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Models of the Atom A Brief History First Thoughts For over 2500 years the model of the atom has been refined. It started with the Greek philosopher Democritus, who reasoned that you could take a piece of matter, cut it in half, cut the half piece in half again, and continue to cut again and again. Eventually, you wouldn’t be able to cut any more. You would have only one particle left, and that particle would be called an atom, a term that means “cannot be divided”. Greek philosophers such as Democritus did not try to prove their theories by doing experiments as scientists now do. Their theories were the result of reasoning, debating and discussion – not of evidence or proof. Today scientists will not accept a theory that is not supported by experimental evidence. But even if Greek philosophers had experimented, they lacked the equipment to prove the existence of atoms. Even as recently as 500 years ago, atoms were still a mystery. A Model of the Atom A long period passed before the theories about the atom developed further. Finally, during the 18th century scientists began debating about the existence of atoms once again. In simple laboratories they were putting substances together to form new substances and taking substances apart to find out what they were made of. They found that certain substances could not be broken down into simpler substances. Scientists came to realize all matter is made up of elements. An element is made of atoms of only one kind. For example, iron is an element made of iron atoms. Silver, another element, is made of silver atoms. Dalton’s concept John Dalton, an English schoolteacher in the early nineteenth century, combined the ideas of elements with the Greek theory of the atom. He proposed the following ideas about matter: 1. Matter is made up of atoms 2. Atoms cannot be divided into smaller pieces 3. All the atoms of an element are exactly alike 4. Different elements are made of different kinds of atoms Dalton pictured the atom as a hard sphere that was the same throughout, something like a tiny marble. Testing the Model Dalton’s theory of the atom was tested in the second half of the nineteenth century. In 1897 a scientist named J.J. Thomson determined that an atom was not the same throughout, but was made up of charged particles smaller than the atom, later these were called electrons. His electron discovering experiment is shown below. Basically, a battery was hooked up to two pieces of metal (called electrodes) inside a gas-filled tube. When turned on a ray of particles streaks from the cathode (negative electrode) to the anode (positive electrode). If a magnet is placed near the stream of particles it deflects them toward it (or away, depending on the magnetic pole). Perhaps the biggest surprise that came from Thomson’s experiment was that atoms were not uniformly solid throughout; inside of it were particles smaller than the atom. Thomson’s Atomic Model Because Dalton’s model did not agree with experimental evidence, it was necessary to change the model of an atom. Thomson revised Dalton’s model from a uniform solid ball to a sphere of positive charge with electrons spread evenly among the positive charge. Below is an example of Thomson’s model – the blue clay is the positive charge and the metal ball bearings represent electrons. The positive clay is equal to the negative charge of the electrons, and so overall the atom is electrically neutral. Rutherford’s Experiments A model is not accepted in the scientific community until it has been tested and the tests support previous observations. In 1906, Ernest Rutherford began an experiment to find out if Thomson’s model of the atom was correct. His experiment involved shooting a stream of particles toward the atoms in a piece of gold. If Thomson’s model was correct, most of the particles should be only slightly deflected because the matter in atoms is spread out over a sphere. But this did not happen. What did happen is that most of the time the stream of particles hit nothing and every once in a while a particle was deflected right back at the source. See the picture of Rutherford’s experiment below. Thomson’s Model Fails The uniform mix of mass in Thomson’s model of the atom did not allow for most particles to pass through unscathed and certainly not for some particles to be deflected straight back. Below are two figures – what scientists thought would happen and what actually happened. Thomson’s model – what was expected What actually happened A Model with a Nucleus The Proton From his experiment Rutherford concluded that almost all of the mass of the atom and all of the positive charge must be crammed into an incredibly small region of space at the center of the atom. He made a new model of the atom and placed protons at the nucleus and electrons, with very little mass, orbiting the nucleus. Rutherford’s model explained not only his experimental results, but also past experimental results too. The Neutron Rutherford’s nuclear model was applauded by other scientists; however some of the data did not fit. For instance, an atom’s electrons have almost no mass. This means according to Rutherford’s model, the only other particle around was the proton, and hence the mass of an atom should be approximately equal to the mass of its protons. The problem was most atoms were twice as massive, sometime more, than the protons present. From this discrepancy it was proposed that there must be another particle in the nucleus, one without an electric charge, and this particle would be called the neutron. Proving the existence of neutrons was not easy because they have no electric charge and they do not react to magnets. It took another 20 years before scientists were able to confirm that atoms contain neutrons. When this occurred in 1926 the model of the atom was yet again revised. Electron Orbits to Clouds Bohr Model With the protons and neutrons firmly tucked away in the nucleus, scientists set out to map how electrons orbit the nucleus. From experiments, it was known that electrons only emit certain types of energy. One of the first successful attempts to model electrons capable of emitting only select energies was done so by Niels Bohr. He suggested that electrons orbit the nucleus in perfect circular orbits. Bohr’s model also stated electrons could only orbit the nucleus in set pathways and they could leap from orbit to orbit, but they were not permitted in the spaces in between. Cloud Model Further experimentation in the 20th century showed that electrons really did not act as particles in simple orbits circling the nucleus. As a matter of fact, it turned out that predicting exactly where an electron is in the atom was a very difficult task – as electrons tend to spread out over an entire space, much like an ocean wave spreads out over a stretch of beach. Scientists determined that it was impossible to know the precise location of an electron at any particular moment, because electrons would often act like waves. The current model of the atom allows for the somewhat unpredictable wave nature of electrons by defining where an electron is most likely to be, but it doesn’t mean the electron is actually there. Electrons travel in a region surrounding the nucleus called the electron cloud. Questions about the reading 1. According to Democritus’ reasoning, what should eventually happen if you cut a piece of paper in half again and again? 2. Starting in the 18th century scientists in simple laboratories started breaking substances apart. What did they discover? 3. Which of Dalton’s ideas about matter are still true today? Which of Dalton’s ideas about matter are false, and why are they false? 4. Why was it such a big deal that Thomson found electrons inside the atom? 5. What particle did Rutherford discover, and how was his model of the atom different from Thomson’s model? 6. Rutherford’s model of the atom simply wasn’t massive enough. What was his model missing? 7. Describe Bohr’s model of how electrons go around the nucleus. 8. As best you can, describe the current model of the atom.