Comparing elements and their compounds

Mendeleev worked with elements that chemists had already discovered. He compared measured relative atomic masses and looked for repeating patterns in physical and chemical properties. He generally arranged elements by increasing relative atomic mass, but gave priority to keeping chemically similar elements together when the mass order disagreed.

A property is an observable or measurable characteristic: for example, density, melting point or how a substance reacts. Similar appearance alone is weak evidence; similar reactions provide stronger chemical evidence.

Compounds supplied another useful comparison. Mendeleev compared the composition and properties of compounds formed by different elements. Elements that formed similar compounds could be placed together even when the elements themselves looked different.

For example, suppose A and B have similar reactions and form oxygen compounds with similar properties, while C reacts differently and forms a different type of oxygen compound. Both lines of evidence support grouping A with B. The decision comes from comparing patterns, not from the names of the elements.

Using the evidence

Consider a second comparison. Two elements both look shiny, but their reactions and the properties of their compounds differ. Their appearance alone would not justify placing them in the same chemical group. Mendeleev considered several pieces of evidence together.