How it began
Early in 1869 the Russian chemist Dmitri Mendeleev was writing an inorganic chemistry textbook for his students at St. Petersburg University, because the only chemistry textbooks available in Russian were translations. He had finished the first volume and was struggling with how to organize the second. He wrote the elements with their atomic weights and typical properties on separate cards and concluded that the properties of the elements depend periodically on their atomic weights. In March 1869 he delivered a full paper to the Russian Chemical Society setting out this periodic law.
Before it existed
Scientists had identified more than 60 elements by Mendeleev's time, and the atom was considered the most basic particle of matter. Through much of the nineteenth century chemists had looked for an organizing principle that covered all the known elements. Mendeleev was far from the first to order the elements by atomic weight or to notice that characteristics recurred on some regular basis, but an overall understanding of how the elements related to one another was slow in coming.
How it works
Mendeleev listed the known elements in order of increasing atomic weight and arranged them in rows so that elements with similar chemical properties fell at regular intervals in the same column. The pattern appeared only if he left blanks, and he proposed that undiscovered elements would fill them, predicting their atomic weights and characteristics. Since Henry Moseley's work in 1913, elements have been ordered by atomic number, the number of protons in the nucleus, and the structure of the table reflects how electrons are arranged in each type of atom.
Who pushed back
Many scientists greeted Mendeleev's first table with skepticism. The discovery of the noble gases in the 1890s added a set of elements nobody had predicted, and the discovery of radioactivity in 1896 seemed poised to destroy the system, because elements had been defined as substances that could not break down into smaller parts. In both cases the system absorbed the new facts: the noble gases fit as a new group, and the study of atomic structure explained how the periodic system worked at the atomic level.
What changed first
The predictive value of the table soon became clear. Gallium was discovered in 1875, scandium in 1879 and germanium in 1886, each with properties that matched what Mendeleev had predicted for the gaps. The periodic system became a tool for research as well as an object of study, and claiming a new element came to mean finding its atomic weight so it could be fitted into the right slot. William Ramsay used the system to predict the atomic weights of further noble gases after finding argon and helium.
What lasted
The periodic table is recognized as one of the great achievements of chemistry and as a concept that unites the physical and life sciences. Its familiar classroom form owes much to Horace G. Deming of the University of Nebraska, whose table first appeared in his 1923 textbook and was spread by scientific supply companies as wall charts and handouts. By the 1950s versions of Deming's table could be found in a majority of chemistry textbooks.
Where it stands today
The United Nations declared 2019 the International Year of the Periodic Table to mark 150 years since the discovery of the periodic law. More than 110 elements are now known, and there are probably around 1,000 different published forms of the table, though the flat, castle-like layout remains the standard in classrooms and textbooks.
