What are atoms

For thousands of years, men have written and guessed about the nature of tiny particles like these, but they did not have any real idea of what an atom is like. Thinking men in ancient Greece, including one named Democritus, 2,500 years ago, asked this question: What would happen if you should cut a piece of iron into smaller and still smaller pieces? You would finally get to a particle so small that you could not cut it further. They called this imaginary particle the atom, which in the Greek language meant "uncuttable." But they did not have any real idea of what an atom is like.

Then, about 1 50 years ago, an Englishman named John Dalton made a great step forward. In fact, he really started the science that has become modern chemistry. The ancient Greeks were on the right track, he said; all matter is composed of atoms, and there are different kinds of atoms, one kind for iron and another kind for gold and a different kind for each other chemical element. But all the atoms of one kind are exactly alike. All iron atoms are the same, all gold atoms are the same, and so on.

Because of this, chemists were able to find out how the weight of one atom compares with the weight of a different atom. It would be impossible to weigh a single atom, because it is too small— it takes billions of atoms to make an ounce. But we know now that hydrogen, which is a gas, has the lightest atom; that oxygen, the life-giving gas in the air, weighs about 16 times as much as hydrogen, and that iron weighs about 56 times as much. The "atomic weight" of every element is known in the sameway.