Steven Elston, age 9, of Eagle, Mich., for his question:
WHAT IS AN ATOM?
Get ready for some big numbers! Each drop of water contains more than 100 billion billion atoms. These are hydrogen atoms and oxygen atoms, and they have come together to form more than 33 billion billion molecules. All matter consists of molecules made up of atoms so tiny they would need to be magnified billions of times to be seen.
The air we breathe, the water we drink, the clothes we wear, in fact everything that takes up space and has weight is called matter. Whether a solid, a liquid or a gas, the smallest piece of matter that keeps the original characteristics of the substance is called a molecule. For example, a sugar molecule is the smallest piece that is still sugar.
With special equipment, scientists can break down a molecule into still smaller parts called atoms. If we broke down a molecule of sugar, for example, we would have 12 carbon atoms, 22 hydrogen atoms and 11 oxygen atoms. If carbon, hydrogen and oxygen atoms were proportioned and arranged differently, however, we would have any number of other substances, such as alcohol or starch.
Atoms are called the building blocks of matter because all substances are composed of them. When atoms of the same kind combine, they form molecules of the chemical elements. Many of the 100 odd elements you are familiar with include calcium, carbon, hydrogen, nitrogen, oxygen and sulfur.
An atom is so small that by comparison a speck of dust would be gigantic. Placed side by side, about 25 million atoms would be needed to span the head of a pin. What's more, atoms are so light it would take billions and billions to weigh a mere ounce.
Most of an atom is empty space. But even so, scientists have identified many of its different parts. Three of the basic components are the protons, neutrons and electrons. Protons and neutrons are found in the center of the atom, while the electrons spend their time orbiting around them. The protons carry a positive electrical charge, and the electrons carry a negative charge. It is this electrical attraction that keeps an atom together.
Scientists have assigned atoms atomic numbers based on the number of protons in the nucleus. For example, hydrogen's atomic number is 1 because each hydrogen atom has one proton. Oxygen, with its 8 protons, has an atomic number of 8. An atom's place in the Periodic Table, an arrangement of all the known chemical elements, is based on its atomic number.