How the model of the atom changed

Fire a bullet at a sheet of tissue paper, and it goes straight through. Nobody would expect anything else. In nineteen oh nine, two scientists in Manchester fired tiny particles at a sheet of gold foil so thin that light came through it. Now and then, one of those particles bounced straight back at them. Ernest Rutherford, who ran the laboratory, later said it was like firing a huge shell at tissue paper and having it come back and hit you. And nothing anyone believed about atoms could explain it.

This is video seven of seven in Atomic structure. It tells how anyone found out what Protons, neutrons and electrons describes, so that one is worth seeing first.

The story starts over two hundred years ago, with John Dalton. Dalton pictured atoms as tiny solid spheres that could not be broken down or changed. About a century later, J J Thomson discovered the electron, a tiny negative particle that comes out of atoms. That ended the solid sphere, because an atom with parts can be broken down. Thomson's new model was the plum pudding. The atom was a ball of positive charge with negative electrons embedded in it. Embedded means sunk into it, like fruit set into a pudding. The positive ball and the negative electrons balanced exactly, so the plum pudding atom was neutral overall, not positive. At this point in the story only one particle had been found, the electron. There are no protons and no neutrons in the plum pudding, because nobody had discovered them yet. Here is a student's description. In the plum pudding model, the electrons float around the outside of a positive ball, and the protons and neutrons are in the middle. That description, though, has three separate problems in it. What are the three mistakes in the student's description? The electrons were embedded in the ball, not floating outside it. Neither protons nor neutrons had been discovered. And the positive charge was spread through the whole ball, with nothing packed in the middle. One examiner's report on the plum pudding model says electrons "were variously described as 'floating' or 'covering' the atom rather than embedded in the atom." It records that "a large number of students lost credit by referring to protons and neutrons in the nuclear model", even though the question showed a timeline where neither had been found. So the safe habit is to check the date. Before you describe any model, ask which particles had been found by then, and use only those.

Now, the experiment from the start of this video. In Rutherford's laboratory, Hans Geiger and Ernest Marsden fired alpha particles at a thin sheet of gold. An alpha particle is small, fast and positively charged. If the plum pudding were right, what should the alpha particles do? Pass straight through, all or nearly all of them. In a plum pudding the positive charge is spread out, with nothing dense enough to stop them. That was what they expected. But what they observed was different. Most of the alpha particles went straight through. A few were pushed aside, through an angle. And a very few bounced straight back. Keep those two apart in an answer. Expected is what the plum pudding predicted. Observed is what happened, and only the observed results tell you about the atom. Next, most went straight through. What does that tell you about the atom? Most of the atom is empty space. Say it that way. Saying there is empty space in the atom is weaker, because it does not say how much. Now a harder one: why did only a very few bounce straight back? Because the thing they hit is tiny, so almost every particle misses it. To throw a fast particle back, it must be dense, holding most of the mass. And it must be positive, to push the positive alpha particles away. So Rutherford replaced the plum pudding with the nuclear model. The positive charge and almost all the mass sit in a tiny nucleus, and the electrons are outside it. How tiny is in the video How big an atom is. Keep the three observations together, in order, as this lesson's handle: most straight through, a few pushed aside, a very few straight back.

Next came Niels Bohr. He suggested that electrons orbit the nucleus in shells, at specific distances from it. His calculations agreed with what experiments showed. Later experiments showed that the positive charge of a nucleus is made of smaller particles, each carrying the same positive charge. These were named protons. About twenty years after the nucleus was found, James Chadwick showed that it also holds particles with no charge, the neutrons. That completed the atom you know from Protons, neutrons and electrons. A separate report on Bohr's part records that many students "did not mention that the electrons were orbiting the nucleus in these shells. Even fewer referred to specific distances from the nucleus. Many confused Bohr with Chadwick and stated that Bohr discovered neutrons." The fix is one name, one idea. Bohr: electrons orbiting in shells at specific distances. Chadwick: the neutron. Now, put the story in order yourself, from the earliest idea. What is the right order: nuclear model, plum pudding, Bohr's shells, solid sphere, neutron? Solid sphere, plum pudding, nuclear model, Bohr's shells, and then the neutron. The electron ended the solid sphere, and the scattering ended the plum pudding. And every change followed one rule. A model was replaced when new evidence appeared that the old model could not explain.

To end the chapter's story, one question from each part of it. What discovery ended the solid sphere model of the atom? The electron. An atom with smaller parts inside it cannot be a solid sphere that never breaks down. Next, back to Manchester: why did a very few particles bounce straight back? They hit the tiny, dense, positive nucleus almost head on. That is why a very few came back off the gold foil, like Rutherford's shell off tissue paper. Try this: a student says Rutherford's model had electrons in shells. What is wrong? Shells came later, with Bohr. Rutherford's nuclear model had the electrons outside the nucleus, with no shells at all.

If you could tell this story to someone, model by model, with the evidence that ended each one, you are comfortable with it, and the thumb is yours to press. Not there yet? Hold on to this one and tell the story aloud once from memory, because saying it is what makes it stick.

That completes our chapter on Atomic structure. Next chapter: The periodic table.

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