Nuclear Twins: The Discovery of the Proton and Neutron


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The Atomic Number Puzzle

Frederick Soddy was puzzled. Radioactive decay changed atoms of one element into atoms of another. Where, Soddy wondered, did these elements fit into the periodic table?

Beginning with John Dalton in the early 1800's, chemists had listed the elements in order of their atomic masses. The atomic mass is the mass of a single atom of the element. It is measured in units in which an atom of carbon weighs exactly 12. On this scale, the atomic mass of hydrogen is approximately 1, that of helium is 4, and so on.

In 1869, Dmitri Mendeleyev noticed regularites in the chemical properties of the elements. He arranged the list of elements into the periodic table of the elements. The elements in each row increase in mass from left to right. The elements in each column are similar to each other. Each element has an atomic number, as well as an atomic mass. Hydrogen has atomic number 1, helium atomic number 2, and so on.

Soddy asked his colleague Alexander Fleck to analyze the decay products of uranium and thorium. He compared Fleck's results to the periodic table. He noticed that when an element emitted an alpha particle, it produced an element two places to the left. An element that emitted electrons produced an element one place to the right. For example, polonium-218, that is, polonium with atomic mass 218, emitted an alpha particle and became lead-214. Lead-214 emitted an electron to become bismuth-214. Bismuth-214 emitted an electron to become polonium-214.

If this seems confusing, imagine how it must have seemed to people observing it for the first time! It took radiochemists years to figure out what was happening in radioactive decay.

When a radioactive element emitted one alpha particle and two electrons, it ended up in the same place in the periodic table, but it was lighter by four atomic mass units. An element could have as many as four different atomic masses. Soddy called these different forms of the same element "isotopes," which means "same place," because they occupied the same place in the periodic table. All isotopes of an element have the same atomic number.

What does its atomic number tell us about an atom?

The Dutch physicist Antonius van den Broek thought that the atomic number was the number of units of electric charge on the nucleus. A unit of charge is the electric charge of a hydrogen ion. An electron has a charge of -1. An alpha particle has charge +2.

According to Van den Broek, the hydrogen nucleus had charge +1. The nucleus of gold, the seventy-ninth element, had charge +79.

Soddy agreed. Van den Broek's prediction explained the sequence of radioactive decays. When an atom of polonium, element number 84, emitted an alpha particle, it lost two units of nuclear charge. What was left was an atom of lead, element number 82. When the lead atom emitted an electron, it gained one unit of charge to become bismuth, element number 83. By emitting another electron, bismuth would gain a unit of charge and become polonium.

The final confirmation of Van den Broek's hypothesis came from the experiments of Henry Gwyn Jeffreys Moseley. Mosely used cathode rays to knock atomic electrons out of their orbits. When an electron jumped back into orbit, it emitted electromagnetic radiation.

According to Bohr's atomic theory, an electron jumping to the innermost orbit of a heavy atom emits X-rays. From the wavelength of the X-rays, Moseley could calculate the charge of the nucleus.

Mosely measured X-rays from 38 elements. He started with aluminum, atomic mass 27, and ended with gold, atomic mass 197. As he moved across the periodic table, one element at a time, he found that the charge on the nucleus increased by one unit. This proved that the charge on the nucleus was equal to its atomic number.

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The Philosopher's Stone.

Another clue to the composition of the atomic nucleus came from alpha-scattering experiments. Ernest Marsden wanted to find out what would happen if he bombarded light nuclei with alpha rays.

In 1914, Marsden accepted a job as a professor in New Zealand. There he did not have the equipment to finish his experiment. Since Marsden was not able to complete the experiment, Rutherford decided to do it himself.

In August, 1914, World War I began. Physicists throughout Europe joined the war effort. During the war years, Rutherford worked on ways of detecting submarines. He had little time for alpha particles.

After the war ended in 1918, Rutherford returned to his experiments. When he shot alpha particles into nitrogen gas, he noticed something strange. Some of the scintillations in his detector did not seem to be coming from either alpha particles or nitrogen atoms. They looked like scintillations made by hydrogen nuclei.

At first, Rutherford thought the nuclei were coming from his radium source. Further experiment showed that they were coming from the nitrogen gas. Rutherford concluded that some of the nitrogen atoms were disintegrating when hit by alpha particles. Rutherford was changing atoms of nitrogen into atoms of oxygen.

For centuries, alchemists had searched for something that would turn less valuable metals into gold. They called this the "philosopher's stone." Finally, Rutherford had discovered how to transmute the elements.

His experiment convinced Rutherford that the nitrogen nucleus had hydrogen nuclei in it. That meant the hydrogen nucleus was an elementary particle. Rutherford named it the proton, from the Greek word "protos," meaning "first."

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A Peculiar Prediction

All nuclei, Rutherford concluded, must be built of protons and electrons. He thought the alpha particle, or helium nucleus, was four protons and two electrons bound together by electric forces. The lithium nucleus had seven protons and four electrons, and so on.

If two electrons could bind four protons, could one electron bind one proton? Rutherford thought it probably could. The hypothetical particle, which he named the neutron, would have unusual properties. It would be able to go right through matter, and even penetrate the nucleus.

J. L. Glasson, a student at the Cavendish Laboratory, tried to find the neutron in a hydrogen discharge tube. The discharge tube was a cathode-ray tube with holes in the cathode. Streams of protons zoomed through the holes at high speed.

Glasson hoped that some of the protons would combine with electrons to form neutrons. If any of the neutrons entered a heavy nucleus, the collision would disrupt either the nucleus or the neutron. Glasson would then be able to detect charged particles coming from the collision.

Glasson did not find any evidence for neutrons.

Another student, J. Keith Roberts, measured the heat produced in a hydrogen discharge tube. If neutrons were being formed, they should produce excess energy which he would measure as excess heat.

Roberts proved that the law of conservation of energy was true for the discharge tube. He did not find any neutral particles.

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James Chadwick and the Beryllium Rays

Rutherford predicted the existence of the neutron in 1920. Twelve years later, his assistant James Chadwick found it. Chadwick had been a student at Manchester University. After graduating in 1911, he stayed at the laboratory doing research for Rutherford.

In 1913, Chadwick went to Berlin, Germany, to work with Hans Geiger. The war broke out the following year. Because Chadwick was an Englishman, he was detained as a civilian prisoner of war. He was allowed to read books and talk to other physicists, but he could not do experiments.

In 1918, when the war ended, Chadwick returned to Manchester. He worked with Rutherford on the transmutation of the elements. In 1919, Rutherford went to Cambridge to become director of the Cavendish Laboratory. Chadwick went with him.

At Cambridge, Chadwick searched for the neutron. He tried in 1923, but did not find it. He tried again in 1928, with no success.

In 1930, the German physicists Walther Bothe and Herbert Becker noticed something odd. When they shot alpha rays at beryllium (atomic number 4) the beryllium emitted a neutral radiation that could penetrate 200 millimeters of lead. In contrast, it takes less than one millimeter of lead to stop a proton. Bothe and Becker assumed the neutral radiation was high-energy gamma rays.

Marie Curie's daughter, Irene Joliot-Curie, and Irene's husband, Frederic, put a block of paraffin wax in front of the beryllium rays. They observed high-speed protons coming from the paraffin. They knew that gamma rays could eject electrons from metals. They thought the same thing was happening to the protons in the paraffin.

Chadwick said the radiation could not be gamma rays. To eject protons at such a high velocity, the rays must have an energy of 50 million electron volts. An electron volt is a tiny amount of energy, only enough to keep a 75-watt light bulb burning for a tenth of a trillionth of a second. The alpha particles colliding with beryllium nuclei could produce only 14 million electron volts.

The law of conservation of energy states that energy can neither be created nor destroyed. It certainly looked as if energy was being created along with the neutral radiation.

Chadwick had another explanation for the beryllium rays. He thought they were neutrons. He set up an experiment to test his hypothesis.

Chadwick put a piece of beryllium in a vacuum chamber with some polonium. The polonium emitted alpha rays, which struck the beryllium. When struck, the beryllium emitted the mysterious neutral rays.

In the path of the rays, Chadwick put a target. When the rays hit the target, they knocked atoms out of it. The atoms, which became electrically charged in the collision, flew into a detector.

Chadwick's detector was a chamber filled with gas. When a charged particle passed through the chamber, it ionized the gas molecules. The ions drifted toward an electrode. Chadwick measured the current flowing through the electrode. Knowing the current, he could count the atoms and estimate their speed.

Chadwick used targets of different elements, measuring the energy needed to eject the atoms of each. Gamma rays could not explain the speed of the atoms. The only good explanation for his result was a neutral particle.

To prove that the particle was indeed the neutron, Chadwick measured its mass. He could not weigh it directly. Instead he measured everything else in the collision and used that information to calculate the mass.

In scattering demonstration 2C, you saw that if you shot a copper penny into a target, one of the target pennies would fly out. If you knew what your target was made of, but did not know what sort of projectile was hitting it, you could calculate the mass of the projectile from the speed of the target particle.

For his mass measurement, Chadwick bombarded boron with alpha particles. Like beryllium, boron emitted neutral rays. Chadwick placed a hydrogen target in the path of the rays. When the rays struck the target, protons flew out. Chadwick measured the velocity of the protons.

Using the laws of conservation of momentum and energy, Chadwick calculated the mass of the neutral particle. It was 1.0067 times the mass of the proton. The neutral radiation was indeed the long-sought neutron.

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The Modern Atom

If a neutron did indeed consist of a proton and an electron, then hydrogen atoms should sometimes turn into neutrons. As Chadwick had learned when he first tried to find them, it was very difficult to make neutrons out of protons and electrons. He began to wonder if the neutron, like the proton, was an elementary particle.

Except for their electric charge, the proton and neutron were nearly identical. Their mass was almost the same, and both were found in the atomic nucleus. They behaved similarly in scattering experiments. Physicists concluded that the neutron and the proton were two versions of the same elementary particle.

The atomic nucleus was not made of protons and electrons. It was made of protons and neutrons. All of the electrons in an atom were outside the nucleus.

An atom of an element with atomic number Z had Z protons in the nucleus and Z electrons in orbit around it. If the atomic mass of a particular isotope was A, then an atom of that isotope had A minus Z neutrons in its nucleus. Electrons, protons, and neutrons explained every atom in the universe.

It seems that whenever scientists think they have something all figured out, a new discovery comes along to contradict their theories. In the year in which Chadwick discovered the neutron, a new particle appeared. The new particle didn't seem to fit into a universe made of electrons, protons, and neutrons. Far from being over, the search for elementary particles had just begun.

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Back to the Electron Centennial Page
On to the next chapter: Probing the Proton


Top of this page
Back to the Electron Centennial Page
On to the next chapter: Probing the Proton