More about Dalton's atomic theory John Dalton , ca. 1803 Elements are composed of atoms with characteristic masses. Compounds are composed of atoms combined in small, whole number ratios. Relationship between elemental composition and atom ratios in molecules. If C and O atoms weigh 12 and 16 units, respectively, the atom ratios in molecules convert to the element mass percentages in compounds.
Chemical reactions rearrange connections between atoms.
Hypothetical properties of the electron. How J. J. Thomson used properties of cathode rays to hypothesize properties of the electron. Subatomic particles important in chemistry. All nuclei with more than 83 protons are unstable. The first mass spectrograph was built in 1919 by F. W. Aston , who received the 1922 Nobel Prize for this accomplishment.