I'm basing my answer according to your question. The person(s) to give the concept of an atom and molecule is Democritus from Greece and Mahashri Kanada from India. However,the first person to officially write down the theory,is the English scientist John Dalton. But most of Dalton's atomic postulates were wrong,so you can say that it JJ Thomson was the actual person to give the correct atomic theory. I hope I am able to help you.