Get Wired 101 Midterm Practice Test

The smallest unit that defines an element is the atom. It’s the basic building block that retains the element’s identity and its characteristic chemical behavior. The identity of an element comes from its atomic number, which is the number of protons in the nucleus, and this identity is manifested when the atom has its electrons arranged in specific ways that drive reactivity and bonding.

A molecule, in contrast, is made of two or more atoms bonded together, so it’s inherently larger and may represent more than one element. An ion is a charged form of an atom or molecule—still fundamentally based on atoms, but it’s distinguished by a gain or loss of electrons, which changes its charge and properties but not the idea that the basic unit of an element is the atom. The nucleus alone is just part of the atom; without the surrounding electrons, you don’t see the element’s chemical properties. So the atom is the smallest piece that carries the defining identity of an element.