Thom is shooting baskets. He’s trying to hit the backboard so the ball will bounce off it and into the basket. If only the backboard was bigger! It would be a lot easier to hit. If the ball misses the backboard, it will just keep going and Thom will have to run after it. Believe it or not, the research that led to the discovery of the nucleus of the atom was a little like shooting baskets.
Narrowing Down the Nucleus
In 1804, almost a century before the nucleus was discovered, the English scientist John Dalton provided evidence for the existence of the atom. Dalton thought that atoms were the smallest particles of matter, which couldn't be divided into smaller particles. He modeled atoms with solid wooden balls. In 1897, another English scientist, named J. J. Thomson, discovered the electron. It was first subatomic particle to be identified. Because atoms are neutral in electric charge, Thomson assumed that atoms must also contain areas of positive charge to cancel out the negatively charged electrons. He thought that an atom was like a plum pudding, consisting mostly of positively charged matter with negative electrons scattered through it.
The nucleus of the atom was discovered next. It was discovered in 1911 by a scientist from New Zealand named Ernest Rutherford, who is pictured in Figure below. Through his clever research, Rutherford showed that the positive charge of an atom is confined to a tiny massive region at the center of the atom, rather than being spread evenly throughout the “pudding” of the atom as Thomson had suggested. You can watch a video about Rutherford and his discovery at this URL:
Go for the Gold!
The way Rutherford discovered the atomic nucleus is a good example of the role of creativity in science. His quest actually began in 1899 when he discovered that some elements give off positively charged particles that can penetrate just about anything. He called these particles alpha (α) particles (we now know they were helium nuclei). Like all good scientists, Rutherford was curious. He wondered how he could use alpha particles to learn about the structure of the atom. He decided to aim a beam of alpha particles at a sheet of very thin gold foil. He chose gold because it can be pounded into sheets that are only 0.00004 cm thick. Surrounding the sheet of gold foil, he placed a screen that glowed when alpha particles struck it. It would be used to detect the alpha particles after they passed through the foil. A small slit in the screen allowed the beam of alpha particles to reach the foil from the particle emitter. You can see the setup for Rutherford’s experiment in the Figure below.
Q: What would you expect to happen when the alpha particles strike the gold foil?
A: The alpha particles would penetrate the gold foil. Alpha particles are positive, so they might be repelled by any areas of positive charge inside the gold atoms.
Assuming a plum pudding model of the atom, Rutherford predicted that the areas of positive charge in the gold atoms would deflect, or bend, the path of all the alpha particles as they passed through. You can see what really happened in the Figure above. Most of the alpha particles passed straight through the gold foil as though it wasn’t there. The particles seemed to be passing through empty space. Only a few of the alpha particles were deflected from their straight path, as Rutherford had predicted. Surprisingly, a tiny percentage of the particles bounced back from the foil like a basketball bouncing off a backboard! You can see an animation of this experiment at the following URL: http://www.clickandlearn.org/gr9_sci/atoms/modelsoftheatom.html.
Q: What can you infer from these observations?
A: You can infer that most of the alpha particles were not repelled by any positive charge, whereas a few were repelled by a strong positive charge.
The Nucleus Takes Center Stage
Rutherford made the same inferences. He concluded that all of the positive charge and virtually all of the mass of an atom are concentrated in one tiny area and the rest of the atom is mostly empty space. Rutherford called the area of concentrated positive charge the nucleus. He predicted—and soon discovered—that the nucleus contains positively charged particles, which he named protons. Rutherford also predicted the existence of neutral nuclear particles called neutrons, but he failed to find them. However, his student James Chadwick discovered them several years later. You can learn how at this URL: http://www.light-science.com/chadwick.html.
The Planetary Model
Rutherford’s discoveries meant that Thomson’s plum pudding model was incorrect. Positive charge is not spread evenly throughout an atom. Instead, it is all concentrated in the tiny nucleus. The rest of the atom is empty space except for the electrons scattered through it. In Rutherford’s model of the atom, which is shown in the Figure below, the electrons move around the massive nucleus like planets orbiting the sun. That’s why his model is called the planetary model. Rutherford didn’t know exactly where or how electrons orbit the nucleus. That research would be undertaken by later scientists, beginning with Niels Bohr in 1913. New and improved atomic models would also be developed. Nonetheless, Rutherford’s model is still often used to represent the atom. You can see an animated version of the model at this URL: http://www.clickandlearn.org/gr9_sci/atoms/modelsoftheatom.html.
- Ernest Rutherford discovered the nucleus of the atom in 1910. He sent a beam of alpha particles toward gold foil and observed the way the particles were deflected by the gold atoms. From his results, he concluded that all of the positive charge and virtually all of the mass of an atom are concentrated in one tiny area, called the nucleus, and the rest of the atom is mostly empty space.
- In Rutherford’s planetary model of the atom, the electrons move through empty space around the tiny positive nucleus like planets orbiting the sun.
Watch this video about Rutherford’s gold foil experiment, and then answer the questions below.
- How did Rutherford observe alpha particles in his experiment? In the modern version of Rutherford’s experiment, which is shown in the video, how are alpha particles observed? Which way do you think is more accurate?
- Based on the animation in the video, draw a sketch showing what happens to alpha particles as they pass through gold atoms.
- How has Rutherford’s gold foil experiment been adopted by modern researchers?
- How did Ernest Rutherford discover the nucleus of the atom?
- Place Rutherford’s discovery in the broader history of the atom. (Hint: See the timeline at the following URL for more information: http://www.clickandlearn.org/gr9_sci/atoms/modelsoftheatom.html.)
- Describe how you could make a three-dimensional version of Rutherford’s planetary model of the atom.