Dmitri Mendeleev, a Russian chemist, is widely credited with developing the first widely accepted version of the periodic table in 1869. The mid-19th century was a time of rapid discovery, with more than 60 elements identified, but no overarching system existed to organize them. Chemists needed a systematic way to classify these building blocks of matter to understand the relationships between them. Mendeleev’s work provided this much-needed framework, transforming chemistry from a collection of facts into a predictive science.
Arrangement by Atomic Weight
Mendeleev’s primary organizing principle was to arrange the known elements in sequential order based on their increasing atomic weight. Atomic weight, or atomic mass, was the most reliable quantitative measurement available to scientists at the time. He wrote the properties of each element onto separate note cards, allowing him to physically sort and rearrange them until a pattern emerged. By ordering the elements from lightest to heaviest, he observed that certain chemical similarities reappeared at regular intervals. This recurrence of properties became known as the periodic law, which formed the basis of his system.
Grouping Elements by Shared Properties
While atomic weight provided the initial sequence, Mendeleev’s true genius was recognizing and prioritizing the concept of chemical periodicity. He broke the sequence of elements into rows, called periods, and aligned these rows so that elements with similar chemical behaviors fell into vertical columns, known as groups. Elements in the same column demonstrated comparable characteristics, such as reactivity, valence, and the formulas of their compounds with oxygen and hydrogen. For instance, elements like lithium, sodium, and potassium, which form similar compounds, naturally aligned in a single group.
This prioritization of chemical similarity meant Mendeleev did not strictly adhere to the increasing atomic weight order in every case. He occasionally reversed the placement of two elements to ensure they were grouped with others that shared their properties. A notable example is his decision to place tellurium (atomic weight 127.6) before iodine (atomic weight 126.9), because iodine’s properties clearly aligned with the halogens (fluorine, chlorine, and bromine). This choice demonstrated his conviction that the periodic recurrence of chemical properties was a more fundamental principle than the numerical order of atomic weights.
The Genius of Leaving Gaps
A revolutionary aspect of Mendeleev’s table was his willingness to leave empty spaces within the sequence of elements. When a known element’s atomic weight did not align with the chemical properties required for a particular spot, he concluded that an undiscovered element must belong there. He boldly predicted the existence and detailed properties of these missing elements, naming them using the Sanskrit prefix “eka-” (meaning “one”). For example, he predicted “eka-aluminum” and “eka-silicon,” extrapolating their atomic masses, densities, and chemical reactivity based on the properties of their neighbors.
The subsequent discovery of these elements served as a powerful validation of his entire system. Gallium, discovered in 1875, matched the predicted properties of eka-aluminum almost perfectly, including an atomic mass of about 68. Similarly, the discovery of germanium in 1886 confirmed the predictions for eka-silicon. These accurate forecasts transformed the table from a mere classification scheme into a predictive tool, cementing its acceptance in the scientific community.
The Shift to Atomic Number
Despite the success of the atomic weight arrangement, a few anomalies, such as the tellurium-iodine reversal, remained unresolved. The underlying reason for the periodic trends was finally revealed over four decades later with the work of physicist Henry Moseley in 1913. Moseley used X-ray spectroscopy to determine the actual number of positive charges in the nucleus, which he defined as the atomic number. His experiments established that the atomic number, which represents the number of protons, is the fundamental property distinguishing one element from another.
Moseley’s findings provided the physical basis for the periodic law, demonstrating that the chemical properties of elements are a periodic function of their atomic number, not their atomic weight. Arranging the elements by increasing atomic number naturally corrected the few remaining order discrepancies in Mendeleev’s table, such as the placement of argon and potassium. The modern periodic table is therefore ordered by increasing atomic number, confirming the structure Mendeleev created while providing the correct physical principle behind it.