Chemists describe pure substances by the type of particles present. Distinguishing discrete molecules from extended lattices (ionic, atomic, or metallic) determines how formulas are interpreted and how particle-level models are drawn.

Pure substances and what “particle” means

A pure substance contains only one type of chemical species throughout (even if that species contains more than one element). In AP Chemistry, the key question is whether the substance is made of discrete units or an extended lattice.

Discrete units: substances made of molecules

Molecular substances are composed of many separate molecules with strong covalent bonds inside each molecule and comparatively weaker forces between molecules (often intermolecular forces). Their chemical formula is a molecular formula that gives the actual number of each type of atom in one molecule (for example, $H_2O$ is one molecule with 2 H and 1 O).

Common cues that a substance is molecular:

• It is composed only of nonmetals (or a nonmetal + metalloid in many covalent compounds).

• The particles can be pictured as separate clusters (molecule-by-molecule).

• The formula can, in principle, be scaled down to one particle that still “exists” as that same unit.

Extended structures: lattices of ions or atoms

Many pure substances are not made of individual molecules.

Two complementary particulate diagrams of solid NaCl: a compact 3D lattice view and an expanded view that clarifies the repeating ion arrangement. The labels emphasize that the structure is an extended array of $\mathrm{Na^+}$ and $\mathrm{Cl^-}$ ions rather than discrete “NaCl molecules.” This is a concrete visual anchor for interpreting NaCl as a formula unit (fixed ratio) within a continuous crystal. Source

Instead, they are continuous repeating structures with no smallest “molecule” that preserves the structure.

A lattice description is used for:

• Ionic solids: a repeating arrangement of cations and anions

• Covalent network solids: a repeating arrangement of atoms connected by covalent bonds

• Metallic solids: a repeating arrangement of metal atoms (often modeled as metal cations in a sea of delocalized electrons)

Because there is no discrete molecule to count, the written chemical formula is interpreted differently.

Formula units and fixed ratios

When a pure substance is an ionic solid (and in some lattice-type solids), chemists use the idea of a formula unit to communicate composition.

A formula unit is not a “piece” you can physically isolate from the crystal; it is a ratio statement that reflects how the lattice repeats.

A 3D particulate model of an ionic crystal lattice, showing alternating ions packed into a repeating array. The visual reinforces why a “formula unit” is a composition ratio (e.g., $1:1$ in NaCl) rather than a separable chunk of the solid. In an actual crystal, the pattern extends throughout the entire solid, with no smallest self-contained molecule. Source

This directly matches the syllabus emphasis: some pure substances are lattices “held together in fixed ratios described by a formula unit.”

Key implications:

• The formula in an ionic compound gives the simplest ratio of ions that results in overall charge neutrality.

• The “particles” in an ionic solid are ions, but the formula unit combines them in the smallest neutral ratio (e.g., one cation with one anion, or multiple ions as needed).

• For covalent network solids (e.g., many silicon/oxygen solids), the formula likewise indicates a repeating ratio, not a discrete molecule.

Interpreting chemical formulas correctly

To decide whether a formula refers to molecules or formula units, focus on structure type:

• Molecular formula interpretation (discrete molecules)

  • The subscripts describe atoms within one molecule

  • Many identical molecules make up the sample

• Formula unit interpretation (extended lattice)

  • The subscripts describe the lowest repeating ratio in the lattice

  • The sample is one continuous network of repeating units

Common student pitfalls to avoid

• Treating an ionic compound’s formula as if it describes a single “molecule” floating around.

• Assuming every solid is a lattice of ions; many solids are molecular (made of molecules packed together) while others are covalent network or metallic.

• Forgetting that “lowest ratio” language applies to formula units: the written formula is a compositional simplification, not necessarily a count of atoms in a discrete particle.