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ATOMS (A short history of the knowledge of the atom) Compiled by Jim Walker Originated: Sept. 1988 Latest revision: 2016 atom n. A unit of matter, the smallest unit of an element. The history of the atomic theory of matter illustrates the thinking process that goes on in the philosophers and scientists’ heads. The models they use do not provide an absolute understanding of the atom but only a way of thinking about matter that can be used to make useful predictions. Unfortunately, the atomic ideas of Democritus had no lasting effects on other Greek philosophers, including Aristotle. In fact, Aristotle dismissed the atomic idea as worthless. People considered Aristotle's opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also. For more than 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter. Not until the early 1800's did people begin again to question the structure of matter. John Dalton Democritus Not until around 460 B.C., did a Greek philosopher, Democritus, develop the idea of atoms. He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms, from the Greek atomos meaning “indivisible”. In the 1800's an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seemed to consist of elementary lumpy particles (atoms). Although he did not know about their structure, he knew that the evidence pointed to something fundamental. We call this the “billiard ball model” of the atom. Dalton was able to figure out that compounds could be made up of the same atoms in different proportions, for example, H2O and H2O2. J. J. Thomson In 1897, the English physicist J.J. Thomson discovered the electron and proposed a model for the structure of the atom. Thomson knew that electrons had a negative charge and thought that matter must have a positive charge. His model looked like raisins in an English “Plum Pudding”. He got his ideas for electrons by studying the beam in a cathode ray tube. The beam could be bent by a magnet or an electric charge which showed that the beam had a negative charge. His experiments changed the idea that the atom was a hard sphere into the idea that it had positive and negative parts. The negative parts (the electrons) were the same in all atoms, but the positive part was different from atom to atom. Ernest Rutherford Radioactivity provided a tool to explore matter, alpha particles. These particles had a positive charge and physicists thought that they consisted of the positive parts of the Thompson atom (now known as the nucleus of atoms). In 1911 Ernest Rutherford thought it would prove interesting to bombard atoms with these alpha rays, figuring that this experiment could investigate the inside of the atom (sort of like a probe). He used Radium as the source of the alpha particles and pointed them at the atoms in gold foil. Behind the foil sat a fluorescent screen on which he could observe the alpha particles impact. The results of the experiments came unexpectedly. Most of the alpha particles went smoothly through the foil. Surprisingly, an occasional alpha bent sharply from its original path, sometimes bouncing straight back from the foil! Rutherford reasoned that they must get scattered by tiny bits of positively charged matter. Most of the space around these positive centers had nothing in them. He thought that the electrons must exist somewhere within this empty space. Rutherford thought that the negative electrons surrounded the small, dense, positive nucleus. The electron can exist in only one of the orbits at any one time. We call his model the planetary model because the electrons had certain “orbits”. Bohr’s model explained the line spectrum observed when electricity is passed through a sample of hydrogen gas. The Hydrogen Spectrum Rutherford's atom The Nuclear Model Erwin Schrodinger Niels Bohr In 1912 a Danish physicist, Niels Bohr came up with a theory that said the electrons have certain allowed energy levels. Bohr essentially said, "Here are some rules that seem impossible, but they describe the way atoms operate, so let's pretend they're correct and use them." Bohr came up with two rules that agreed with experiment: RULE 1: Electrons can orbit only at certain allowed distances from the nucleus. RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. A powerful model of the atom was developed by Erwin Schrodinger in 1926. Schrodinger generated a mathematical model for the distribution of electrons in an atom known as wave functions or “orbitals”. The electron is a wave. It is difficult to imagine a physical model of electrons as waves, but we have accepted it as the “electron cloud” model of the atom. This is quantum mechanics. The Schrodinger model describes the regions in space where electrons are most likely to be found. Instead of trying to tell us where the electron is at any time, the electron cloud only tells us where it might be.