What is the smallest unit of an element that cannot be chemically subdivided?

• A. Atom
• B. Molecule
• C. Nucleus
• D. Ion

Answer: (A)

The concept being tested is what single piece of an element actually carries its chemical identity. An atom is the smallest unit that retains the element’s properties in chemical reactions. It holds a nucleus made of protons and neutrons and electrons that surround the nucleus; chemistry is governed by how those electrons interact and bond with other atoms. You can split an atom into subatomic particles, but those parts don’t behave like the element in chemistry, so they aren’t used to describe chemical substances. A molecule, by contrast, is a collection of two or more atoms bonded together, which is larger than a single atom. The nucleus is only the center of the atom and doesn’t on its own account for chemical behavior. An ion is just a charged form of an atom, arising from gaining or losing electrons, but it is still the same elemental atom at its core.