Final answer:
The smallest particle of an element that retains the properties of that element in a chemical reaction is an atom. Atoms are composed of subatomic particles such as protons, neutrons, and electrons, with the number of protons defining the element.
Step-by-step explanation:
The smallest particle of an element that retains the properties of that element in a chemical reaction is a(n) atom. Atoms are the fundamental building blocks of matter and the smallest unit of an element that still possesses the element's properties. Atoms are composed of even smaller particles known as subatomic particles, which include protons with a positive charge, neutrons with no charge, and electrons with a negative charge. The number of protons (also known as the atomic number) determines the element to which an atom belongs, while the protons and neutrons together contribute to the mass number of an atom. Differences in neutron number can create isotopes of the same element, which still behave as that element in chemical reactions.