What is the simplest structural unit of an element or compound?

The simplest structural unit of an element or compound is an atom. An atom is the basic building block of matter, consisting of a nucleus made up of protons and neutrons, with electrons orbiting around the nucleus.

When discussing molecules, these are formed when two or more atoms bond together, either from the same element or different elements. For example, a molecule of water consists of two hydrogen atoms bonded to one oxygen atom. Though molecules are important structural units, they are not the simplest units because they are composed of multiple atoms.

Terms like molar mass refer to the weight of a given quantity of a substance and do not describe a structural unit. The term "element" refers to a pure substance made up of only one type of atom, but it does not define the simplest unit in the structural sense.

Thus, the correct and simplest structural unit that can exist independently and retains the properties of the substance is an atom.