In 1913, Frederick Soddy described isotopes, which were forms of an atom of one element that contained different numbers of neutrons. Several discoveries expanded the understanding of atoms. The model and its validation in 1908 by Jean Perrin supported atomic theory and particle theory.īohr's model explained the spectral lines of hydrogen but didn't extend to the behavior of atoms with multiple electrons. In 1905, Albert Einstein postulated that Brownian motion was due to the movement of water molecules. Avogadro's law made it possible to accurately estimate the atomic masses of elements and made a clear distinction between atoms and molecules.Īnother significant contribution to atomic theory was made in 1827 by botanist Robert Brown, who noticed that dust particles floating in water seemed to move randomly for no known reason. In 1811, Amedeo Avogadro corrected a problem with Dalton's theory when he proposed that equal volumes of gases at equal temperature and pressure contain the same number of particles. His oral presentation (1803) and publication (1805) marked the beginning of the scientific atomic theory. He proposed that each chemical element consists of a single type of atom that could not be destroyed by any chemical means. Dalton's law of multiple proportions drew from experimental data. These theories didn't reference atoms, yet John Dalton built upon them to develop the law of multiple proportions, which states that the ratios of masses of elements in a compound are small whole numbers. Ten years later, Joseph Louis Proust proposed the law of definite proportions, which states that the masses of elements in a compound always occur in the same proportion. In 1789, Antoine Lavoisier formulated the law of conservation of mass, which states that the mass of the products of a reaction is the same as the mass of the reactants. It took until the end of the 18th century for science to provide concrete evidence of the existence of atoms.

Thomson's atomic theory