HENRY GWYN JEFFREYS MOSELEY (1887-1915)

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1914 – Manchester, England

‘Moseley’s law – the principle outlining the link between the X-ray frequency of an element and its atomic number’

ca. 1910s --- Physicist Henry Gwyn Jeffreys MOSELEY --- Library Image by © Bettmann/CORBIS

MOSELEY

Working with ERNEST RUTHERFORD’s team in Manchester trying to better understand radiation, particularly of radium, Moseley became interested in X-rays and learning new techniques to measure their frequencies.
A technique had been devised using crystals to diffract the emitted radiation, which had a wavelength specific to the element being experimented upon.

In 1913, Moseley recorded the frequencies of the X-ray spectra of over thirty metallic elements and deduced that the frequencies of the radiation emitted were related to the squares of certain incremental whole numbers. These integers were indicative of the atomic number of the element, and its position in the periodic table. This number was the same as the positive charge of the nucleus of the atom (and by implication also the number of electrons with corresponding negative charge).

By uniting the charge in the nucleus with an atomic number, a vital link had been found between the physical atomic make up of an element and its chemical properties, as indicated by where it sits in the periodic table.
This meant that the properties of an element could now be considered in terms of atomic number rather than atomic weight, as had previously been the case – certain inconsistencies in the MENDELEEV version of the periodic table could be ironed out. In addition, the atomic numbers and weights of several missing elements could be predicted and other properties deduced from their expected position in the table.

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DMITRI MENDELEEV (1834-1907)

1869 – Russia

‘The properties of elements are periodic functions of their atomic weights’

Arrange the atoms in order of their atomic weight (relative atomic mass) and elements are also arranged in order of their properties. This arrangement of the elements is called the periodic table.

In the modern periodic table elements are no longer arranged by their atomic weight but by a more fundamental quantity; ‘atomic number’.

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DIMITRY IVANOVICH MENDELEYEV

The atomic number of an element is the number of protons in the nucleus of one of its atoms; the number of neutrons, which contributes to atomic weight, is ignored. The modern periodic law is that ‘The properties of elements are periodic functions of their atomic numbers’.

In 1860 Dmitri Ivanovich Mendeleev attended a chemistry conference in Karlsruhe where the Italian Stanislao Cannizzaro’s speech announcing his rediscovery of the distinction between atoms and molecules (originally announced in 1811 by AVOGADRO) made a profound impression.

The German chemist Johann Wolfgang Döbereiner (1780-1849) had recognised mathematical patterns in elements that had similar properties. He found that adding the atomic weights of calcium (40) and barium (137) and dividing the total in two left a value close to the weight of strontium (88). Finding this same pattern repeated for lithium, sodium and potassium, and for chlorine, bromine and iodine confirmed the relationship, which he termed the Law of Triads.

In 1862, French scientist Alexandre Beguyer de Chancourtois developed a way of representing the elements by wrapping a helical list around a cylinder.

A repeating pattern in natural phenomena is a strong indication that there exists a simple, compact description.
The periodic table suggests that the distinct atoms of the elements may be described in terms of significantly fewer building blocks than the number of the individual elements. Atoms, then, were made of significantly fewer subatomic building blocks.

In 1869 the 35-year-old Mendeleev published a table of the 61 elements then known. His list of elements – ‘On the Relation of the Properties to the Atomic Weights of Elements’ – occupied a grid where the atomic weight increased as you went down a column (periods) and the elements in any particular row (groups or families) shared similar properties and valencies (metals and gases, for instance).

Mendeleev had to juggle the order of a few elements, assuming their weights to have been incorrectly measured, and predicted that some undiscovered elements would fill the gaps in the table, based on the properties of the elements surrounding the gaps.
The modern periodic table has been turned sideways.

By 1886, with the discoveries of gallium, scandium and germanium with the properties he had foretold, his prediction was fulfilled. By 1925, chemists had successfully identified all the 92 elements they believed to exist in nature. The first artificial element, neptunium, was synthesised in 1940. Many more elements have been made since then.

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‘The Genesis of a Law of Nature’ – Mendeleev

EDWIN McMILLAN (1907- 90) GLENN SEABORG (1912- 99)

1940 – USA

‘Elements heavier than uranium in the periodic table (transuranium elements) are made artificially. Uranium (U, atomic number 92) is the heaviest element known to exist naturally in detectable amounts on the Earth’

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SEABORG & McMILLAN

In 1933 ENRICO FERMI showed that the nucleus of most elements would absorb a neutron.
In 1940 McMillan, a nuclear physicist, produced and identified the first artificial element, neptunium (Np, 93). In 1943 Seaborg, a chemist, succeeded in creating plutonium (Pu, 94).

So far more than 20 synthetic elements have been created. All are unstable, decaying with half-lives ranging from a year to a few milliseconds.
At least thirteen transuranium elements have been named after scientists:-
curium (Cm, 96: Marie and Pierre Curie [1944]), einsteinium (Es, 99: Albert Einstein [1952]), fermium (Fm, 100: Enrico Fermi [1952]), mendelevium (Md, 101: Dmitri Mendeleev [1955]), nobelium (No, 102: Swedish chemist Alfred Nobel (1833-96), known for his bequest for the foundation of the Nobel Prizes [1956]), lawrencium (Lr, 103: Ernest O. Lawrence, a physicist best known for development of the cyclotron [1961]), rutherfordium (Rf, 104: Ernest Rutherford [1968]), seaborgium (Sg, 106: Glenn Seaborg [1974]), bohrium (Bh, 107: Niels Bohr [1981]), meitnerium (Mt, 109: Lise Meitner [1982]); roentgenium (Rg, 111: Wilhelm Conrad Röntgen was first created in 1994 by the GSI Helmholtz Centre for Heavy Ion Research near Darmstadt in Germany [1994]), copernicium (Cn, 112: named after astronomer Nicolaus Copernicus [1996]), flerovium (Fl, 114 named after Soviet physicist Georgy Flyorov [2012]).

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