The simplest structural unit of an element or compound is called an atom.

Here's why:

* Elements: Elements are made up of only one type of atom. For example, gold (Au) is made up of only gold atoms.

* Compounds: Compounds are formed when two or more different types of atoms bond together. For example, water (H₂O) is a compound made of two hydrogen atoms and one oxygen atom bonded together.

While molecules can also be considered building blocks, atoms are the fundamental, indivisible particles that make up everything.