In 1911, as understanding of the nature of atoms was growing rapidly, Ernest Rutherford performed an experiment that cleared up some important misconceptions. He showed that
· The positive charge in atoms is concentrated in a small region, the nucleus, and
· The nucleus is very-very small compared to the overall size of the atom.
Rutherford's experiment is sketched below. ↓
He was able to make a reasonably-focused beam of positively-charged alpha (α) particles (helium nuclei containing two protons and two neutrons, the product of certain kinds of radioactive decay), and to shoot the beam at a thin foil of gold. He set up detectors behind and all around the foil sheet.
Because gold atoms are large and the foil was thin, he could be sure that his beam would pass through (if it did) just a few gold atoms.
What he found was stunning. Most of the α-particles passed straight through the foil as though there was nothing there at all. A few passed through with some deflection, and only a few bounced off the foil, as we would expect particles to bounce off a solid material.
From the ratio of the number of particles that bounced back (because they hit the heavy gold nucleus) to the total number emitted by the alpha particle source, Rutherford was able to deduce that the nucleus must be extremely small.
We now know that the typical nucleus is on the order of 1 femtometer (1 fm = 10-15 m) in diameter, while the diameter of, say, an oxygen atom is about 1.2 angstroms (1 Å = 10-10 m).
That means there are roughly five orders of magnitude of space between the nucleus and the nearest electron.
Think about that for a minute. The diameter of Earth is 6371 Km and the mean distance to the Moon is 384,400 Km. That's about 60 times the diameter of Earth. If the Moon were an electron and the Earth its nucleus, it would have to be about 1000 times farther away to be an accurate model of a hydrogen atom.
In that example, all of the major planets of our solar system would fit between Earth and our moon. If you were to enter an atom in an electron-sized ship, you would have to travel a very great distance before the nucleus became visible.
Now that's an amazing thing considering most matter we know of is made of atoms. It means that most of everything – even concrete or steel – is empty space.
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