Antoine Lavoisier, born August 26, 1743, French chemist and one of the fathers of modern chemistry. He brought chemical experimentation from the philosophical alchemists to the scientific chemists. He stated the first version of the law of conservation of mass, co-discovered, recognized, and named oxygen (1778), as well as hydrogen, disproved the phlogiston theory, introduced the metric system, wrote the first extensive list of elements, and helped to reform chemical nomenclature all before being beheaded on 8 May 1794 in Paris, at the age of 50.
Amedeo Avogadro, born August 9, 1776, was an Italian chemist known for his work on the theory of molarity and molecular weight. While he didn’t discover the number of particles in one mole of a substance, 6.022×1023, is now know as Avogadro’s number.