1869
The Russian scientist Mendeliev established a tabular classification based on atomic mass, but in which the elements were arranged in horizontal 'periods' each containing eight elements; it was found that the resulting vertical columns or groups of elements in the classification showed significant similarities and trends in their properties. Mendeliev's extraordinarily imaginative classification, which at the time of its formulation necessarily contained gaps and inconsistencies, has subsequently been developed into the modern periodic table of the elements, one of the corner stones of contemporary chemistry.
1909
Rutherford's students Geiger and Marsden bombarded a thin gold foil with a beam of a-particles from a radioactive source; the scattered particles were detected by a fluorescent screen. As illustrated schematically on the left, the results were quite unexpected and extraordinary: whereas the vast majority of the particles went through the foil with only small or negligible deflections, a small proportion were deflected by large angles and a few bounced back! These observations could only be explained by postulating a model in which most of the atom was empty space, with a nucleus with a positive electrical charge and which contained nearly all the mass of the atom. The nuclear model, the basis of the modern understanding of atomic structure, was born.
1912
Lawrence Bragg analyzed diffraction patterns from crystals to deduce the position of atoms within crystal lattices. The first crystals studied had simple structures: NaCl (shown on the left), KCl and ZnS. By 1928 the technique revealed the structure of minerals and is now used to determine the structure of enzymes with and other macromolecules with thousands of atoms.
1913
Niels Bohr provided an explanation for atomic spectra based on quantised orbits for electrons. This model was the forerunner of quantum mechanics.
1926
Schrödinger discovered the basis of quantum mechanics in an equation describing electrons as waves. Quantum mechanics provides a simple rationale for chemical interactions but demands tremendous computational resources, Routine application of quantum mechanical methods has therefore followed the widespread availability of computers.
1953
Watson and Crick produced a double helix model for the structure of DNA based on the pairing of bases.
1965
The first enzyme structure determined using x-ray diffraction. For the first time the molecular basis of an enzyme catalysed reaction was revealed.