The periodic table: how to read it and why it's organized this way

The ordering rule: atomic number

The periodic table lists chemical elements in order of increasing atomic number, the number of protons packed into each atom’s nucleus. It wasn’t always organized this way. When Dmitri Mendeleev published his table in February 1869, the only tool available for comparing elements was atomic weight, an element’s average mass: he laid out cards for the known elements in order of increasing atomic weight and noticed that certain types of element recurred at regular intervals.

The real shift came in 1913, when the physicist Henry Moseley used X-rays to experimentally measure the atomic number of every element. Plotting the square root of the frequency of the X-rays each element emitted against its atomic number, he got a perfectly straight line: a measurable, repeatable method that explained why some elements needed to be reordered relative to Mendeleev’s weight-only classification. Since then, atomic number, the count of protons in the nucleus, has been the official criterion of the modern periodic table.

Periods and groups: rows and columns with a purpose

The table’s horizontal rows are called periods, and every period ends with a noble gas. The vertical columns are called groups, numbered 1 through 18: that numbering, now the worldwide standard, has been recommended by IUPAC since 1988, replacing older systems built on Roman numerals and letters such as A and B.

A group’s membership follows the atom’s own structure: elements sharing a column also share the same number of electrons in the atom’s outermost energy level, the valence electrons, and that’s what gives them similar chemical properties. Ground-state electron configuration, together with atomic number, is among the values that define each element and its place in the table — the same concept that comes up when talking about electrons in electric current.

Real-world example: when Mendeleev left a gap in his 1869 table, its position among the known elements let him predict the properties of an element still unknown, which he provisionally called eka-aluminium. In 1875 gallium was discovered, and its chemical properties matched the forecast almost exactly. The same happened by 1886 for scandium and germanium: three confirmations that earned Mendeleev’s table lasting recognition.

Before Mendeleev: nearly a century in the making

In 1789 Antoine Lavoisier had already grouped the known elements into four categories: gases, nonmetals, metals and “earths.” In 1829 Johann Döbereiner identified the first “triads” of elements with similar properties. In 1860 the Karlsruhe Congress supplied more accurate atomic-weight data, a technical groundwork that made later attempts possible: in 1862 Alexandre-Émile Béguyer de Chancourtois published a three-dimensional cylindrical layout, the telluric helix, which already showed the periodicity of elemental properties, and in 1865 John Newlands proposed the “law of octaves,” noting similarities between elements whose atomic weights differed by seven units.

Between 1864 and 1870 Julius Lothar Meyer produced several periodic tables: his 1868 version listed elements by atomic weight, with those of equal valence arranged in vertical columns, work developed independently and arriving at something nearly identical to Mendeleev’s, though published later. Mendeleev’s specific contribution was deliberately leaving gaps for undiscovered elements and predicting their properties, a predictive power none of the earlier attempts had managed to demonstrate.

A table still being updated

The International Union of Pure and Applied Chemistry (IUPAC), the world’s official authority on naming and classifying elements, keeps the periodic table current. The table’s last four elements, numbers 113, 115, 117 and 118, were officially named in November 2016, after their discoveries were verified by IUPAC together with the International Union of Pure and Applied Physics (IUPAP). The official table now runs up to element 118, oganesson.

The numerical data tied to each element gets revised too. The latest official release of the IUPAC table is dated 4 May 2022 and folds in the most recent standard atomic weight values, calculated by the Commission on Isotopic Abundances and Atomic Weights, an IUPAC body established in 1899. The same logic behind groups shows up elsewhere too: in tracing where the carbon plants use in photosynthesis comes from, or in the role carbon dioxide plays in the greenhouse effect.