In 1897 British physicist Joseph John (J. J.) Thomson (1856–1940) discovered the electron in a series of experiments using a cathode ray tube at Cambridge University. He said that there were "bodies much smaller than atoms" that had a negative charge. He called the negative particles electrons. Thomson took the model of the atom one step further with his plum-pudding model. While you might not be familiar with plum-pudding, you are probably very familiar with a chocolate cookie. Thomson theorized that if there were negative particles in the atom, there must also be a positive charge to make an atom neutral. His model described the atom as being basically positive (the cookie) with areas of negative that can be broken off (the chips).
One of Thomson's students, Ernest Rutherford built on his research and further developed our modern concept of the atom. One of his first discoveries was that radiation was not a single substance but three. While he was at McGill University in Canada, he found that radiation could be influenced by an electrical charge. The radiation that was attracted to a negative charged, he called alpha (positive charge). The radiation that was attracted to a positive charge he called beta (negative charge). Later gamma radiation was found to be unaffected by any charge (neutral).
After receiving the Nobel Prize, Rutherford moved to Manchester and attracted many young scientists to his research group. Under his direction, they continued to study the atoms and radiation. The Gold Foil Experiment was to become one of the most famous. In this experiment, alpha particles (+) were fired at a very thin sheet of gold foil. It was expected that the particles would go straight through the foil like bowling balls through tissue paper. Imagine their surprise when some of the particles seemed to be deflected, and even bounced back off. This observation led to the conclusion that the atom is mostly empty space, with a small, hard positively charged center. Rutherford called this the nucleus. This group would go on to discover the proton in 1917.
Later Rutherford and a fellow Danish physicist, Neils Bohr, developed a new model of the atom. They were bothered by the concept that if electrons were small and negative, and the nucleus was dense and positive, why don't the electrons simply fall into the nucleus? They developed a model that described the atom as having a small positive nucleus with the electrons orbiting the nucleus in energy levels. In this model, the electrons are held in the atom by the electrostatic attraction between the positive nucleus and negative electrons, but are constantly being pushed outward by their angular momentum. This is the familiar planetary model that is commonly taught to young students.
During this same time, Max Planck was studying electromagnetic radiation in Berlin. Among his many discoveries, he found that EMR is quantized. That it is carried by small, discrete packets that he called photons. These small quantities cannot be divided, they are the smallest amount of energy. This can be very difficult for us to conceptualize. We live in a world where we can divide amounts easily into smaller amounts. You don't have to use an entire gallon of gas at a time, you can just use a little, then a little more. But when you start to look at the world on the subatomic level, you reach a point where it is all or nothing. Think of money. We normally think in terms of dollars. That's the level we are comfortable with, that we use everyday. Dollars can be broken down into quarters, dimes, nickels and even pennies. We don't think of paying for everything in pennies because they are very small in relation to what we are buying- but imagine buying something very small. Can you pay for something worth a portion of a penny? No, you either use a whole penny or not. Photons are the pennies of the electromagnetic spectrum.
Bohr and Rutherford incorporated this concept into their model. Now an electron could not move from one level to another, it had to jump or make a quantum leap. When an electron absorbs a photon, it will jump to a higher energy level, when an electron emits a photon, it will fall to a lower energy level. Again, this is difficult for us to imagine. We think in terms of something orbiting, but in reality electrons don't orbit, they jump.
Werner Heisenberg, Max Born and Wolfgang Pauli took these ideas and formed the quantum mechanical model of the atom.
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