SIR JAMES CHADWICK (1891-1974)

1932 Manchester, England

‘Discovery of neutrons – elementary particles devoid of any electric charge’

In contrast with the Helium nuclei (alpha rays) which are charged, and therefore repelled by the electrical forces present in the nuclei of heavy atoms, the neutron is capable of penetrating and splitting the nuclei of even the heaviest elements, creating the possibility of the fission of 235uranium

Assistant to ERNEST RUTHERFORD, Chadwick’s earlier work involved the showering of elements with alpha particles. The picture that gradually emerged was one of a nucleus that contained a very heavy particle with a positive electric charge. This particle was christened the proton, the hydrogen building block envisaged by WILLIAM PROUT.
A spin-off of this was the deduction that the nucleus of the hydrogen atom, the positively charged proton with an atomic weight of one was present in larger quantities in the nucleus of every other atom.

Rutherford and Geiger had shown that a helium atom and an alpha particle were the same thing, apart from the positive electric charge carried by the alpha particle.

A helium atom seemed to consist of a nucleus of a pair of protons circled by two electrons. However, a helium nucleus seemed to weigh as much as four protons. The mass of the known components of an atom did not add-up. Protons seemed to account for around half of the weight and were matched in number by an equal amount of negatively charged electrons to counter their positive charge. But the weight of an electron was one-thousandth that of a proton, so approximately half of the atomic weight of the element was unaccounted for.
Chadwick solved the conundrum in 1932 when he re-interpreted the results of an experiment carried out by IRENE and FREDERIC JULIOT-CURIE (Irene was the daughter of PIERRE and MARIE CURIE).
The couple had found in 1932 that when beryllium was showered with alpha particles, the resultant radiation could force protons out of substances containing hydrogen. Chadwick suggested that neutrally charged sub-atomic units, which he named neutrons, with the same weight as protons, could force this reaction and therefore were what made up the radiation that the Curies called gamma rays. Rutherford had hinted at the existence of such a particle in 1920.

The explanation was widely accepted and the riddle of `atomic weight’ had been solved: a similar number of neutrons to protons in the nucleus of an element would make up the remaining fifty per cent of the previously ‘missing’ mass.

photo portrait of FREDERICK SODDY ©

FREDERICK SODDY (more)

The discovery of the neutron made sense of the observation that many elements come in a variety of forms, each with differing radioactive properties such as decay rate. Each form consisted of atoms with a different mass. Frederick Soddy christened these variants ‘isotopes’ in 1911. The idea that each element might be a mixture of atoms of different atomic weights explained why the atomic weights of a handful of elements were not simple multiples of the atomic weight of hydrogen, the most notorious example being chlorine whose atomic weight was 35.5 times that of hydrogen. Most of the variant forms of each element turned out to be radioactively unstable. An element such as chlorine, with more than one stable isotope, is rare.

The various isotopes of an element were merely atoms with the same number of protons in their nucleus but with a different number of neutrons.

artistic representation of atomic disintegration

Thus every atom was composed of electrons, protons and neutrons. The protons and neutrons clung together in a central clump – the atomic nucleus – while the electrons circled in a distant haze. The neutrons were responsible for increasing the weight of the elements without adding any electrical charge. Two protons and two neutrons made a helium nucleus; eight protons and eight neutrons an oxygen nucleus; 26 protons and 30 neutrons an iron nucleus; 79 protons and 118 neutrons a gold; and 92 protons and 146 neutrons a nucleus of uranium. When a radioactive nucleus expelled an alpha particle, it lost two neutrons and two protons and consequently became a nucleus of an element two places lower in the periodic table. When a radioactive nucleus emitted a beta particle, however, a neutron changed into a proton, transforming the nucleus into that of an element one place higher in the periodic table.

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MARCUS OLIPHANT (1901-2000)

1934 – UK

‘Hydrogen has three isotopes: hydrogen-1 (ordinary hydrogen: one proton), hydrogen-2 (deuterium: one proton, one neutron) and hydrogen-3 (tritium: one proton, two neutrons)’

They each have one single proton (z = 1), but differ in the number of their neutrons. Hydrogen has no neutron, deuterium has one, and tritium has two neutrons. The isotopes of hydrogen have, respectively, mass numbers of one, two, and three. Their nuclear symbols are therefore 1H, 2H, and 3H. The atoms of these isotopes have one electron to balance the charge of the one proton. Since chemistry depends on the interactions of protons with electrons, the chemical properties of the isotopes are nearly the same.

MARK OLIPHANT

The lightest rare gas, helium, exists in nature in two forms – two isotopes

The usual form is represented as 4He, where the figure 4 stands for the number of nucleons in the atomic nucleus (two protons and two neutrons). In the unusual form, 3He, the atomic nucleus has only one neutron, so it is lighter. In helium that occurs naturally the heavier isotope is more frequent than the lighter one by a factor of about 10 million. That is why it is only in the last 50 years that it has been possible to produce large amounts of 3He, at nuclear power stations, for example. At normal temperatures the gases of the two isotopes differ only in their atomic weights.

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ENRICO FERMI (1901- 54)

1942 – USA

Fermi established his reputation with his concept of radioactive beta decay, the theory that a proton could be created from a neutron via the shedding of an electron (a beta particle) and an antineutrino.

The Joliot-Curies had announced their discovery that radioactive isotopes could be generated artificially by showering certain elements with alpha particles in 1934. Fermi realised that the newly discovered neutrons would be even better suited to this purpose as their lack of charge would allow them to slip into elements’ nuclei without resistance.

Fission chain reaction

Fission chain reaction

Fermi established the concept of ‘slow-neutrons’ by placing a piece of solid paraffin in front of the target element during bombardment. Working his way through the elements he created a number of new radioactive isotopes.

He was awarded the 1938 Nobel Prize for physics and later the significance of his work when applied to uranium was realised. Using his neutron-bombarding technique in a series of experiments with 235uranium, Fermi and NIELS BOHR confirmed that a nuclear chain reaction could almost certainly be created as the basis of an atomic bomb.

By Dec 2 1942 his team had created an ‘atomic pile’ of graphite blocks, drilled with uranium, which went on to produce a self-sustaining chain reaction for nearly half an hour.

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