Final answer:
An atom is the smallest particle of a pure substance that retains all the properties of that substance. Atoms are fundamental building blocks of matter, and their composition includes protons, electrons, and neutrons, with the number of protons defining the element.
Step-by-step explanation:
The particle that is the smallest part of a pure substance and retains all of the properties of the pure substance is known as an atom. An atom is the smallest unit of an element that still has the properties of that element and can enter into a chemical combination. Atoms are incredibly tiny, typically around 100 picometers in diameter, which is roughly one ten-billionth of a meter. Despite their small size, they are the fundamental building blocks of matter and every element consists of identical atoms which define the substance's unique characteristics.
Moreover, atoms consist of even smaller particles like electrons, protons, and neutrons. The number of protons in an atom's nucleus determines the element it represents, thus giving each element its unique properties. Atoms are indispensable in the study of chemistry as they are the basic units of ordinary matter and engage in chemical reactions that form the basis of our tangible world.