more dramatic.

If your interest runs to atoms, the Periodic Table of Chemical Elements will fascinate you. Though it looks like a coded top secret document, it is as simple as ABC. Each square gives basic data on one of the known elements. For example, squares H, O and U belong to hydrogen, oxygen and uranium. Each has its atomic number, plus a mass number which tells the total weight of its basic atom.

This remarkable chart was started when researchers assumed that all atomic particles carried either a positive or negative electrical charge. The atomic number gives the positive protons in the atomic nucleus. Hydrogen, the smallest atom, has one. Oxygen has eight, and uranium has 92. In a normal atom, the number of positive protons is matched by its number of negative electrons.

The mass number is the total weight of all known and unknown particles inside an atom. The heavy proton is one mass unit, and the tiny electron is negligible. The hydrogen atom, with one proton and one electron, has a mass number of one plus a tiny fraction. But a quantity of hydrogen gas weighs slightly more than it should  if each nucleus has only one proton.

Solving this problem led to the discovery of both neutrons and isotopes. The neutral neutron has neither a positive nor negative charge. All atoms, except regular hydrogen, have neutrons in their nuclei. Isotopes are overweight atoms with extra neutrons. Among about 6,000 atoms of hydrogen gas there is an isotope of hydrogen two. It has a proton, an electron and a neutron.

The isotope hydrogen two is heavy hydrogen, alias deuterium. Tritium, hydrogen three, is a hydrogen isotope with two unexpected protons. We now know that all elements have isotopes. Their reactions differ from regular atoms, and many are useful in biological research. Some isotopes are radioactive. For example, uranium 235 is a radioactive isotope used as a fuel to produce nuclear energy.