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Unit 12: Kinetics and Nuclear Chemistry—Rates of Reaction

Section 9: Lise Meitner and the Discovery of Fission

Lise Meitner (1878–1968) was born in Vienna. As a girl, she was denied admission to secondary school in Vienna. Despite rampant sexism at all levels of academia, she earned her PhD in 1905 and eventually became the first woman professor of physics in Germany in 1926. (Figure 12-16)

Lise Meitner

Figure 12-16. Lise Meitner

© Wikimedia Commons, Public Domain.

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Lise Meitner

Figure 12-16. Lise Meitner

Lise Meitner, along with her nephew, Otto Frisch, discovered the process of nuclear fission and that energy is released when nuclei split.

In the 1930s, Meitner and her research partner, Otto Hahn (1879–1968), began trying to produce elements heavier than uranium (the heaviest natural element) by bombarding uranium nuclei with neutrons. After Hitler's rise to power, Meitner and many other prominent Jewish scientists fled Germany. Meitner took refuge in Sweden with her nephew Otto Frisch (1904–1979), also a physicist. Hahn continued the experimental work the two had been working on, and corresponded with Meitner about the results. Hahn found a strange result: Bombarding uranium with neutrons did not produce heavier elements at all; in fact, after exposure to neutrons, Hahn found lighter elements (barium and krypton) in the uranium sample. Puzzled, he sent his findings to Meitner in Sweden. She and her nephew concluded that neutron bombardment made uranium nuclei split apart in a process called "fission":

$↖ 238↙92$U + $↖ 1↙0$n → $↖141↙56$Ba + $↖ 92↙36$Kr + 3$↖ 1↙0$n

Meitner and Hahn noticed that the products of the fission reaction had a lower mass than the reactants. This startling result, which seemingly contradicts the Law of Conservation of Mass, occurs because some of the mass is converted into energy according to Albert Einstein's famous equation E = mc2.

Protons and neutrons release energy when they cluster together to make a nucleus because this nuclear formation is a more stable arrangement than having all the particles separate. The energy released is called the "binding energy," and the more stable the resulting nucleus, the more energy is given off. Because nickel and iron are the most stable of all the nuclei, they also have the greatest binding energies. (Figure 12-17)

Nuclear Binding Energy

Figure 12-17. Nuclear Binding Energy

© Science Media Group.

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Nuclear Binding Energy

Figure 12-17. Nuclear Binding Energy

A graph of nuclear binding energy for selected nuclei. Note how the nuclear binding energy per nucleon is highest for nickel, which is why nickel and iron are the two most stable elements.

Where does this energy come from? Energy cannot be created from nothing, and in this case the energy is created from the particles themselves. Some of the mass in the particles is converted into energy—this means that the particles themselves become lighter. The amount of mass lost is called the "mass defect." So, the mass of a carbon nucleus (12.00000 u) is actually less than the sum of six protons and six neutrons (12.09894 u); the mass defect of carbon is 0.09894 u, or approximately 0.82 percent. The more stable the nucleus, the greater the binding energy, and the greater the mass defect. Any nuclear reaction in which the products have a higher mass defect/binding energy than the reactants will release energy.

Uranium fission is just such a process. Much more energy is released by nuclear reactions than chemical reactions. After the discovery of fission, it wasn't long before scientists began working on how to harness this powerful new source of energy.

Glossary

Binding energy

The energy released when a nucleus is broken into individual protons and neutrons.

Fission

The splitting of an atomic nucleus into smaller particles.

Mass defect

The mass lost by nuclear particles when they come together to form a nucleus.

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