The Periodic Table

By the 1800s dozens of elements were known but in no useful order. Dobereiner spotted triads and Newlands spotted that every eighth element repeated (Law of Octaves), but neither pattern covered all elements, so chemists kept searching for the true rule.

In 1869 Mendeleev arranged elements by increasing atomic mass and put look-alikes in the same column. His genius was to leave gaps for elements not yet found and predict their properties; gallium and germanium turned up almost exactly as predicted.

In 1913 Moseley showed that atomic number, not mass, is the real basis for the order. The Modern Periodic Law states that the properties of elements are a periodic function of their atomic number. This fixed the few mass-order anomalies in Mendeleev's table.

Every trend comes from two ideas: more protons pull electrons in harder, and more shells push outer electrons further out. So across a period atoms shrink and pull electrons more strongly; down a group atoms grow and lose electrons more easily, which is why Group 1 metals get more reactive downwards.