Mendeleev and the empty-box bet

A classification based on properties, not just mass

In 1869, the Russian chemist Dmitri Mendeleev devised his periodic table by arranging the known elements in order of increasing atomic mass, whilst grouping them into families with similar properties. Only around sixty elements were known at the time.

The stroke of genius: leaving spaces empty

To ensure that his classification by groups remained consistent, Mendeleev had to leave spaces empty in his periodic table, in places where no known element matched the expected properties. Rather than forcing an existing element into the wrong slot, he chose to bet that a yet-unknown element would fill the gap.

Predictions verified

Mendeleev went a step further: he predicted the precise properties of the missing elements (atomic mass, density, reactivity) based on the position of the empty slot in the table. A few years later, the discovery of gallium (1875), scandium (1879) and then germanium (1886) confirmed his predictions with remarkable accuracy, which definitively validated his approach.

Why this is important in science

This episode illustrates a central idea of the scientific method: a good model does not merely describe what we already know; it must also enable us to predict what we do not yet know, and these predictions must be verifiable through experimentation.

A common misconception

It is sometimes believed that Mendeleev ‘invented’ the missing elements at random. In reality, his predictions followed logically from the position of the empty slot in the table: knowing the column (group) and the row (period) was sufficient to anticipate the chemical properties of the element to come.