Molecularity Terminology – What Do You Call a One-Atom Molecule? In basic chemistry and kinetic theory, a molecule that consists of a single atom (for example, He, Ne, or Ar in the gaseous state) is termed as which type of molecule?

Introduction / Context:
Describing substances by the number of atoms per molecule (molecularity in the informal sense) is useful in thermodynamics, spectroscopy, and reaction kinetics. Noble gases exist as single, discrete atoms under standard conditions, so recognizing the correct term aids in interpreting specific heats and transport properties.

Given Data / Assumptions:

• Molecule contains exactly one atom.
• Ideal-gas behavior assumed for simple property discussions.
• Examples include He, Ne, Ar (noble gases), and atomized metals at very high temperatures.

Concept / Approach:

Single-atom molecules are termed monoatomic (sometimes written monatomic). Diatomic molecules have two atoms (e.g., O2, N2), triatomic have three (e.g., CO2, H2O), and polyatomic is a broader term for more than two atoms. This classification affects degrees of freedom and hence specific heat values at a given temperature range (e.g., monoatomic ideal gases have Cv ≈ 3/2 R, Cp ≈ 5/2 R under calorically perfect assumptions).

Step-by-Step Solution:

Verification / Alternative check:

Thermodynamic tables list specific heats for monoatomic gases consistent with 5/2 R at moderate temperatures, supporting the monoatomic classification.

Why Other Options Are Wrong:

Diatomic, triatomic, and tetratomic imply 2, 3, and 4 atoms per molecule, respectively. 'Polyatomic' is a generic term for many-atom molecules, not a single-atom case.

Common Pitfalls:

Confusing atomic number with atoms per molecule; misclassifying halogens (e.g., Cl2) as monoatomic—many elemental gases are diatomic, not monoatomic.

Final Answer:

monoatomic