HOW COME? There's more than meets the eye to an atom

How did scientists figure out that matter is made of atoms, since they couldn't see them? asks a reader.

Even though we can't see atoms just by looking, the idea of their existence has been around for thousands of years. Scientists and philosopher Democritus, who lived in ancient Greece, thought that if we divided matter into smaller and smaller parts, we'd eventually reach the tiniest possible amount of any element -- a single atom.

While he couldn't prove his theory, Democritus turned out to be right.

But while atoms are impossibly small, they are also more than 99 percent empty space. If the Sun were the nucleus of an atom, the electrons in the atom's outermost orbits would be more than 10 billion miles away -- nearly three times the distance of dwarf planet Pluto.

Although Democritus couldn't show that atoms actually existed, the idea came up again and again over the centuries. Why? Because it helped explain what scientists were seeing in experiments.

In the early 1800s, British chemist John Dalton found that when two substances combine in a chemical reaction, their weights, as in a food recipe, come in fixed proportions. This suggested that there were identical, equal-weight atoms of one substance linking up with different weight atoms of another substance. Atoms, Dalton thought, were tiny round balls. Later experiments convinced scientists that each atom, whether of calcium, mercury, or other elements, were made of even teensier common parts.

In the early 1900s, scientist Ernest Rutherford devised experiments that showed what an atom must be like on the inside. Scientists in England shot high-speed, positively charged particles, discharged from radioactive radon, at a thin gold foil. Some of the particles whizzed right through. Others veered off to the sides. Some even ricocheted back at experimenters. Rutherford said it was like firing a heavy mortar shell at tissue paper, and having it bounce back.

Why didn't all the positively charged particles simply pass through the foil? The scientists realized that many must have been electrically repelled. Rutherford's experiment helped prove that atoms contain a nucleus made of positively charged particles -- so-called protons.

Experiments later proved that electrically neutral particles, or neutrons, are also packed into atoms' centers. Whizzing around the nucleus, at a (relatively) great distance, are negatively charged particles. These negative electrons "swarm" around the nucleus, attracted to the positive protons like bees to nectar.