Periodic Table: the home of pure substances
All the known elements in the universe are brought together in a single table, a tool that took more than a century to build. And it may not be finished yet.
The image that represents the universe
It is called the Periodic Table of Chemical Elements. And, as the name suggests, in a single picture it presents all the pure substances that humanity has ever identified. In a long process of discovery – which, with the evolution of technology, may not be over yet – 118 elements have been identified, described and baptised as the basis of everything that exists.
Celebrating the International Year of the Periodic Table in 2019, UNESCO said that it "is more than just a guide or catalogue of all known atoms". It is a tool that can be used to predict the properties of elements, even those that have not been discovered yet, by comparing them with existing ones. Its columns and rows indicate elements that share similar characteristics, making it easier to understand trends in the behaviour of materials, even for students in the classroom.
For scientists, it establishes patterns that help control experiments on how elements react and associate, allowing new compounds to be created. It is therefore essential in industrial areas such as pharmaceuticals, chemistry and new energies, but also for studying the environment, either in terms of pollution, soil composition or water quality.
And as simple as the format of the Periodic Table may seem today (it can be printed on an A4 sheet of paper), it is the result of more than a century of work by various scientists and of humans’ constant curiosity about what makes up the visible and invisible world.
The creation and construction of the chemistry bible
The International Year of the Periodic Table marked 150 years since Dmitri Mendeleev published his columnar classification of elements with similar properties. Initially, the Russian chemist used atomic mass as a criterion for organisation. However, in 1913, British physicist Henry Moseley used X-rays to discover the atomic number (the number of protons in the nucleus of an atom), and the table took on its current configuration: the numbers of the elements, from 1 (hydrogen) to 118 (oganesson), correspond to their proton charge.
However, many other names contributed to the story behind the discovery of the chemical elements. In the 5TH century BC, the Greek philosopher Empedocles developed an initial theory that everything in nature was based on four main elements: Earth, Water, Fire and Air. Then, around 800 BC, the Arab alchemist Jabir ibn Hayyan isolated arsenic and antimony, although it took until 1669 for the first chemical discovery of an element, phosphorus, to be made by the German alchemist Henning Brand.
In 1789, Antoine Lavoisier tried to group the elements into metals and non-metals and, 40 years later, the German physicist Johann Wolfang Döbereiner observed similarities in the chemical and physical properties of certain elements, which he put together in groups of three. In 1860, the first international chemistry conference in Germany published a list of elements and their atomic masses, with the consensus that hydrogen would be given an atomic weight of 1 and that the other elements would be decided by comparison.
Sir William Ramsay added the noble gases – so called because they are found in nature in isolated form, and it is very rare to see them combined with other elements – to Mendeleev's table at the end of the 19TH century. In 1945, new radioactive elements were discovered as part of the Manhattan Project. And finally (or maybe not), in 2016, four new names were added: tennessine (Ts), nihonium (Nh), moscovium (Mc) and oganesson (Og).
A journey that is not over yet
The most recent version of the Periodic Table is from 2022, on the recommendation of the International Union of Pure and Applied Chemistry (IUPAC), the globally recognised authority on developing standards for naming chemical elements and compounds. However, it may not be the last, given the creation of artificial elements, which began in 1940 with neptunium and plutonium, and account for 28 elements.
The most recent discoveries were synthesised in the laboratory and these elements are called superheavy because they contain a much higher number of protons in their nucleus than the chemical elements found in nature, at least on Earth. Nihonium has the atomic number 113 and was discovered in 2003, the same year as moscovium, which has the number 115. Tennessine, created in 2010, is number 117 and oganesson, synthesised in 2002, has the number 118, the last in the table, which means that it is the known substance with the most protons in its nucleus. They are all highly radioactive elements that disintegrate so quickly that researchers only have seconds to study them. The most stable of the atoms of moscovium, for example, lasts 0.65 seconds.
The search for new elements continues, but it becomes increasingly difficult to synthesise them because although it is possible to add more protons to existing elements, when this happens, they tend to become more and more unstable. There were attempts to synthesise new elements that go as far as the 127th element, called ununseptium, but they are still merely hypothetical. However, studies with computerised models suggest that it may be possible for a 173rd element to exist. After this number, the nucleus would collapse, creating quantum chaos.
While we wait for new substances to emerge that will require a revision of the Periodic Table, it is still important to celebrate the legacy of the existing one. After all, as UNESCO puts it, it is "an essential window to the universe that helps expand our knowledge of the world around us".
