Dmitri Mendeleev published the first widely accepted version of the Periodic Table of Elements in 1869, which organized the 63 known elements according to two primary principles: increasing atomic weight and the regular recurrence of chemical properties, known as periodicity. This arrangement demonstrated that the properties of elements were not random but followed a predictable pattern. Mendeleev’s great insight was not simply in classifying the elements that were already known, but in realizing that his newly created system was incomplete. The table contained deliberate, empty spaces that suggested the existence of elements yet to be discovered.
The Logic Behind the Gaps
Mendeleev’s methodology for creating the periodic system was rigorous, demanding that elements with similar chemical behaviors align in the same vertical columns, or groups. When he ordered elements by atomic weight, he found that sometimes the next heaviest element did not possess the required chemical properties to fit into the next available spot in the sequence. To maintain the integrity of the chemical groupings, he chose to leave a gap, asserting that an undiscovered element belonged there instead.
He used a systematic naming convention for these placeholder elements that relied on the Sanskrit prefix “Eka,” meaning “one”. The prefix indicated that the predicted element was positioned one place down from a known element in the same group. For example, a missing element below Boron was named Eka-Boron, and one below Aluminum became Eka-Aluminum. This naming system allowed him to communicate the precise location and anticipated chemical properties of the missing element to the scientific community.
The Prediction of Eka-Aluminum (Gallium)
Eka-Aluminum’s prediction became one of the most compelling pieces of evidence supporting the validity of Mendeleev’s system. He placed this missing element directly under aluminum and forecast its characteristics: an atomic weight of approximately 68, a density of about 5.9 grams per cubic centimeter, and a low melting point. He also anticipated its oxide formula would be Ea2O3.
In 1875, just four years later, French chemist Paul-Émile Lecoq de Boisbaudran discovered Gallium using spectroscopic analysis. Gallium’s properties were an almost perfect match for Mendeleev’s forecasts.
Gallium’s measured atomic weight (69.7) and density (5.91 g/cm³) closely aligned with the predictions (68 and 5.9). Gallium also had a low melting point of 29.8 degrees Celsius. The compounds, such as the oxide Ga2O3, followed the predicted formulas, confirming the accuracy of Mendeleev’s theoretical framework.
The Prediction of Eka-Silicon (Germanium)
The prediction of Eka-Silicon, placed directly below silicon, provided another significant test of the periodic law. Mendeleev predicted it would be a dark gray metal with an atomic weight of about 72 and a density of 5.5 grams per cubic centimeter. He detailed its compounds: the oxide (EO2) would have a density of 4.7 g/cm³, and the volatile chloride (ECl4) would have a density of 1.9 g/cm³ and a low boiling point.
In 1886, German chemist Clemens Winkler isolated Germanium from the mineral argyrodite. Germanium’s properties confirmed Mendeleev’s predictions, including an atomic weight of 72.6 and a density of 5.32 grams per cubic centimeter.
The properties of Germanium’s compounds were consistent with the forecasts: the oxide, GeO2, had a density of 4.70 g/cm³. The chloride, GeCl4, was a volatile liquid with a boiling point of 86 degrees Celsius and a density of 1.88 g/cm³. The discovery of Germanium, following Gallium, solidified the acceptance of Mendeleev’s periodic system.
The Prediction of Eka-Boron (Scandium)
Mendeleev predicted Eka-Boron would occupy the space below boron, having an atomic weight close to 44 and a density near 3.5 grams per cubic centimeter. He anticipated it would exhibit properties characteristic of its placement, such as forming specific types of salts.
The element was discovered in 1879 by Swedish chemist Lars Fredrik Nilson, who named it Scandium. Scandium had an atomic weight of 44.95, close to the predicted value of 44. The discovery of Scandium, along with Gallium and Germanium, provided the third major pillar of evidence validating Mendeleev’s Periodic Law.
Although the predictions for Eka-Boron were slightly less precise than those for Eka-Aluminum, Scandium’s overall characteristics confirmed the established pattern. The successful identification of Scandium demonstrated that Mendeleev’s table was a powerful predictive framework for uncovering the fundamental order of the elements.