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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ERNEST RUTHERFORD (1871-1937)

1911 Manchester, England

‘The atom contains a core or nucleus of very high density and very concentrated positive charge. Most of the atom is empty space, with the electrons moving about the tiny central nucleus’

Early photograph of ERNEST RUTHERFORD

ERNEST RUTHERFORD

Working under JJ THOMSON (1856-1940) at the Cambridge Cavendish Laboratory and later at the McGill University in Montreal, in 1898 Rutherford put forward his observation that radioactive elements give off at least two types of ray with distinct properties, ‘alpha’ and ‘beta’ rays.

In 1900 he confirmed the existence of ‘gamma’ rays, which remained unaffected by a magnetic force, whilst alpha and beta rays were both deflected in different directions by such an influence. Although both displayed the ability to stab through solid matter, alpha rays were far less penetrating than beta rays.
He proved through experimental results that they were helium atoms missing two electrons.

Alpha Beta Particles, Gamma Rays in a Magnetic Field

Alpha Beta Particles, Gamma Rays in a Magnetic Field

Alpha rays are in fact positively charged helium atoms that become true helium when they slow down and their charge is neutralised by picking up electrons.
Beta rays were later shown to be made up of electrons, and gamma rays to have a shorter wavelength than X-rays.

diagram showing comparative penetrations of Alpha Beta Gamma radiation

Alpha Beta Gamma radiation

In Montreal, Rutherford worked with Frederick Soddy and showed that over a period of time, half of the atoms of a radioactive substance could disintegrate. During the process the substance spontaneously transmuted to other elements. During radioactive decay, one kind of atom (radium) was ejecting another kind of atom (helium).

Working with other elements, Rutherford and Soddy found that each radioactive element had its own characteristic ‘half-life’. After one half-life, a sample retained only half its original radioactivity, after two half-lives a quarter, after three half-lives an eighth. The half-life of thorium emanation, now known as radon, was close to a minute. The half-lives of other radioactive elements ranged from a split-second to many billions of years. That of radium was 1620 years, while uranium had a half-life of 4.5 billion years.

The concept of half-life provides a way of measuring the age of rocks. As radioactive atoms decay they emit alpha particles. As these are essentially helium atoms, the amount of helium gas accumulates within the pores and fissures of a sample of a uranium mineral as a measure of how many atoms have decayed. Heating samples to drive off their helium and measuring the amount gives an indication of their age.
In order to provide more reliable dates, measuring the amount of lead, the ultimate decay product, compared with the amount of uranium, eliminates the errors introduced by the escape of some of the helium decay product to the air.

Dating rocks in this way gives an estimate of the age of the Earth, and by implication also the Sun, of around 4.5 billion years.

A radioactive atom is simply a heavy atom, which happens to be unstable. Eventually it disintegrates by expelling an alpha, beta or gamma ray. What remains is an atom of a slightly lighter element. A radioactive atom may decay more than once. Uranium, for instance, transforms itself into a succession of lighter and lighter atoms, one of which is radium, until it achieves stability as a non-radioactive atom of lead.

English: Radioactive decay modes

Working with HANS GEIGER (1882-1945), Rutherford developed the Geiger counter at Manchester University in 1908. This device measured radiation and was used in Rutherford’s work on identifying the make-up of alpha rays.

While he was at McGill, Rutherford had experimented firing alpha particles at a photographic plate. He had noticed that, while the image produced was sharp; if he passed the alpha particles through thin plates of mica, the resulting image on the photographic plate was diffuse. The particles were clearly being deflected through small angles as they passed close to the atoms of mica.
In 1910 his team undertook work to examine the results of directing a stream of alpha particles at a piece of platinum foil. While most passed through, about one in eight thousand bounced back – that is, deflected through an angle of more than 90 degrees.

Deflection of alpha Particles by Thin Metal Foil

Deflection of alpha Particles by Thin Metal Foil

In 1911 he put forward the theory that the reason for the rate of deflection was because atoms contained a minute nucleus that bore most of the weight, while the rest of the atom was largely ’empty space’ in which electrons orbited the nucleus much as planets orbit the Sun. The reason that one in eight thousand alpha particles bounced back was because they were striking the positively charged nucleus of an atom, whereas the rest simply passed through the spacious part.

But what was an atomic nucleus made of?
At 100,000th the size of the atom, it would take decades of painstaking experiments to discover.

In 1919, working in collaboration with other scientists, Rutherford artificially induced the disintegration of atoms by collision with alpha particles. In the process the atomic make-up of the element changed as protons were forced out of the nucleus. He transmuted nitrogen into oxygen (and hydrogen) and went on to repeat the process with other elements.

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