Final answer:
Democritus introduced the concept of indivisible particles called 'atomos' without empirical evidence, whereas John Dalton developed an experimentally backed atomic theory that described atoms as the smallest part of an element with unique mass and properties that determine how they combine in compounds.
Step-by-step explanation:
The concept of the atom has evolved significantly from the time of the ancient Greek philosopher Democritus to the scientific era of John Dalton. Democritus, who developed ideas in the 5th century BCE, speculated that all matter was composed of indivisible particles called atomos, and he believed that the properties like color and taste were a result of the different combinations and connections of these atomos. Changes in matter were thus due to the rearrangement of these particles.
Fast forward to the early nineteenth century, John Dalton, a British scientist, significantly advanced the atomic concept by anchoring it in quantitative experiments. Dalton's atomic theory was backed by experimental evidence, such as the formulas of simple chemicals and the behavior of gases. He posited that all elements are composed of atoms, and that these atoms were the smallest indivisible parts that maintain the identity of an element, characterized by mass and combining ratios in compounds. Dalton's theory represented a substantial development in the understanding of the atom because it moved beyond philosophical speculation to a theory grounded in empirical data.