His exact contributions are difficult to disentangle from those of his mentor Leucippus, as they are often mentioned together in texts. Their speculation on atoms, taken from Leucippus, bears a passing and partial resemblance to the nineteenth-century understanding of atomic structure that has led some to regard Democritus as more of a scientist than other Greek philosophers. Democritus, wikipedia
I don't know if this is only in my home country or is widespread in the world. In here, Democritus is the person that's known to be the "pioneer" of atomic theory. However, Leucippus is well known to be his master and the teachings of Democritus are almost unanimously been linked with that of Leucippus. Then why is Democritus credited with atomic theory invention? Why isn't the student-and-master relationship applicable in here? (If both master and student have brought up an idea into development, shouldn't the master be credited more?)
Fun trivia: The SE spell checker finds Democritus legit, while Leucippus is a spelling mistake! This should mean that Democritus is more credited everywhere.