Development of the periodic table

Although elements such as gold, silver, tin, copper, lead and mercury have been known since earliest times, the first scientific discovery of an element occurred around 1669. Hennig Brand, a German alchemist, treated urine to a series of processes that resulted in the production of the element phosphorus.

Over the next 200 years, a great deal of knowledge about elements and compounds was gained. By the middle of the 19th century, about 60 elements had been discovered.

Scientists began to recognise patterns in the properties of these elements and set about developing classification schemes.

1862

French geologist Alexandre-Emile Béguyer de Chancourtois plotted the atomic weights of elements on paper tape and wound them, spiral like, around a cylinder. The design put similar elements onto corresponding points above and below one another. He called his model the telluric helix or screw.

1864

English chemist John Newlands noticed that, if the elements were arranged in order of atomic weight, there was a periodic similarity every 7 elements. He proposed his ‘law of octaves’ – similar to the octaves of music. Noble gases had yet to be discovered, which is why Newland’s table had a periodicity of 7 rather than 8.

1868

Lothar Meyer compiled a periodic table of 56 elements based on a regular repeating pattern of physical properties such as molar volume. Once again, the elements were arranged in order of increasing atomic weights. (Meyer’s work was not published until 1870.)

1869

Russian chemist Dmitri Mendeleev produced a periodic table based on atomic weights but arranged ‘periodically’. Elements with similar properties appeared under each other. Gaps were left for yet to be discovered elements.

1894

William Ramsay discovered the noble gases and realised that they represented a new group in the periodic table. The noble gases added further proof to the accuracy of Mendeleev’s table.

1913

Henry Moseley determined the atomic number of each of the known elements. He realised that, if the elements were arranged in order of increasing atomic number rather than atomic weight, they gave a better fit within the ‘periodic table’.

1928

Amateur French scientist Charles Janet uses mathematical patterns to investigate the electron configuration of elements. He groups elements into blocks named after their atomic orbitals: s-block (sharp), p-block (principal), d-block (diffuse) and f-block (fundamental).

1944

Glenn Seaborg proposed an ‘actinide hypothesis’ and published his version of the table in 1945. The lanthanide and actinide series form the two rows under the periodic table of elements.

The periodic table today

Most school science laboratories have a copy of the periodic table pinned to a wall somewhere.

Close inspection of the table shows the following distribution of types of element.

Most of the elements are metals. Metalloids are elements that have some of the physical properties of metals but some of the chemical properties of non-metals. Antimony, for example, conducts electricity but its chemistry resembles that of the non-metal phosphorus.

Scientists are constantly working on discovering new materials and further investigating the properties of existing elements. The periodic table can be reviewed and new elements can be added, but only added after rigorous scientific examination. The International Union of Pure and Applied Chemistry (IUPAC) verifies the additions of new elements and at the end of 2015 the 7th period of the periodic table of elements was completed.