Final answer:
The tiniest bit of a pure elemental substance that still retains the properties of that element is an atom, which is composed of even smaller particles including protons.
Step-by-step explanation:
The tiniest, most minuscule bit of a pure elemental substance is an atom. An atom is the smallest unit of ordinary matter that retains the unique properties of a chemical element. Atoms form the building blocks of all matter in the universe, and although they are incredibly small, atoms contain even smaller particles known as protons, neutrons, and electrons. The number of protons in the nucleus of an atom determines which element it is. Therefore, while a proton is a subatomic particle that contributes to the makeup of an atom, it is not enough on its own to retain the properties of an elemental substance.