Chemistry – Avogadro’s Hypothesis (Terminology) According to Avogadro’s hypothesis, the smallest particle of an element or a compound that can exist independently is called _____.

Introduction / Context:
Avogadro’s hypothesis states that equal volumes of gases, at the same temperature and pressure, contain equal numbers of particles. The related terminology emphasises that for many substances the independently existing particle is not a single atom but a molecule (e.g., O2, N2).

Given Data / Assumptions:

• We refer to the tiniest independently existing particle of an element or compound.
• Examples: oxygen gas exists as O2 molecules; water as H2O molecules.
• Cations and anions are charged species, not the universal form of independent existence.

Concept / Approach:
In gaseous state, Avogadro treated the basic countable entities as “molecules,” whether the substance is an element (often diatomic) or a compound. Atoms are subunits of molecules for many substances, though monoatomic gases (e.g., noble gases) are special cases where the molecule equals a single atom.

Step-by-Step Solution:

Verification / Alternative check:
For helium (He), the “molecule” is one atom; for oxygen (O2), the molecule has two atoms. The umbrella term remains molecule across substances, aligning with Avogadro’s counting entity.

Why Other Options Are Wrong:

• a cation / an anion: Ions; not universally representative of all substances.
• an atom: Too narrow; many substances exist naturally as multi-atom molecules.

Common Pitfalls:
Equating “smallest particle” with “atom” in all contexts. Chemistry distinguishes atoms (building blocks) from molecules (independent chemical entities).

Final Answer:
a molecule