As previously mentioned, chemistry is distinct from physics in its focus on matter at the molecular level. The taxonomy of the elements is the method that provides the underlying language of the field, describing the basic, indivisible building blocks of the known matter of the universe. It is a list that intermittently expands as new elements are discovered – and our understanding of the fabric of our world incrementally increases.
A chemical element defines types of atoms that all share the same quantity of protons in their atomic nuclei. Hydrogen, for example, is classified as element No. 1, since hydrogen atoms contain a single proton. Carbon, by contrast, has six protons, and oganesson, 118. At the time of writing, a total of 118 elements have been discovered, of which 94 occur naturally and the remainder are synthetic. Most elements have a number of isotopes too – that is to say, variants where the number of neutrons in each atom changes even as the number of protons stays constant.
Elements such as gold, silver and forms of carbon, including diamonds, are relatively abundant in form. However, most elements appear in compounds – groups of different elements in which the atoms are attached by chemical bonds. The most famous compound is water (H20, where each molecule is formed of two atoms of hydrogen combined with one of oxygen).
In antiquity matter was often thought to be composed of a combination of four basic elements – air, earth, fire and water. Although mining and early extractive methods such as smelting required a fairly advanced grasp of elemental difference (for instance, between gold and copper), achieving an effective system of categorization has been a drawn-out (and ongoing) process.
Today, for instance, elements may be divided between conductive metals, non-conductive non-metals and semi-conductive metalloids. Alternatively, they may be categorized by their state (solid, liquid or gas) at a particular temperature and pressure, or according to whether they occur naturally or are the synthetic result of man-made nuclear reactions.
The periodic table
Undoubtedly the most famous mode of cataloguing is the periodic table, the creation of Russian chemist Dmitri Mendeleev and first published in 1869.
Containing sixty-six elements at that stage, the table arranges the elements into rows (periods) by increasing atomic number, while the columns group elements that share particular physical and chemical properties. Mendeleev also proved highly effective at predicting the nature of several then unknown elements. While virtually all of the known naturally occurring elements had been identified by 1900, the most recent additions (four of them) were added only in 2015. The table continues to be tweaked and expanded as new knowledge is acquired. Kosuke Morita, who led a Japanese team credited with finding one of the newest ones (element 113), immediately looked to the future, explaining that he now planned to ‘look to the uncharted territory of element 119 and beyond’.
While the elements make up all of the universe’s ‘ordinary matter’, it is thought that some 95 per cent of the universe comprises unobserved ‘dark matter’ and ‘dark energy’ (see here) consisting of something other than the chemical elements we list in the periodic table.