Dmitri Mendeleev published the first widely accepted version of the periodic table of elements in 1869, establishing a framework that organized the known chemical world. His arrangement was based on the “Periodic Law,” which stated that element properties recur periodically when arranged by increasing atomic weight. This work was not simply a catalog of the 63 elements known at the time, but a systematic tool that intentionally highlighted missing pieces. The table contained spaces where no known element fit the established pattern, prompting Mendeleev to predict the existence and properties of several undiscovered elements.
The Logic Behind the Gaps
Mendeleev’s primary organizing principle was to group elements with similar chemical behaviors into vertical columns, known as groups. He observed that as atomic weight increased, element properties repeated in a regular, cyclical fashion. This principle forced him to make a choice when an element’s atomic weight did not align with the chemical properties of the group it would naturally fall into.
To maintain the integrity of the chemical families, Mendeleev prioritized recurring properties over a strict ordering by atomic weight. This required him to leave deliberate gaps in the sequence. If an element’s atomic weight suggested it belonged in a certain spot, but its properties did not match the column, he left the space vacant. He reasoned that an undiscovered element must exist with the correct atomic weight and expected chemical behavior to fill that specific spot.
This systematic approach turned the periodic table from a mere classification scheme into a powerful predictive instrument. The gaps were not errors but calculated placeholders for future discoveries. By interpolating the properties of the known elements directly above and below a vacant space, Mendeleev could forecast the characteristics of the element yet to be found.
Naming the Missing Elements and Predicting Their Properties
To name his hypothetical elements, Mendeleev used the Sanskrit prefix eka, meaning “one,” to denote that the predicted element was one space below a known element in the same group. His three most famous predictions were Eka-Boron, Eka-Aluminum, and Eka-Silicon, which he detailed in his 1871 paper.
Eka-Aluminum was predicted to have an atomic weight of approximately 68 and a low melting point. Mendeleev calculated its density to be 5.9 grams per cubic centimeter and specified that it would form an oxide with the formula E₂O₃. He also anticipated that the element would be a soft metal and its chloride compound would have the formula ECl₃.
Eka-Silicon was the second major prediction, positioned below silicon in Group IV. He predicted an atomic weight of around 72 and a density of 5.5 grams per cubic centimeter. Since it was located in the carbon-silicon family, he predicted it would form a refractory oxide with the formula EO₂. He also suggested the element would be a dark gray metal or metalloid with a high boiling point. These forecasts provided a clear target for chemists to begin their search.
Proof of Concept: The Discovery of Eka-Elements
The validation of Mendeleev’s Periodic Law came with the discovery of elements that precisely filled his predicted gaps. The first success arrived in 1875 when French chemist Paul-Émile Lecoq de Boisbaudran discovered a new element, Gallium, using spectroscopic analysis. Gallium was found to have an atomic weight of 69.7 and a density of 5.91 grams per cubic centimeter, matching Mendeleev’s predictions for Eka-Aluminum with accuracy.
The similarities extended to chemical behavior: Gallium’s oxide was Ga₂O₃, exactly matching the predicted formula E₂O₃. The measured properties of Gallium, including its low melting point, were close to the forecasts, immediately elevating the status of Mendeleev’s table. Following this, Scandium was isolated in 1879 by Lars Fredrik Nilson, perfectly fitting the spot and properties predicted for Eka-Boron.
The final confirmation came in 1886 with the isolation of Germanium by Clemens Winkler, which corresponded to Eka-Silicon. Winkler’s analysis showed Germanium had an atomic weight of 72.6 and a density of 5.32 grams per cubic centimeter. Its oxide formula was confirmed as GeO₂, aligning almost perfectly with Mendeleev’s prediction of EO₂. The successive discovery of these elements, whose properties closely mirrored the forecasts, transformed the periodic table from a theoretical classification into a confirmed map of the elements.