All of the following are ionic compounds except
Ionic compounds are built from cations and anions arranged in an extended lattice, and they characteristically dissociate into ions in water as electrolytes. A molecular covalent compound consists of discrete molecules held together by shared-electron bonds and does not dissociate into ions under ordinary aqueous conditions.
A representative multiple-choice form for “all of the following are ionic compounds except” uses options such as: KBr, CaCl2, NH4NO3, and CCl4. The exception is the molecular covalent compound.
Answer selection for a typical set of choices
| Option | Formula | Ionic or molecular? | Bonding description | Electrolyte behavior in water |
|---|---|---|---|---|
| A | KBr | Ionic | K+ and Br− ions form an ionic lattice (formula units, not molecules). | Strong electrolyte; dissociation gives K+(aq) and Br−(aq). |
| B | CaCl2 | Ionic | Ca2+ with two Cl− ions; extended ionic solid. | Strong electrolyte; dissociation gives Ca2+(aq) and 2 Cl−(aq). |
| C | NH4NO3 | Ionic | NH4+ and NO3− ions; covalent bonds exist inside each polyatomic ion, but the compound itself is ionic. | Strong electrolyte; dissociation gives NH4+(aq) and NO3−(aq). |
| D | CCl4 | Molecular covalent (exception) | Nonmetals share electrons in C–Cl bonds; discrete tetrahedral molecules dominate. | Nonelectrolyte; no significant ion production in water. |
Chemical reasoning behind the exception
The exception is the compound that cannot be represented as a stable collection of separate cations and anions. CCl4 contains only nonmetals and forms covalent bonds by electron sharing; its structure is a set of neutral molecules rather than an ionic lattice. In contrast, KBr and CaCl2 pair a metal with a nonmetal, and NH4NO3 pairs two polyatomic ions (ammonium and nitrate), all producing an ionic solid that dissociates into ions in water.
Electronegativity and composition as supporting evidence
Electronegativity difference (\(\Delta \chi\)) supports, but does not replace, the structural definition of ionic compounds. Typical values illustrate why CCl4 is not ionic:
- KBr: large \(\Delta \chi\) between K and Br favors ion formation and an ionic lattice.
- CaCl2: large \(\Delta \chi\) between Ca and Cl favors Ca2+ and Cl−.
- CCl4: smaller \(\Delta \chi\) for C–Cl supports covalent bonding and neutral molecules.
- NH4NO3: \(\Delta \chi\) within NH4+ and NO3− describes covalent bonds inside the ions, while ionic attraction between ions holds the crystal together.
Common pitfalls
- “No metal” interpreted as “not ionic”: NH4NO3 contains no metal but is ionic because it is composed of NH4+ and NO3−.
- Δχ treated as an absolute rule: Bond polarity trends correlate with Δχ, while ionic compounds are defined structurally by ions in a lattice; polyatomic salts require ion-level thinking rather than single-bond thinking.
- Molecules confused with formula units: Ionic solids are described by empirical ratios (formula units), while covalent substances such as CCl4 are described by discrete molecules.
Summary statement
CCl4 is the “except” choice in a typical set because it is a molecular covalent compound, while KBr, CaCl2, and NH4NO3 are ionic compounds that dissociate into ions as electrolytes in aqueous solution.