Physics 2000 Science Trek The Periodic Table

Evidence for the Structure of Atoms

How do we know what the structure of atoms is like?

Our current picture of the atom wasn't created all at once; it was built up and improved step by step based on experimental evidence and some extremely clever insights.

Some ancient Greek philosophers speculated that everything might be made of little chunks they called "atoms." The name comes from a Greek word meaning "uncuttable"; atoms were supposed to be unbreakable, the smallest possible units of anything.

In later centuries, scientists more or less left the atom idea on the shelf, until the early nineteenth century, when the chemist John Dalton came out with an updated atomic theory of his own.

This is the same John Dalton I mentioned in our discussion of atomic weight. As I said then, he and others had noticed that elements in chemical reactions combine in certain definite proportions; this, Dalton guessed, had to mean that the elements were made of tiny, unbreakable chunks that always stick together in the same ways--two hydrogen chunks plus an oxygen chunk always makes water, for example.

So Dalton's guess was right.

It was, and it wasn't. The scientific world soon accepted that atoms did exist...but were they really unbreakable?

Meanwhile, a lot of apparently unrelated experiments about electricity had been going on. By the nineteenth century, scientists knew a fair amount about how electricity behaved--but what was it, exactly? Some kind of fluid? Waves? A bunch of little particles?

In 1897, J. J. Thomson answered this question. He found that cathode rays were bent in certain directions by electric and magnetic fields, and therefore, he thought, must be made up of negatively charged particles of some sort. Later, those particles were named electrons.

To find out more about Thomson and his experiments, you can take a look at the following links:

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