Nuclear Fission: The Nucleus Ability to Split into Two Different Nuclei


By Ron B. Davis Jr.Georgetown University

Almost everybody knows that it’s possible for an atomic nucleus to change its identity, not only by shedding a small, high-energy particle but also by splitting itself into two different elements. This process of breaking atomic nuclei into two smaller pieces is known as ‘nuclear fission.’ Isotopes that can undergo fission are called ‘fissile.’

An image of Otto Hahn and Lise Meitner
Lise Meitner and Otto Hahn discovered nuclear fission. (Image: Unknown/Public domain)

The Discovery of Nuclear Fission

The discovery of nuclear fission was ultimately advanced by Lise Meitner, an Austrian-born physicist who had migrated to Germany in 1912. There, she built relationships with some of the preeminent chemists of the day, including Otto Hahn. But Meitner was forced to flee Germany in 1938, relocating to Sweden to work at the Nobel Institute for Physics.

Meanwhile, Hahn continued his work, using the recently discovered neutron as a projectile, shooting beams of neutrons at large atoms, like uranium. Adding more neutrons would create larger isotopes even further from the stability curve. He expected those highly unstable isotopes might undergo radioactive decay to produce new elements.

Much to Hahn’s surprise, his chemical analysis of element-92 uranium radiated with neutrons, showing evidence of element 56, barium. Hahn wondered how this might be possible. Barium has 36 fewer protons than uranium. To reach element 56 from element 92 through radioactive decay would require a sequence of at least eighteen alpha particle emissions, and somehow bypassing stable elements like lead along the way.

Desperate to find answers, he sent Meitner a letter describing his findings. Meitner’s interpretation of the results was ground-breaking. As a nuclear physicist, it was clear to her that only one explanation could possibly account for the drastic loss of mass in Hahn’s experiments. The nucleus wasn’t emitting eighteen alpha particles—it was breaking—nearly in half—in one concerted step!

So if Hahn’s uranium atoms were in fact fissile and had broken into element-56 barium, then where was the other piece? Well, the periodic table holds the answer. Uranium’s 92 protons minus barium’s 56 protons, leaves behind a nucleus with 36 protons. This corresponds to a krypton atom—a noble gas that could easily escape the confines of his experiment and simply float away.

This article comes directly from content in the video series Understanding the Periodic TableWatch it now, on Wondrium.

What Free Neutrons Can Do

So, fission creates two atoms from one. But fission also produces excess neutrons. Remember that heavier elements need a higher neutron to proton ratio to be stable enough to form at all. So, when such a large atom, with a greater proportion of neutrons, breaks, a few of those neutrons are no longer needed. That’s why so-called ‘free neutrons’ are very commonly produced in fission reactions.

A neutron can strike a fissile uranium nucleus, causing it to undergo fission. One of the products of that reaction is a pair of more stable elements, barium, and krypton, causing the release of large quantities of energy. The lighter barium and krypton nuclei need fewer neutrons to achieve stability, allowing the release of some ‘free neutrons’.

An image of the first technetium 99m generator
Technetium is not obtainable from nature. (Image: Brookhaven National Laboratory/Public domain)

If there are additional fissile nuclei nearby, those ‘free neutrons’ can continue the entire process, causing the fission of more uranium, producing more energy, producing more neutrons, which perpetuate the cycle in a nuclear chain reaction.

Heavy, Unearthly, Radioactive

All elements have radioactive isotopes. But large elements, beginning at element 83 and continuing to the end of the table, have only radioactive isotopes. One of the rare violations of this overall division of which elements are stable is located near the center of the table. Element number 43, technetium is a lone outpost of instability surrounded by stable elements.

Technetium is the lightest element on the table that cannot be obtained in nature. There is effectively zero natural abundance on Earth, so no average atomic mass can be reported. Instead, the table only reports the atomic mass of its most stable isotope, technetium-98.

An image of the Sun
Technetium is a heavy element built up in stars via nuclear fission. (Image: NASA/Public domain)

With the help of 20th-century technology, though, technetium can be made, atom by atom. Technetium was first obtained in 1937, some 70 years after Mendeleev’s prediction. It was found in a piece of molybdenum foil used in other nuclear experiments that created free neutrons.

The Long-lasting Starchild

Technetium is sometimes used for its radioactivity. For example, freshly prepared technetium-99m is a source of gamma rays for medical imaging. Technetium’s most stable isotopes are Tc97 and Tc98, both of which have a half-life of just over 4 million years. This may sound like a long time, but from the perspective of a star’s lifespan, it is the blink of an eye.

In 1952, American astronomer Paul W. Merrill set out to prove that stars were indeed the birthplace of heavy elements, and technetium helped him to demonstrate this. Knowing technetium’s emission spectrum, and that its half-life was shorter compared to the age of most stars, Merrill demonstrated that the technetium emission lines from certain long-lived stars were remarkably strong.

Merrill argued that since technetium’s half-life was minuscule compared to the age of the stars in his study, it must have been actively forming within each star itself. It was this discovery that is widely pointed to as the first direct evidence that stars are the birthplaces of heavy elements.

So, technetium’s unusual tendency to break down became the key to proving that stars are where it is built up, bringing our elemental story full-circle.

Common Questions about Nuclear Fission

Q: Who was Lise Meitner?

Lise Meitner was an Austrian-born physicist who, in 1912, migrated to Germany, where she contacted a number of prominent chemists of the time, such as Otto Hahn. In 1938, she moved to Sweden, where she worked at the Nobel Institute for Physics. The discovery of nuclear fission was eventually advanced by Meitner.

Q: How was nuclear fission discovered?

The discovery of nuclear fission began with Otto Hahn’s investigation. He used the newly discovered neutrons as projectiles shooting them at large atoms. During the chemical analysis of uranium, Hahn noticed the presence of the element barium. According to the nuclear decay, this was impossible. Eventually, Lise Meitner came up with nuclear fission as the interpretation of the results, and the process was discovered.

Q: How do chain reactions occur in the nuclear fission process?

In nuclear fission, fissile nuclei break into smaller stable nuclei with a lower ratio of neutrons to protons. Meanwhile, some ‘free neutrons’ are leftover, leading most fission reactions to promote additional fission reactions. Thus nuclear chain reactions can happen.

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