Who was the first to state the concept of an atom?

Leucippus(theory) and Democritus (Atomic model).

They were Greek philosophers and the first proponents of the atomic theory. Leucippus first proposed it and Democritus adapted it but it was opposed in the 5th century BC. It was Democritus who developed the first atomic model, which was a round sphere without any other components. 

List of guarantees for people who get medical care.