Anthony Gardner, age 13, of Frankfort, Kentucky, for his question:

Which is the smallest atom?

The size and the weight of an atom are linked to its mass. In the language of science, the mass of an object is the amount of matter contained in its volume. Matter is served in particles, in atoms and molecules that are constructed from still smaller particles of matter. A cup of water is lighter than a cup of lead because the lead contains more matter. These mass problems are somewhat simpler down in the small world of atomic particles. Every proton has the same mass of weight. It is 1,836 times heavier than every electron. The mass of an atom is the sum of the mass of all its particles.

The atom with the smallest number of particles is hydrogen. The typical hydrogen atom has a nucleus of one proton plus an orbiting electron. Its size is too small to be measured with existing instruments. But we know that ordinary hydrogen has the smallest number of particles, and the smallest atomic mass. It seems reasonable to assume that an atom of ordinary hydrogen is the smallest whole atom in the universe.