What is the smallest particle of a substance that retains all the properties of that substance?

The smallest particle of a substance that retains all the properties of that substance is an atom. An atom is the fundamental building block of all matter and is the smallest unit of an element that maintains the chemical properties characteristic of that element.

When discussing substances, atoms combine to form molecules, which are groups of two or more atoms bonded together. However, a molecule may not represent the smallest unit of a substance if it contains more than one atom; thus, while molecules are essential in understanding the composition of substances, they do not necessarily represent the smallest distinguishable unit that retains the full properties of the substance.

On the other hand, compounds are substances composed of two or more different types of atoms bonded together, and elements are pure substances made up of only one type of atom. Therefore, while related, they do not define the smallest particle that retains the properties of the substance itself, which is specifically the atom.