Explain how the interpretations of Avogadro led to an understanding of the diatomic nature of molecules such as hydrogen, oxygen, nitrogen and chlorine and to the formulae of the compounds hydrogen chloride, water and ammonia.
Amedeo Avogadro was an Italian chemist in the late 18th/early 19th century. His work with gasses lead to Avogadro's Law which states that different volumes of different gasses at constant temperature and pressure will have different masses. In other words, one liter of hydrogen will have a different weight or mass than one liter of ammonia. This is due to the fact that different gasses have different molecular weights, so he essentially developed the notion of molecular weight. He was also one of the first chemists to distinguish between atoms and molecules by saying that gasses were composed of molecules which in turn were composed of different atoms fused together in unique combinations. When combined with Gay-Lussac's Law about the ratios of elements to one another, this helped scientists to determine which individual elements composed various gasses that they commonly studied like hydrogen, oxygen, ammonia, and water vapor. They were also able to determine the ratios of these elements to one another and determine the molecular formulas of these gasses.
The number that bears his name (Avogadro's number) is a constant that denotes the number of atoms in one mole of a substance (6.022 x 10^23). Avogadro did not actually discover this number. It was named in his honor in the early 20th century.