John Dalton, in the late 1700's is credited with first relating observed chemical phenomena to the idea of individual atoms. His theory stemmed from his study of combining substances through chemical reactions to make new substances. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atoms of different elements can combine with one another in simple whole number ratios to form compounds.
Chemical reactions occur when atoms are separated, joined, or rearranged.
However, atoms of one element are not changed into atoms of another by a chemical reaction. Though even more recent experimentation has revealed fundamental particles beyond electrons, protons, and neutrons, the two models mentioned in the Virginia Science Standards of Learning for Physical Science, require familiarity only with these three particles. Electrons were first discovered by scientists who were more interested in electricity than atomic structure. In 1897, he found cathode rays could be deflected by magnets or electrically charged plates. Through his experiment, Thomson showed cathode rays are collections of small negatively charged particles, which he named electrons .