When, in 1899, the close friend and colleague of Marie and Pierre Curie, André Louis Debierne, announced the discovery of actinium, he could not imagine that this element would become the first of a group of key elements for understanding atomic structure. A theoretical basis, an initially empirical periodic table, for the systematic study of the atom had been laid down many years earlier by Dmitri Ivanovich Mendeleev in 1869. It turned out to be a tool that could be used to predict what Mendeleev discerned as “missing elements,” elegantly confirmed by the discoveries of gallium in 1875, scandium in 1879, and finally germanium in 1886. Over the course of the following 60 years, a series of discoveries were made that began to reveal the modern picture of the structure of the atom. In chronological order, these were cathode rays, emission spectra, canal rays (protons), X‐rays, radioactivity, the electron, α, β, and γ rays, Planck's law, the photoelectric effect, the atomic nucleus, isotopes, Bohr model of atomic structure, atomic number, and the neutron. It gradually became clear that the number of nuclear protons equaled the nuclear charge and conferred on each atom its unique identity. This allowed scientists to determine how many elements existed in nature, namely 92. It also allowed them to devise experiments to push the envelope beyond 92—to actually create new elements by bombarding and combining the existing atomic nuclei, thus expanding the original periodic table to 118 elements. The impact of these discoveries has changed the course of history. The story of Debierne's discovery, actinium, and the 29 elements that follow it are the subject of this article and this volume.
