Basic chemical terminology – smallest independently existing, chemically recognizable unit The smallest quantity of a substance that can exist by itself in a chemically recognizable form is called a:

Introduction / Context:
Precise terminology in chemistry underpins stoichiometry and material characterization. The question asks for the smallest amount of a substance that, by itself, preserves the substance’s chemical identity in composition and properties.

Given Data / Assumptions:

• The unit must “exist by itself,” i.e., as a discrete entity.
• It must be “chemically recognizable,” meaning it exhibits the substance’s characteristic composition and bonding.
• Applies to covalently bonded substances and also to diatomic elements in their natural form.

Concept / Approach:
A molecule is a discrete aggregate of atoms held together by chemical bonds that represents the smallest particle of a pure substance (elemental or compound) capable of independent existence while retaining its chemical properties. An atom by itself may or may not represent the substance’s standard form (e.g., oxygen exists as O2 molecules in nature). Elements/compounds are bulk classifications, not smallest units.

Step-by-Step Solution:
Identify the requirement of independent existence: points to a bonded unit rather than an abstract formula unit.Ensure chemical recognizability: composition and structure correspond to the substance (e.g., H2O molecule for water).Therefore, “molecule” fits the definition best.Note: for ionic solids, the basic repeating unit is a formula unit in a lattice, but as a discrete “by itself” entity, we still refer to molecules for molecular substances.

Verification / Alternative check:
Diatomic elemental gases (H2, N2, O2, Cl2) demonstrate that the recognizable chemical form is molecular, not atomic, under standard conditions.

Why Other Options Are Wrong:

• Element: a class of atoms with the same atomic number; not a discrete smallest particle.
• Compound: a substance composed of two or more elements; again not the smallest particle.
• Atom: for many substances this does not represent the stable independently existing form (e.g., O atoms do not persist as free entities at STP).
• Radical: a reactive group; not generally stable by itself nor representative of the complete substance.

Common Pitfalls:
Equating “atom” with smallest unit universally; while true in a physical sense, the question emphasizes chemically recognizable and independently existing form, which is a molecule for molecular substances.

Final Answer:
molecule