Democritus proposed the first particle theory of matter in 430 B. C.E. He claimed that matter was made from small, indivisible bits. He referred to these bits as atomos, a Greek word meaning "uncuttable.” What is the modern definition of an atom? Group of answer choices

the smallest particle of matter

the smallest particle of a pure substance

the smallest particle that can still be considered an element

a unit of matter with a mass equal to one gram