Tim Frank, age 11, o Milwaukee, Wis., for his question:

WHICH IS THE LIGHTEST ELEMENT KNOWN?

Nobody on earth knows which is the heaviest atomic element, what it is like or where it may be found. But scientists are quite sure that the lightest of all elements is hydrogen. They know this because the nucleus of the basic hydrogen atom has but one proton. And no atom can be a genuine atom without at least one proton.

All the chemical elements are made of atoms, and a complete atom is a balanced unit of positive and negative charges. These are its major atomic particles. The major positive particles are protons in the tightfisted nucleus. Normally the central nucleus is orbited by an equal number of negative electrons.

All protons are alike in weight and electric charge. The nature and weight of each atomic element is determined by its number or protons. This is its atomic number on the periodic table of chemical elements.  These numbers progress in whole digits, for each atomic element has one more proton than the next lightest and one less than the next heaviest one.

Hence the key to the lightest of all elements must be the number of protons in all its basic atoms. And since the smallest whole number is one, the lightest element must be the one with atomic number one. It is, of course, hydrogen, which is the lightest and also the simplest of all atoms.

Though the atomic numbers are whole numbers, the atomic weights are not. The weights include all the particles in the complete atoms  and opposite charges, the proton is much heavier. The atomic weight of the basic hydrogen atom is 1.0080  one proton plus a lightweight electron.

Under ordinary earth conditions, pure hydrogen is a gaseous element, about 14 times lighter than air. It can be frozen solid at minus 435 degrees Fahrenheit and at minus 423 it boils to its gaseous state. Hydrogen also combines with oxygen to form molecules of liquid water and forms certain solids by combining with other elements.

Though basic hydrogen is the lightest of all elements, we now know that a rare nucleus also may contain a neutral neutron particle. This is an isotope atom of heavy hydrogen, alias deuterium. Another isotope called tritium has two neutrons, in addition to the single proton which makes it a hydrogen atom.