John Dalton formulated his atomic theory in the early 1800s based on careful chemical measurements and observations. His theory proposed that (1) all matter is composed of tiny indivisible atoms, (2) atoms of the same element are identical in mass, (3) atoms of different elements have different masses, and (4) atoms combine in simple whole number ratios. Dalton's atomic theory provided the foundation for modern chemistry and physics by establishing that elements combine at the atomic level. While Dalton's model has been refined over time, it represented a revolutionary shift in understanding the basic nature of matter.