Dmitri Mendeleev, a Russian chemist, developed the periodic table in 1869, providing a foundational structure for chemistry. His work organized the known chemical elements into a coherent system that revealed underlying relationships between them. Where previous attempts at classification were incomplete, Mendeleev established a definitive order. This organization demonstrated a fundamental law of nature: the properties of elements follow a distinct, repeating pattern. The resulting table remains the basis of modern chemical study.
The State of Chemistry Before Mendeleev
Before the 1860s, chemistry lacked a unified system to explain elemental behavior. Scientists had identified approximately 63 elements, but there was no consensus on their accurate atomic weights or how they should be classified. This lack of a standardized framework made it difficult to predict the properties of newly discovered substances.
Several earlier attempts were made to find an organizing principle. German chemist Johann Döbereiner proposed the Law of Triads in 1829, grouping elements like chlorine, bromine, and iodine based on similar chemical properties. Döbereiner noted that the atomic weight of the middle element in a triad was often the average of the other two.
Later, in 1864, English chemist John Newlands arranged the elements by increasing atomic weight and observed a repetition of chemical properties every eighth element, calling it the Law of Octaves. However, Newlands’ system failed for heavier elements, forcing him to place dissimilar elements into the same group. These efforts showed that a pattern existed, but the governing principle remained undefined.
Assembling the Elements
Mendeleev focused on two measurable characteristics: atomic weight and chemical properties. He created a card for each known element, noting its atomic weight and specific properties like valency and reaction patterns. This tactile method allowed him to constantly rearrange and compare the elements, searching for patterns.
He established atomic weight as the primary ordering principle, arranging elements in sequential rows from lightest to heaviest. The breakthrough occurred when he realized that chemical properties showed a regular, repeating nature. When elements with similar properties, such as the alkali metals, were stacked vertically, a pattern emerged that he termed “periodicity.”
Mendeleev used this periodicity to construct the table, placing elements with analogous chemical behavior into the same vertical columns, or groups. This organization meant that properties changed gradually across a row, but repeated when starting the next row. For instance, he placed lithium, sodium, and potassium in the same column because they were all highly reactive metals.
The Power of Prediction
Mendeleev prioritized the periodic pattern of chemical properties over strict adherence to atomic weight. He recognized that some elements’ atomic weights would place them in chemically dissimilar groups. To maintain the integrity of the vertical groups, he intentionally swapped the order of elements like tellurium and iodine, ensuring each fell into the correct chemical family.
This willingness to violate the atomic weight sequence allowed him to leave deliberate gaps in his table. He reasoned that these empty spaces must correspond to elements that had not yet been discovered. He used the properties of surrounding known elements to predict the specific characteristics of these missing substances.
For Eka-aluminum, positioned below aluminum, Mendeleev predicted an atomic weight of approximately 68 and a density of 5.9 grams per cubic centimeter. He forecasted its oxide would have the formula E2O3. For Eka-silicon, located below silicon, he predicted an atomic weight of about 72 and a density of 5.5 grams per cubic centimeter, forming an oxide with the formula EO2. These detailed predictions were the ultimate test of his system.
Confirmation and Scientific Acceptance
Mendeleev’s table was initially met with caution, as it challenged accepted atomic weight values and included placeholders for hypothetical elements. However, the discovery of a new element in France six years later provided the first confirmation of his theory. In 1875, Paul-Émile Lecoq de Boisbaudran discovered gallium.
When gallium’s properties were measured, they aligned almost perfectly with Mendeleev’s predictions for Eka-aluminum. Gallium was found to have an atomic weight of 69.7 and a density of 5.91 grams per cubic centimeter, forming an oxide with the formula Ga2O3. This close correspondence was too precise to be coincidence.
The discovery of a second element, germanium, in 1886 by Clemens Winkler provided further compelling evidence. Germanium’s measured properties, including an atomic weight of 72.6 and a density of 5.32 grams per cubic centimeter, were nearly identical to the predictions made for Eka-silicon. The success of these predictions silenced the critics and established the periodic table as the fundamental organizing principle of chemistry.