What is the smallest part of an element that has all the properties of that element?

The smallest part of an element that retains all the properties of that element is the atom. An atom is the basic building block of matter and consists of protons, neutrons, and electrons. Each element is defined by the number of protons in its nucleus, known as its atomic number. This unique arrangement of subatomic particles gives each element its distinct physical and chemical properties, such as reactivity, conductivity, and phase at room temperature.

In contrast, a compound is a substance made up of two or more different elements that are chemically bonded together; thus, it does not represent a single element. A molecule may refer to a group of atoms bonded together, which can be either elements of the same type or different types. Therefore, it is not the smallest unit of a single element. An ion, on the other hand, is an atom or molecule that has gained or lost one or more electrons, resulting in a charged particle, which also modifies the properties of the original element.

Thus, the atom is the fundamental unit that encapsulates all the essential characteristics of an element, making it the correct choice.