Given a scientific passage and an answer, generate a question for the given answer.

Passage: Inside of elements, you will find identical atoms. An atom is the simplest and smallest particle of matter that still has chemical properties of the element. Atoms are the building block of all of the elements that make up the matter in your body or any other living or non-living thing. Atoms are so small that only the most powerful microscopes can see them. Answer: an atom
The simplest and smallest particle of matter that still has chemical properties of the element is called?