Atom definition.

• A. The smallest portion of an element that exhibits all chemical properties; Consists of nucleus and electrons.
• B. The smallest unit of matter that cannot be subdivided.
• C. The smallest unit of a compound.
• D. A charged particle.

Answer: (B)

Atoms are the basic building blocks of elements—the smallest unit that still carries the identity and properties of that element. In many chemistry contexts, they’re described as indivisible by chemical means, which is why this definition fits best here: it captures the idea that you can’t break an element down into simpler substances through ordinary chemical reactions. Subdividing an atom leads to subatomic particles, but those parts don’t define the element’s chemical behavior the way the whole atom does.

The other statements miss the full concept: the smallest unit of a compound is a molecule (not an element’s building block), a charged particle refers to ions or electrons rather than whole atoms in general, and describing the atom as consisting of nucleus and electrons focuses on internal structure rather than the role of the atom as the fundamental unit of an element.