Dmitri Mendeleev, a Russian chemist, faced the challenge of organizing the approximately 63 elements known in 1869 into a logical system. He sought a framework that moved beyond a simple list, one that could reveal underlying relationships between the elements. Arranging the elements in increasing order of atomic weight provided a starting point, but it was insufficient for a flawless, predictive system. Mendeleev recognized the periodic law: chemical properties repeat at regular intervals. This insight led him to develop a table that grouped elements with similar behavior, requiring a more sophisticated organizing principle than mass alone.
Beyond Atomic Weight: The Importance of Valence
Atomic weight served as the primary method for ordering the elements, yet Mendeleev observed that chemical behavior sometimes contradicted this sequence. He realized that a more fundamental property must be dictating the elements’ family groupings and repeating patterns, which he termed “periodicity.” This property was the element’s combining capacity, or valence, defined by the fixed ratios in which elements form compounds.
Mendeleev meticulously studied the stable compounds that elements formed with the two most reactive elements, oxygen and hydrogen. These elements readily combined with almost every other element, making them universal reference points. The consistent chemical formulas of these compounds indicated how many atoms of one element bonded with another, providing a numerical value for valence. By prioritizing this chemical behavior over strict atomic weight, Mendeleev established a second, more reliable principle for his classification system.
Defining Groups Through Hydride Formulas
The most direct way Mendeleev used chemical formulas to define his vertical groups was through the stoichiometry of an element’s maximum hydride. A hydride is a compound where an element combines with hydrogen, and the resulting formula directly indicated the group number of the element. For instance, elements in Group I, such as lithium (Li) and sodium (Na), consistently formed hydrides with the formula \(\text{RH}\) (e.g., \(\text{LiH}\)), where R represents the element.
Moving across the table, elements in Group IV, like carbon (C) and silicon (Si), formed hydrides with the consistent formula \(\text{RH}_4\) (e.g., \(\text{CH}_4\) and \(\text{SiH}_4\)). Elements in Group V, such as nitrogen (N) and phosphorus (P), formed \(\text{RH}_3\) (e.g., \(\text{NH}_3\)). Mendeleev tracked the maximum valence through this progression of formulas (\(\text{RH}\), \(\text{RH}_2\), \(\text{RH}_3\), and \(\text{RH}_4\)). He also used a parallel system based on oxides (e.g., \(\text{R}_2\text{O}\), \(\text{RO}\), \(\text{R}_2\text{O}_3\), \(\text{RO}_2\)) to classify elements that did not readily form stable hydrides. This dual approach confirmed the periodicity of chemical properties that defined his vertical columns.
Using Formulas to Predict and Adjust Element Placement
The consistency of the hydride and oxide formulas gave Mendeleev the confidence to treat his table not just as a record of known elements but as a predictive tool. When he encountered a situation where a known element’s atomic weight suggested one position but its chemical formula suggested another, he trusted the chemical properties. The most famous example of this was the reversal of tellurium (Te) and iodine (I).
Tellurium had a slightly higher atomic weight than iodine, which would have placed it after iodine in a strictly weight-ordered table. However, tellurium formed an oxide with the formula \(\text{TeO}_3\), aligning it with Group VI elements oxygen and sulfur. Iodine formed compounds similar to the Group VII elements chlorine and bromine, leading Mendeleev to reverse their positions.
The gaps in the table where a specific hydride or oxide formula was missing allowed Mendeleev to predict undiscovered elements. For example, he left a space under silicon, which was in the \(\text{RH}_4\) group, and predicted a new element he called Eka-Silicon. He forecast that this element would form a hydride (\(\text{EsH}_4\)) and an oxide (\(\text{EsO}_2\)). When germanium was discovered in 1886, its properties, including its oxide formula \(\text{GeO}_2\) and a volatile chloride \(\text{GeCl}_4\), matched Mendeleev’s predictions almost perfectly.