Experiments by J.J. Thomson in 1897 led to the discovery of a fundamental building block of matter one hundred years ago, the British physicist J.J. Thomson was venturing into the interior of the atom.

At the Cavendish Laboratory at Cambridge University, Thomson was experimenting with currents of electricity inside empty glass tubes. He was investigating a long-standing puzzle known as "cathode rays." His experiments prompted him to make a bold proposal: these mysterious rays are streams of particles much smaller than atoms, they are in fact minuscule pieces of atoms. He called these particles "corpuscles," and suggested that they might make up all of the matter in atoms. It was startling to imagine a particle residing inside the atom--most people thought that the atom was indivisible, the most fundamental unit of matter.

It took more experimental work by Thomson and others to sort out the confusion. He found out that the rays are made up of electrons: very small, negatively charged particles that are indeed fundamental parts of every atom.

In 1911 Ernest Rutherford who performed many experiments to explore radioactivity did an experiment in which he discovered that the atom must have a concentrated positive center charge that contains most of the atom's mass. He suggested that the nucleus contained a particle with a positive charge the proton. Atoms of different elements have different numbers of protons giving their nuclei different charges. That meant the hydrogen nucleus (it has one proton) was an elementary particle. Rutherford named it the proton, from the Greek word "protos," meaning "first."

In 1932, James Chadwick, an English physicist who had worked with Rutherford, detected neutrons and measured their mass in an invisible game of billiards. He fired the neutrons at a block of paraffin wax, which has a high concentration of hydrogen and is therefore rich in protons. Some of the neutrons collided with protons in the wax and knocked them out. Chadwick could then detect these protons and measure their energy. Using his knowledge of energy and momentum, he was able to work out the mass of the neutrons from the range of energies of the protons that they knocked out. He found that its mass was slightly more than that of a proton. Chadwick, like Rutherford, used an ingenious method to probe into what cannot be seen.