Carol barber, age 11, of scott8yi11e, va.,, for her question:

What is atomic weight?

The atom, of course, is too small for our eyes to see, and a single atom is far too light to be weighed on a pair of scales. But scientists needed to know the weights of the various atoms in order to compare them. So they figured round  about wavs to get the weight of the tiny atom. These atomic weights appear on the fascinating periodic table of the elements.

Each of the chemical elements has an atomic number and a mass number. The mass number is the atomic weight of an average atom. The atomic weight of the elment Oxygen is 16. This means that an average oxygen atom has 16 units of atomic weight. This weight includes the mass. Of all the particles and fragments from which the oxygen is made. You will wonder what feather light unit of weight is used to weigh the tiny atom. Perhaps it is a milligram, which is a 1000th part of a gram or about one 32nd part of an ounce. Not at all. It takes trillions of trillions of whole oxygen atoms to weigh it. Even a milligram. A single atom is so light that we have no weight unit to but this does not matter. Scientists only needed to compare the weights of the various atoms with each other. They needed a unit light enough to show that some atoms are lighter and some heavier than others. So they worked out a unit of atomic weight to meet their needs. They chose the element oxygen and gave it an atomic weight of 16. On this basis, a single oxygen atom has 16 units of atomic weight. The atoms of all the other elements weigh so many units more or some units less than an atom of oxygen. This atom contains eight proton particles, from which it gets its atomic number 8. It's atomic weight or mass number is 16.     A proton is a unit of matter like all other protons, and all the atomic numbers are whole numbers. The unit of atomic weight, however, is man made, and an atom may weigh so many whole units plus part of a unit. For this reason, most of the atomic weights are not whole numbers. Hydrogen, with atomic number one, has an atomic weight of 1.008. Copper, atomic number 29, has an atomic weight of 63.84.     The elexamts can be arranged in a list, each one heavier than the one before it.  This gives the number of protons, and the number of neutrons increases with the number of protons. The bulk of atomic weight is in the neutrons and protons. Heliuun with two protons has two neutrons. Its atomic number is two, its atomic weight is 4.003. Uranium, atomic number 92, has an atomic weight of 238.4.