public health system. We continue to work to keep COVID-19 out, but we are ready if it arrives in our community. 2.1 Early Ideas in Atomic Theory By the end of this section, you will be able to: State the postulates of Dalton’s atomic theory Use postulates of Dalton’s atomic theory to explain the laws of definite and multiple proportions The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers, the scientists of their day. In the fifth century BC, Leucippus and Democritus argued that all matter was composed of small, finite particles that they called atomos, a term derived from the Greek word for “indivisible.” They thought of atoms as moving particles that differed in shape and size, and which could join