A scientist named Johann Döbereiner discovered trends in the reactivity of groups of elements. Certain sets of three elements, such as lithium, sodium, and potassium, showed similar chemical properties, and Döbereiner noticed that the average of the atomic masses of the atoms of the heaviest and lightest elements in the triad gave the atomic mass of the midweight element. For example, the mean of the atomic masses of lithium and potassium is (3 + 19)/2 = 11, which is the atomic mass of sodium. Due to differing numbers of neutrons in each element and the existence of different isotopes, this law isn’t always strictly true, but it does tend to work well, especially for lighter elements. For reasons we hope to explain in the coming questions, this trend plays an important role in the structure of the periodic table.