Jöns Jacob Berzelius (August 20 1779-July 16 1848). Swedish chemist, inventor of modern chemical notation, and the discoverer of selenium, thorium, and cerium. Along with John Dalton and Antoine Lavoisier, considered one of the fathers of modern chemistry.
Trained as a medical doctor at the University of Uppsala, in 1807 he became a professor at the University of Stockholm. Not long after arriving there he wrote a chemisty textbook for his medical students, from which point a long and fruitful career in chemistry began. While conducting experiments in support of the textbook he discovered the Law of Constant Proportions, which showed that inorganic substances are composed of different elements in constant proportions by weight. From this, by 1828 he compiled a table of the relative atomic weights (with oxygen set to 100) of all elements then known. Taken together, this work was a strong confirmation of the atomic hypothesis; that inorganic chemical compounds were composed of atoms combining in whole number amounts. In discovering that the atomic weights were also not integer multiples of hydrogen's, Berzelius also disproved Prout's Hypothesis that elements were built up from atoms of hydrogen.
In order to aid his experiments, he developed a system of chemical notation in which the elements were given simple written labels -- such as O for oxygen, or Fe for Iron -- and proportions were noted with numbers. This is the same basic system as is used today, the only difference being that where we would use a subscript number (i.e., H20), Berzelius would use a superscript.