Kim Buttrick, age 13, of Charleston, West Virginia, for her question:

What is meant by an isotope?

In movie making, a lot of time is spent preparing the scene for the cameras. During these tedious times, a look alike stand in may substitute for the star of the show. An isotope is a sort of stand in on an infinitesimally small scale. It is an almost perfect copy of an iron atom or an atom of some other chemical element. The atom and its isotope are alike in basic features    except weight. The stand in is a trifle heavier than the star.

The most important feature of every atom is the number of protons in its nucleus. This is its atomic number. All the atoms of an element have the same number of „mss, each bearing an identical charge of positive electricity. The next element on the list has either one more proton or one less. Each element has its own atomic number, different from all the others. So if one of its atoms loses or gains a proton, it becomes a different atom that belongs to another element.

This shows the importance of the number of protons wadded together in the atomic nucleus. In a normal atom, this number is matched by an equal number of orbiting, negative electrons. When scientists began to understand the structure of the atom, they thought that it was built entirely from positive protons and negative electrons. They had ways to measure these opposite charges    but their atomic weights did not appear to be what they should be. The proton is a heavy particle and the weight of the electron is negligible. The heavy protons give the atomic mass number    and the hydrogen atom which has only one proton should have a mass number one. So it does    or just about.

However, slight imperfections drive scientists to distraction. They found that hydrogen in bulk weighs just a trifle more than it should    if every one of its atoms had only one proton. If a few atoms had an extra nucleus particle, it did not reveal either a positive or a negative charge. It must be an electrically neutral particle  ¬and so it was. At last it was found and named the neutron.

The average hydrogen atom does not have a neutron. But a few do and they are called isotopes. Heavy hydrogen, alias deuterium, has a mass number of two because its nucleus has one proton plus a neutron. Later, ordinary hydrogen was found to contain a few atoms with two protons. This heavier, mass three isotope is tritium. We now know that all atomic elements contain a few isotope atoms, and some elements have a dozen or more different isotopes. All atoms of each element have the same number of protons and the same atomic number. But their isotope stand ins have larger mass numbers because of those extra neutrons.

Separating isotopes from ordinary atoms is very difficult.. But it is worthwhile because their extra weight and other properties make them useful. Deuterium replaces ordinary hydrogen in heavy water. Other isotopes are used to trace hidden biological processes. In some cases, extra neutrons make the nuclei unstable. Some of these radioactive isotopes turned out to be valuable nuclear fuels.