Dmitri Mendeleev published his periodic system of elements in 1869, at a time when chemistry was struggling to organize the roughly 63 elements known to exist. Many scientists had attempted to find a logical arrangement, but none had produced a system that could consistently account for all known substances. Mendeleev’s enduring success came from his unique, multi-layered methodology that went beyond simple classification. His approach not only brought coherence to the elements but also demonstrated a profound underlying order in nature.
The Foundation: Ordering by Atomic Weight
Mendeleev’s initial step involved a practical, hands-on approach to organization. He created a set of index cards, one for each of the known elements at the time. On each card, he meticulously recorded the element’s symbol, its measured atomic weight, and its various chemical and physical properties, such as density and how it reacted with oxygen and hydrogen. He began his arrangement by listing the elements primarily in ascending order of their atomic weight. This metric served as the foundational organizing principle for his list of elements. The atomic weight provided a numerical sequence, starting with the lightest element, hydrogen.
Grouping Elements by Recurring Properties
The true insight occurred when Mendeleev moved beyond a simple linear list to a two-dimensional table. He observed that when elements were ordered by atomic weight, their chemical behavior did not change randomly, but instead showed a repeating pattern. This pattern, which he termed “periodicity,” was based on the recurrence of properties like valency, which describes an element’s combining power with others. By aligning elements that exhibited similar chemical characteristics, such as the type of compounds they formed, he created vertical columns, or groups, of chemically related elements. The horizontal rows represented the sequence of elements where properties gradually changed until the pattern repeated itself in the next row.
Predicting the Unknown: Gaps in the Table
Mendeleev’s most revolutionary action was his willingness to break the sequence and leave blank spaces in his developing table. He did this whenever the element whose atomic weight placed it next in the sequence did not possess the chemical properties required to fit into the next available vertical group. Instead of forcing a mismatch, he concluded that an element was simply missing and had not yet been discovered.
He assigned provisional names to these missing elements using the Sanskrit prefix “Eka-,” meaning “one,” to indicate the element below a known element in the same group. For instance, he predicted the existence of Eka-Aluminum and Eka-Silicon. When the element gallium was discovered in 1875, its properties matched Mendeleev’s predictions for Eka-Aluminum with remarkable accuracy. Similarly, the discovery of germanium in 1886 confirmed the forecast for Eka-Silicon, validating his entire system and proving the power of the periodic law.
Prioritizing Properties Over Atomic Weight
Mendeleev demonstrated his confidence in the periodic law by deliberately violating his own ordering principle in a few instances. This occurred when the chemical properties of an element strongly suggested placement in a specific group, even if its measured atomic weight was slightly out of order. He believed that the chemical behavior of an element was a more fundamental characteristic than its numerical weight.
The most famous example involves Tellurium (Te) and Iodine (I). Tellurium had a higher atomic weight (approximately 127.6) than Iodine (approximately 126.9). However, Mendeleev placed Tellurium before Iodine because Tellurium’s properties aligned with the oxygen family, while Iodine’s properties matched the halogen family. This decision highlighted that the recurring pattern of chemical properties was the ultimate logic governing the table’s structure.