Robbie Butler, age 10, of Princeville, Illinois, for his question:

Have they figured out the size of an atom?

The assorted atoms come in various sizes but all of them are too small for the human eye to see. No ordinary microscope is powerful enough to enlarge them to be counted one by one. Electron microscopes that magnify two million times reveal separate molecules as shadowy pinheads    and there is more than one little atom in each little molecule.
Our elever scientists have figured out several ways to measure the sizes of the bitsy atoms. For example, suppose you have a pile of square bricks, each weighing one pound. You want to figure their sizes without measuring all their sides. Stack them neatly into a square container, then weigh the box and its contents. Subtract the weight of the box, then each pound of the total weight equals one brick    so you know the number of your bricks. Now measure the sides of the box and figure out its exact size. Compare this overall size with your number of bricks and you have figured out the size of each brick.
Of course this is a round about way to measure the size of a one pound square brick. It would be simpler to figure its size by measuring its sides. However, atoms are too small for this simple treatment    but this did not stop our clever scientists from solving the problem. They figured out several indirect ways to measure them. For example, they know that a drop of water is cram full of molecules made of hydrogen and oxygen atoms. They know the weight of each molecule and the weight of the drop. So they can figure how many molecules it takes to fill one drop of water. And each molecule has three atoms. The astonishing number of atoms in a certain sized drop of water can be written as figure five followed by 21 zeros.
Before they could figure the sizes of the atoms, scientists had to find.and compare their weights, one with another. This problem can be solved with gases. A pint bottle holds the same number of hydrogen atoms, or oxygen atoms or molecules of carbon dioxide. This reliable number makes it possible to compare the weights of the different atoms. When you know the weights of the basic atoms, you can estimate their individual sizes by estimating them in bulk. However, this is very tricky work that calls for delicate instruments and precise conditions of pressure and temperature.   
A sparkling white grain of ordinary table salt is made of many tiny cube shaped crystals. Each crystal is a neatly interlocked lattice of molecules    and each molecule contains one atom of sodium and one of chlorine. There are about a million atoms in each neat row, a million rows in each neat layer and a million layers in the neat crystal cube. Each atom, then, is about a million million million times smaller than a salt crystal. The number of atoms in the crystals in a white grain of salt is about 10 plus a tail of 24 zeros. Several different methods are used to cross check the measurements and scientists agree that a line up of 100 million average sized atoms measures about one inch.
We can never hope to see the infinitesimally small atom. But maybe we can use a scale of measurements to imagine it. You know the size of a football. It is about 1,000 times bigger than a grain of salt. The grain of salt is about 1,000 times bigger than a tiny bacterium, which is too small for your eyes to see. But that tiny bacterium is about 1,000 times wider than the infinitesimal atom.