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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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



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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


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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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 ©


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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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’

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: named after Wilhelm Conrad Röntgen was first created in 1994 by the GSI Helmholtz Centre for Heavy Ion Research near Darmstadt, 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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