The history of the periodic table: from alchemy to element 118

Ancient elements and alchemy

Ten of today’s elements were known long before anyone used the word element in its modern sense: gold, silver, copper, iron, tin, lead, mercury, sulfur, carbon and antimony. No one knows who found them first. Ancient Greek thinkers, following Empedocles and Aristotle, believed instead that everything was a mixture of four elements (earth, water, air and fire), an idea that lasted for nearly two thousand years.

Alchemists in the Islamic world and medieval Europe tried to turn cheap metals into gold. They failed, but they refined laboratory methods such as distillation. In 1661 Robert Boyle challenged the four-element theory in The Sceptical Chymist, urging chemists to trust experiments, and in 1669 the alchemist Hennig Brand discovered phosphorus while hunting for the philosopher’s stone, one of the earliest elements with a recorded discoverer.

Lavoisier’s list of simple substances (1789)

Modern chemistry took shape with the French chemist Antoine Lavoisier, who showed through careful weighing that mass is not lost in chemical reactions and gave oxygen and hydrogen their names. In his 1789 Elementary Treatise on Chemistry he defined an element in practical terms: a substance that no known chemical method could break down into anything simpler.

The book listed 33 of these “simple substances,” sorted into gases, nonmetals, metals and earths. Most were true elements, but light and heat were on the list too, and earths such as lime and silica later proved to be compounds. Even so, Lavoisier gave chemists a clear, testable definition of an element and a first list to build on.

Dalton and the weighing of atoms

Around 1803 the English teacher John Dalton proposed that each element is made of its own kind of atom with its own weight. Atoms were far too small to weigh, so he worked out relative weights from the proportions in which elements combine, taking hydrogen as 1, and published one of the first tables of atomic weights.

Many of Dalton’s numbers were off because he had to guess formulas; he assumed, for example, that water was HO rather than H₂O. The Swedish chemist Jöns Jacob Berzelius measured atomic weights far more precisely and introduced the letter symbols still used today, but disagreements lasted until the 1860 Karlsruhe Congress, where Stanislao Cannizzaro showed how to obtain a consistent set of atomic weights. Mendeleev and Lothar Meyer were both in the audience.

Döbereiner’s triads (1829)

One of the first clear hints that atomic weight and chemical behavior are linked came from the German chemist Johann Wolfgang Döbereiner. In 1829 he described triads, groups of three similar elements in which the middle element’s atomic weight is close to the average of the other two: calcium, strontium and barium; chlorine, bromine and iodine; and lithium, sodium and potassium.

Triads revealed natural families of elements, but only a handful of elements fit, and there was no way to connect one triad to the next.

A spiral and an octave (1862–1865)

In 1862 the French geologist Alexandre-Émile Béguyer de Chancourtois wrote the elements in order of atomic weight along a spiral wound around a cylinder, so that similar elements lined up one above another. His telluric screw, named after tellurium, was one of the first arrangements to show properties repeating, but it was published without its diagram and went largely unnoticed.

In 1865 the English chemist John Newlands noticed that, in order of atomic weight, every eighth element seemed to repeat the properties of the first, like the notes of a musical scale. His law of octaves worked for the lighter elements but broke down after calcium, and in 1866 the Chemical Society in London declined to publish it.

Mendeleev and Lothar Meyer (1869–1870)

Dmitri Mendeleev, a chemistry professor in St. Petersburg, was writing a textbook when he went looking for the order behind the elements. In early 1869 he arranged the elements then known by atomic weight in a grid that lined up similar elements, and in March his paper was presented to the Russian Chemical Society. He stated a periodic law: arranged by atomic weight, the elements show properties that repeat at regular intervals.

What set his table apart was its boldness. Mendeleev put chemistry ahead of strict weight order, left empty spaces for elements he believed were still undiscovered, and argued that some accepted atomic weights were wrong.

In Germany, Lothar Meyer reached a very similar table independently and published it in 1870, along with a graph showing atomic volume rising and falling in waves as atomic weight increases. In 1882 the Royal Society in London awarded its Davy Medal to both men.

Predictions confirmed (1875–1886)

Mendeleev described three missing elements in detail, calling them eka-aluminium, eka-boron and eka-silicon, from the Sanskrit eka, meaning “one,” for one place beyond a known element. They turned up as gallium, found by Paul-Émile Lecoq de Boisbaudran in 1875, scandium, found by Lars Fredrik Nilson in 1879, and germanium, found by Clemens Winkler in 1886.

The matches were striking. For eka-silicon Mendeleev had forecast an atomic weight of about 72 and a density of about 5.5 grams per cubic centimeter; germanium’s are 72.6 and 5.3. Doubters were won over, and the periodic table became the central map of chemistry.

The noble gases find a place (1890s)

In 1894 Lord Rayleigh and William Ramsay discovered argon, an unreactive gas hidden in ordinary air. By 1898 Ramsay had also isolated helium on Earth and, with Morris Travers, discovered neon, krypton and xenon; radon followed in 1899 and 1900. None of these gases fit any existing column.

The answer was a whole new group between the halogens and the alkali metals, first called group 0 and now group 18. It fit the periodic pattern neatly, and in 1904 Ramsay received the Nobel Prize in Chemistry and Rayleigh the Nobel Prize in Physics.

Moseley and atomic number (1913)

A few pairs of elements, such as argon and potassium or cobalt and nickel, still sat out of order by weight. In 1913 the young English physicist Henry Moseley, working in Ernest Rutherford’s laboratory in Manchester, measured the X-rays given off by different elements and found that their frequencies rose in even steps from one element to the next.

Each step matched one more unit of positive charge in the atom’s nucleus. This atomic number became the true basis of the table and showed exactly which elements were still missing. Moseley was killed in action at Gallipoli in 1915, at the age of 27.

Seaborg, the actinides and the superheavy elements (1940–2016)

Scientists also learned to make elements that are extremely rare or absent in nature. Technetium, made in 1937, was the first artificially produced element, and in 1940 neptunium and plutonium became the first elements beyond uranium. In 1944 Glenn Seaborg proposed that the elements from actinium onward form their own series, the actinides, much like the lanthanides. Published in 1945, the idea moved them to a separate row below the main table and gave the table its modern shape.

Since then, laboratories in the United States, the Soviet Union and later Russia, Germany and Japan have made ever heavier elements, often only a few atoms at a time, by fusing atomic nuclei. A joint working group of IUPAC and IUPAP, the international unions for chemistry and physics, now judges each claim. In 2016 the last four confirmed elements were named nihonium, moscovium, tennessine and oganesson, completing the seventh row. The search for elements 119 and 120, which would open an eighth row, goes on.