While the concept of atoms had been around for centuries, it was John Dalton in the early 19th century who offered the first scientifically compelling evidence for their existence.

He developed Dalton's atomic theory which included the following key points:

* All matter is composed of atoms, which are indivisible and indestructible.

* Atoms of a given element are identical in size, mass, and other properties.

* Atoms of different elements differ in size, mass, and other properties.

* Chemical reactions involve the combination, separation, or rearrangement of atoms.

Dalton's theory was based on experimental observations and explained many chemical phenomena. While not all aspects of his theory have stood the test of time (for example, we now know that atoms can be divided into subatomic particles), his work provided a crucial foundation for the modern understanding of atoms.