Deeper Into the Atom

The discovery of the atom's true nature began in 1897 when the British physicist Joseph John Thompson discovered that atoms contain lightweight, negatively charged particles. Thompson theorized that these particles—electrons—were embedded in a positively charged sphere containing most of an atom's mass (quantity of matter).

In 1911, the work of another British physicist, Ernest Rutherford, led to a better view of the atom. Rutherford fired alpha particles (helium atoms stripped of their electrons) at thin sheets of gold foil and found that most of the particles passed through the foil while some were deflected. From this discovery, Rutherford concluded that the massive positive portion of an atom is concentrated in a very small central nucleus. The electrons, he said, circle the nucleus at enormous speed, leaving a great deal of empty space around the nucleus.

Physicists continued to develop their view of the atom. In 1913, the Danish theorist Niels Bohr theorized that an atom's electrons are confined to certain shells, or orbits, around the nucleus, an idea that was later confirmed and refined. The following year, Rutherford proposed that the proton, a massive, positively charged particle that had been discovered in 1902, must be a part of the nucleus. And in 1932, another British researcher, James Chadwick, found that the nucleus also contains other particles with a mass close to that of the proton but with no electric charge. These neutral particles were dubbed neutrons.

For a while the subatomic world looked simple enough, with just three basic parts—electrons, protons, and neutrons—required to make an atom. But as physicists soon learned, they had only begun to understand how matter is constructed. Further experiments with machines called particle accelerators, which slam beams of particles together or into stationary nuclei, showed that atoms were quite complex.