Atoms are the building blocks of matter. Everything around us is made up of atoms. The atom is more than a million times smaller than the thickness of a human hair. The smallest speck that can be seen under an ordinary microscope contains more than 10 billion atoms. Even though atoms are incredibly tiny, they are made up of even more minute particles: protons, neutrons, and electrons. These are called subatomic particles. Each element has a definite number of subatomic particles, which make up the center of the atom, called the nucleus.

In the spring of 1897 J.J. Thomson demonstrated that the beam of glowing matter in a cathode-ray tube was not made of light waves, as "the almost unanimous opinion of German physicists" held. Rather, cathode rays were negatively charged particles boiling off the negative cathode and attracted to the positive anode. These particles could be deflected by an electric field and bent into curved paths by a magnetic field. They were much lighter than hydrogen atoms and were identical "what ever the gas through which the discharge passes" if gas was introduced into the tube. Since they were lighter than the lightest known kind of matter and identical regardless of the kind of matter they were born from, it followed that they must be some basic constituent part of matter, and if they were a part, then there must be a whole. The real, physical electron implied a real, physical atom: the particulate theory of matter was therefore justified for the first time convincingly by physical experiment. They sang success at the annual Cavendish dinner. Armed with the electron, and knowing from other experiment that what was left when electrons were stripped away from an atom was much more massive remainder that was positively charged, Thomson went on in the next decade to develop a model of the atom that came to be called the "plum pudding" model.
The Thomson atom, "a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification" like raisins in a pudding, was a hybrid: particulate electrons and diffuse remainder. It served the useful purpose of demonstrating mathematically that electrons could be arranged in a stable configurations within an atom and that the mathematically stable arrangements could account for the similarities and regularities among chemical elements that the periodic table of the elements displays. It was becoming clear that the electrons were responsible for chemical affinities between elements, that chemistry was ultimately electrical. Thomson just missed discovering X rays in 1884. He was not so unlucky in legend as the Oxford physicist Frederick Smith, who found that photographic plates kept near a cathode-ray tube were liable to be fogged and merely told his assistant to move them to another place. Thomson noticed that glass tubing held
"at a distance of some feet from the discharge-tube" fluoresced just as the wall of the tube itself did when bombarded with cathode rays, but he was too intent on studying the rays themselves to purse the cause. Rontgen isolated the effect by covering his cathode-ray tube with black paper. When a nearby screen of florescent material still glowed he realized that whatever was causing the screen to glow was passing through the paper and intervening with the air. If he held his hand between the covered tube and the screen, his hand slightly reduced the glow on the screen but in the dark shadow he could see his bones. Rontgen's discovery intrigued other researchers beside J.J. Thomson and
Ernest Rutherford. The Frenchman Hernri Becquerel was a third-generation physicist who, like his father and grandfather before him, occupied the chair of physics at the Musee Historie in Pairs; like them also he was an expert on phosphorescence and fluorescence. In his case, particular of uranium. He heard a report of Rontgen's