The Subatomic Particles
In the 1870s, British physicist J.J. Thompson devised an experiment to investigate a long unexplained phenomena. It had been found that a wire carrying an electric current through a vacuum tube produced a strange type of radiation. There were two theories about the nature of this radiation, called 'cathode rays'. It was believed to be either a new type of wave form, or a stream of tiny particles.
Thompson's experiment depended on balancing the electric and magnetic properties of a moving charged particle. He determined that cathode rays were indeed streams of tiny, electrically charged particles (electrons) and he calculated the ratio of the electric charge to its mass. Whatever metal he used in his experiment to produce the rays, he always got the same result.He concluded that although different elements are made of different atoms, all atoms contained identical electrons.
Since atoms are electrically neutral, and all atoms contained electrons, there must obviously be some form of positive matter, to balance the negative charge of the electrons. Thompson developed a model of the atom, often called the 'muffin model', based on his experimental findings. He suggested that the atom was a lump of positive matter, like the dough of the muffin, with electrons spaced evenly throughout it, like blueberries in the muffin.
In 1909, Hans Geiger and Ernest Marsden, working under Ernest Rutherford, carried out experiments in which a beam of newly discovered 'alpha particles' was directed onto and through a thin gold foil. As expected, most of the particles went right through the foil, while a few were deflected to the sides. But, surprisingly, some were reflected from the foil on the same side that the beam hit. Since the alpha particles were positively charged, and 7000 times more massive than an electron, they would brush right by electrons unaffected. If, as Thompson believed, all the postive matter was spread evenly through the atom, the alpha particles should have gone through the foil undeflected.
Rutherford devised a new model of the atom, in which the positive matter was concentrated into a small ball in the center of the atom, called the nucleus, and surrounded by the electrons. But this left many questions unanswered. For instance, since opposite charges are attracted to each other, why didn't the electrons fall into the positively charged nucleus?
In 1932, the neutron, a neutrally charged particle with the same mass as the proton, was discovered by James Chadwick, and incorporated into the atomic model. The periodic table was revised to its modern version, with the ranking by atomic number and not weight. It was discovered that the neutron could be split into a proton, an electron, and a massless, neutrally charged particle called a neutrino.There were now three 'elementary particles'; the electron, the proton, and the neutrino, which made up all atoms. Once again, the physicists and chemists believed they had found the 'simple' explanation for the world.
Quarks: The Search for the Elementary Particle
The History of the Atom
The Periodic Table
The Subatomic Particles
The Quark Theory
Links and References