It was against this backdrop that in 1897 Marie Skłodowska Curie started her thesis on the origin and properties of “uranic rays” discovered by Becquerel. The Search for New Natural Elements through Atomic Properties In red are the missing radioelements with mass lower than uranium. Discovered were Ge in 1896, Ne, Kr and Xe in 1898, Hf and Re in 19. In yellow are the missing nonradioactive elements. All lanthanides were known except Pm (radioactive) and Lu discovered in 1907 (only La could be presented). However, unifying and generally accepted chemical concepts were still missing. Until this point in late 1895, chemistry was still much less developed than physics despite the existence of a chemical industry (acids, bases, salts, glasses, metallurgy, colorants, pharmacy, and perfumery), rapidly expanding chemical knowledge, and chemical theories for certain fields. Still, this classification was purely empirical.

In 1895, 80 elements already had been identified (see figure below). A final proof of the validity of the Mendeleev concept was the discovery of the elements gallium in 1875, scandium in 1879, and germanium in 1886. With a limited number of empty places in the chart, Mendeleev predicted the existence of yet-undiscovered elements, such as eka-aluminium and eka-silicium, and their expected properties. Mendeleev developed a chart showing that homologue elements have large differences in atomic weights and different elements of similar atomic weight exhibit large differences in properties. Significant advances were achieved by Dmitri Mendeleev in 1869 and Julius Lothar Meyer in 1870 in ranking the nearly 60 known chemical elements according to a periodic law, linking relative atomic weights of the elements to their chemical properties. However, the theory of atomism in chemistry was accepted with difficulty. The atoms were too small to measure their weight directly, but relative atomic weights could be determined starting with hydrogen as the lightest one. Around 1805–1808, following John Dalton’s work, a basic scientific concept emerged which held that each chemical element was ultimately composed of hard, solid particles (atoms) of specific, invariable mass (atomic weight), and that all substances were composed of such atoms. The experimental chemistry of elements, substances which cannot be decomposed and which combine in fixed ratios, was developed by Antoine Lavoisier. Chemistry after the Discoveries of Polonium and Radium

Isolated radium in 1910