The story of atoms begins about two thousand years ago, in ancient Greece. There, a wise man named Democritus started to think about what things were made of. Democritus reasoned that you could take a little piece of anything at all—wood, metal, even candy—and cut it in half. Then you could cut the halves in half. Then you could cut those pieces in half. And so on. But finally, Democritus said, you would get to particles so tiny that they could not be divided any more. He called these smallest particles atoms, from the Greek word atomos which means indivisible.
It was hard for people to believe that everything in the world was made of atoms. They soon forgot about Democritus and his strange ideas. Atoms were forgotten for almost two thousand years.
But during the eighteenth century, European scientists again became interested in the structure of things and began to experiment. There were some curious discoveries.
Chemists found that a substance like water could be separated into two other substances, hydrogen and oxygen. But hydrogen and oxygen couldn't be broken down into any other chemicals no matter what the chemists did. They could easily change table salt into sodium and chlorine. But no matter how hard they tried, they weren't able to break down sodium and chlorine into anything else.
So scientists decided that most of the things around us—water, salt, wood, animals—were made of combinations of substances. They called the combinations chemical compounds. They called the substances that couldn't be broken down elements.
We now know of more than one hundred elements. In the year 1800, about half that number had been discovered. And it was at about this time that an Englishman, John Dalton, came along with the first really scientific atomic theory.
Dalton said that all elements are made of atoms, and the atoms of any particular element are always the same. An atom of carbon is always like every other atom of carbon. And, Dalton went on, atoms of different elements have different weights. An atom of carbon weighs more than an atom of hydrogen. An atom of oxygen weighs more than an atom of carbon.
Dalton noticed that atoms combined in different ways according to their weights. Water is made of hydrogen and oxygen. But it takes two hydrogen atoms with one oxygen atom to make water. This smallest unit of water is called a molecule. A molecule is the smallest possible amount of any chemical compound.
Another contribution that Dalton made was the use of shorthand symbols to stand for chemical elements. Today, we use a system which grew out of Dalton's.