Actinium's name, from the Greek for ray, was chosen to mark the powerful radiation it emits, so strongly that the pure element visibly glows in darkness from the ionisation of the surrounding air. Beyond its own properties, actinium lends its name to a whole family: the actinides, the fifteen elements from actinium through lawrencium, which include the naturally important thorium and uranium and the entirely synthetic transuranium elements, and which occupy the lower of the two detached rows beneath the main body of the periodic table.
Actinium has no ancient or alchemical tradition, being a product of the study of radioactivity at the turn of the twentieth century; its significance is that of a namesake and a marker, the ray-giving head of one of the great series of the modern table. The actinide series divides in practice into two very different halves: the first few members, actinium through uranium, occur naturally on Earth in workable if often minute quantities, while everything past uranium, the transuranium elements from neptunium onward, exists only because twentieth-century physicists learned to manufacture new nuclei in reactors and accelerators, a split that makes actinium's own header position a hinge between elements found in the ground and elements built entirely by human hands.