Final answer:
John Dalton, an English chemist, proposed the modern atomic theory around 1803, establishing foundational concepts about the composition of matter and behavior of atoms.
Step-by-step explanation:
The scientist who came up with the modern atomic theory is John Dalton, an English chemist. Dalton proposed this theory around 1803, basing it on experimental results rather than philosophical speculation. His theory introduced several key concepts: that all elements are composed of atoms, which always combine in fixed ratios, and that chemical compounds always contain the same proportion of elements by mass. Despite later discoveries that disproved some aspects of Dalton's theory, such as the indivisibility of atoms and the identical nature of all atoms of an element, his work remains a foundational step in the development of atomic theory.