What is the simplest unit of an element called?

The simplest unit of an element is called an atom. An atom is the basic building block of matter and is the smallest unit that retains the chemical properties of the element. Each element on the periodic table is comprised of atoms, which consist of a nucleus containing protons and neutrons, and electrons that orbit around the nucleus.

In contrast, a molecule is formed when two or more atoms bond together, which may involve atoms of the same element or different elements. A particle is a more general term that could refer to atoms, molecules, or subatomic particles, but it does not specifically denote the simplest unit of an element. A compound is a substance formed from two or more different elements that are chemically bonded in fixed proportions, which also evidences that it is more complex than a single atom. Thus, while molecules and compounds represent combinations of atoms, an atom stands alone as the fundamental unit of an element.