1. Early nuclear physics ↑
1.1 Alpha rays ↑
New Zealand physicist Ernest Rutherford and British chemist Frederick Soddy were the first to show that one element can naturally radiate enough matter to turn into another. They did this in 1902, after experimenting with thorium.
Rutherford showed that you can also turn one element into another by adding alpha rays to atoms, rather than removing them, in 1919. He did this by firing alpha rays at nitrogen atoms. This produced oxygen and a positively charged particle, which Rutherford identified as a proton.
We now know that alpha rays - helium nuclei - contain two protons and two neutrons. When Rutherford fired alpha rays at nitrogen atoms, a proton was knocked out of the nitrogen, and the helium nuclei merged with what was left to make oxygen.
1.2 Neutrons ↑
Rutherford discovered the nucleus of the atom in 1911, and predicted the existence of the neutron in 1920, shortly after he discovered the proton. The neutron was finally discovered by British physicist James Chadwick in 1932.
Earlier that year, French physicists Irène and Frédéric Joliot-Curie (Marie and Pierre Curie's daughter, and her husband), had been investigating a new type of neutral radiation, which had been discovered by German physicists Walther Bothe and Herbert Becker, in 1930.
Rutherford and his student, Chadwick, were convinced that this radiation was composed of neutrons, and Chadwick soon proved this by measuring the mass of the neutral particles that formed the radiation.
The discovery of neutrons explained how isotopes form. Isotopes are elements that occupy the same place in the periodic table but have slightly different masses. Soddy had first suggested the existence of isotopes in 1913, and it was now clear that heavier isotopes contain more neutrons.
1.3 Fission and fusion ↑
The atom was split for the first time weeks after Chadwick's discovery of the neutron, in what would later be called nuclear fission. This was achieved by Rutherford's colleagues, Irish physicist Ernest Walton and British physicist John Cockcroft. Walton and Cockcroft fired protons at lithium atoms in order to split them into two helium nuclei.
Australian physicist Mark Oliphant became the first to fuse hydrogen isotopes together in 1934. Oliphant fused 'heavy' hydrogen nuclei (hydrogen nuclei containing one proton and one neutron, rather than just one proton), and this created helium nuclei.
2. Nuclear energy ↑
2.1 Nuclear binding energy ↑
British physicist Robert d'Escourt Atkinson and Dutch-Austrian-German physicist Fritz Houtermans had first suggested that a large amount of energy could be released by fusing small nuclei together in 1929, three years before the discovery of the neutron.
It was later shown that both fusion and fission release energy. Energy is produced as long as the new nuclei are more tightly bound, and hence more stable, than the old. Whether fusion or fission makes the nucleus more stable depends on the binding energy per proton and neutron.
Binding energy is the energy required to keep two protons or neutrons bound together, and the same amount of energy is needed to tear them apart. This was later explained by quantum chromodynamics, a quantum field theory of the strong nuclear force.
The existence of binding energy means that the total mass of a proton and neutron combined is less than the mass of an individual proton plus an individual neutron.
The binding energy adds to the mass of the nucleus because energy is related to mass via German-Swiss-American physicist Albert Einstein's theory of special relativity. Special relativity states that the rest mass (m0) of an object equals e/c2, since e = m0c2, where e is energy, and c is the speed of light.
The binding energy per proton and neutron varies, depending on how many protons and neutrons make up the nucleus. The most stable nucleus is iron, elements heavier than this undergo fission in order to become more stable, and lighter elements undergo fusion.
When an atom becomes more stable, it moves to a lower energy state, and so energy is released.
2.2 Nuclear chain reactions ↑
In 1934, American physicist Leo Szilard realised that neutrons could be used to mediate a self-sustaining nuclear chain reaction, which would be able to generate vast amounts of energy. This could happen if a nuclear reaction released high velocity neutrons that went on to collide with other atoms, splitting them and releasing yet more neutrons.
Italian physicist Enrico Fermi claimed to have created the new heavy elements ausonium (now known as neptunium) and hesperium (now known as plutonium) by firing neutrons at lighter elements in 1934.
Influenced by Fermi's results, German chemists Otto Hahn and Fritz Strassmann began performing similar experiments in Berlin. By 1938, they had created a new heavy element, barium, after bombarding uranium with neutrons.
That same year, German-American physicist Hans Bethe and Indian-American physicist Subrahmanyan Chandrasekhar showed how stars are fuelled by nuclear fusion chain reactions.
Fission chain reaction of uranium, which splits into krypton, barium, and three separate neutrons. Image credit: modified by Helen Klus, original image by Fastfission/Public domain.
In 1956, Japanese-American physicist Paul Kuroda suggested that nuclear fission might exist elsewhere in nature, since nuclear chain reactions only require natural materials. This was proven in 1972, when evidence of natural self-sustaining nuclear chain reactions was found in uranium mines in Oklo, Gabon. The nuclear reaction is thought to have occurred about 1.5 billion years ago.
2.3 The Manhattan Project ↑
Szilard and Fermi had moved to Manhattan, New York, in 1938. After hearing of Hahn and Strassmann's fission experiments, Szilard, Fermi, and American physicist Herbert Anderson successfully showed that uranium could mediate a nuclear chain reaction the following year.
This discovery prompted Szilard to write to other scientists, and ask them to refrain from publishing work on nuclear physics in case the Nazi government became aware of the possibilities. Not everyone agreed, and so Szilard drafted a letter, warning that Nazi Germany might be attempting to build an atomic bomb. This was signed by Einstein, and delivered to American President Franklin D. Roosevelt, on 2nd August 1939.
Szilard's letter resulted in the Manhattan Project, a top-secret research and development project that began in 1942. Szilard and Fermi created the first artificial nuclear chain reaction that year, with Chicago Pile-1, the first nuclear reactor.
The first atomic bomb was detonated in the Trinity Test, which was conducted in the New Mexico desert in July 1945. In August of that year, two more atomic bombs, one created from uranium fission and one from plutonium, were detonated over the Japanese cities of Hiroshima and Nagasaki.
After World War II, many countries developed both nuclear weapons and nuclear power plants.
2.4 Nuclear power plants ↑
In a nuclear power plant, a nuclear reactor creates a controlled nuclear fission chain reaction. This produces heat, which is used to heat water, creating steam. The steam then drives a turbine, which is connected to an electric generator. Electricity was generated by a nuclear reactor - Experimental Breeder Reactor-I - for the first time in 1951.
Diagram of a nuclear power plant. Image credit: U.S.NRC./Public domain.
Nuclear energy is controversial because it is a sustainable energy source, however it uses dangerous materials, and accidents can have long term and tragic consequences. This was highlighted by the Chernobyl accident of 1986 and the Fukushima Daiichi accident of 2011.