Charles Pennington, age 10, of Atlanta, Georgia, for his question:

Are all atoms the same weight?

We know that there are more than a hundred different atoms ¬and weight is one of the ways used to classify them. In a chemical element, all the atoms are of the same kind. Gold is an element, so is silver. Iron and hydrogen also are chemical elements. All of them are listed very neatly on the Periodic Table of Chemical Elements. Each has a square of its own with shorthand notes to give its weight and other information.

The weight of an atom depends upon the number of particles it has. The lightest atomic element is hydrogen, which has only one proton and one electron. The next lightest is helium, with two protons and two electrons. Iron is much heavier because it has 26 protons, 26 electrons plus some neutrons. Atoms with more particles, such as uranium, are still heavier. Sometimes stray atoms have more particles than their kinfolk. For example, the atoms in heavy hydrogen have an extra neutron that ordinary hydrogen does not have. So some atoms are lighter than others and some overweight.