An ionic compound is a substance made of cations (usually from a metal) and anions (usually from a nonmetal) held together in a crystal lattice by Coulombic attraction between opposite charges, giving it high melting points and conductivity when melted or dissolved.
An ionic compound forms when atoms with a large electronegativity difference, typically a metal and a nonmetal, transfer electrons instead of sharing them. The metal loses electrons to become a cation, the nonmetal gains them to become an anion, and the opposite charges pull the ions into a repeating 3D crystal lattice. There's no single "molecule" of NaCl sitting around. It's a giant ordered grid of Na⁺ and Cl⁻ ions, which is why we write formula units (the simplest ratio) instead of molecular formulas.
The strength of the attraction follows Coulomb's law. Bigger charges and smaller ions mean a stronger lattice. That one idea explains almost every property AP Chem asks about. High melting and boiling points happen because you have to overcome strong Coulombic attractions to pull the lattice apart. Solids don't conduct because the ions are locked in place, but melt the compound or dissolve it in water and the ions become mobile charge carriers. Ionic solids are also brittle, since shifting the lattice lines up like charges that repel and crack the crystal.
Ionic compounds live at the heart of Unit 2 (Topic 2.1, Types of Chemical Bonds), where learning objective 2.1.A asks you to explain how the elements in a bond determine the bond type. Electronegativity trends (EK 2.1.A.1) tell you when electron transfer wins out over sharing, so a metal on the far left bonding with a nonmetal on the far right screams "ionic." But the term doesn't stay in Unit 2. In Unit 3, LO 3.10.A has you explain why ionic compounds dissolve in polar solvents like water (ion-dipole attractions) and not in nonpolar ones, and Topic 3.1's Coulombic-interaction logic explains why interparticle forces in ionic solids dwarf the intermolecular forces between molecules. Then in Unit 4, LO 4.2.A makes you split soluble ionic compounds into their ions to write net ionic equations. If you can't recognize what's ionic and what dissociates, you can't write the equation.
Keep studying AP Chemistry Unit 4
Coulomb's Law (Units 1-2)
Coulomb's law is the engine behind every ionic property. Higher ion charges and shorter distances between ions mean stronger attraction, which is why MgO (2+ and 2-) melts at a far higher temperature than NaCl (1+ and 1-). When an MCQ asks you to rank lattice strength or melting points, you're really just applying Coulomb's law.
Solubility and Ion-Dipole Forces (Unit 3)
When an ionic compound dissolves in water, the lattice trades ion-ion attractions for ion-dipole attractions with polar water molecules. That's the "like dissolves like" logic of LO 3.10.A in action. It also explains why dissolved ionic compounds conduct electricity: free-floating ions carry the charge.
Net Ionic Equations (Unit 4)
Writing a net ionic equation starts with knowing which ionic compounds are soluble strong electrolytes. Those get written as separated ions, while insoluble ionic solids stay together as one formula unit. Misclassify a compound and the whole equation, including the spectator ions you cancel, falls apart.
Covalent (Molecular) Bonding (Unit 2)
Ionic and covalent bonding are two ends of the same electronegativity spectrum from Topic 2.1. Similar electronegativities share electrons; very different ones transfer them. The properties diverge sharply, since molecular compounds tend to have low melting points and don't conduct, even when dissolved.
Ionic compounds usually show up as a "properties detective" question. A typical MCQ stem describes a mystery substance with a high melting point, brittleness, and conductivity when dissolved or molten but not as a solid, then asks you to identify the bonding type or the likely composition (metal cation plus nonmetal anion). You're expected to reason from properties back to bonding using LO 2.1.A, not just memorize labels. In Unit 4, MCQs and FRQs ask you to write balanced net ionic equations, which means correctly dissociating soluble ionic compounds into ions and keeping insoluble ones intact, while conserving both mass and charge (EK 4.2.A.2). Ranking questions also pair ionic compounds with Coulomb's law, asking which lattice is strongest based on ion charge and size.
Ionic compounds transfer electrons between a metal and a nonmetal and exist as a lattice of ions; covalent compounds share electrons between nonmetals and exist as discrete molecules. The giveaway is in the properties. Ionic compounds have high melting points and conduct when molten or dissolved, while molecular compounds usually melt at low temperatures and don't conduct at all because they have no mobile charged particles. Watch the boundary case too: bonds between atoms of unequal but not wildly different electronegativity are polar covalent, not ionic.
An ionic compound is a crystal lattice of cations and anions held together by Coulombic attraction, usually formed between a metal and a nonmetal with a large electronegativity difference.
Coulomb's law predicts lattice strength: higher ion charges and smaller ions mean stronger attractions, higher melting points, and harder solids.
Ionic compounds conduct electricity only when their ions can move, so molten or dissolved ionic compounds conduct but the solid does not.
Soluble ionic compounds dissociate completely into ions in water, which is why they get written as separated ions in complete and net ionic equations.
Ionic compounds dissolve in polar solvents like water through ion-dipole attractions, but they generally won't dissolve in nonpolar solvents.
On the exam, identify a mystery substance as ionic from its property fingerprint: high melting point, brittleness, and conductivity in solution.
It's a substance made of cations and anions held in a crystal lattice by Coulombic attraction, typically formed when a metal transfers electrons to a nonmetal. NaCl, MgO, and CaF₂ are classic examples tested in Topic 2.1.
Only when the ions can move. Solid ionic compounds don't conduct because the ions are locked in the lattice, but molten or dissolved ionic compounds conduct well. That solid-versus-solution distinction is a favorite MCQ trap.
Ionic compounds form by electron transfer between a metal and a nonmetal and exist as ion lattices; covalent compounds form by electron sharing between nonmetals and exist as molecules. Practically, ionic compounds have high melting points and conduct in solution, while molecular compounds usually don't conduct at all.
Mostly, but bonding is a spectrum based on electronegativity difference (EK 2.1.A.1). A huge difference (like Na and Cl) gives ionic bonding, while a smaller difference gives polar covalent character, so the exam wants you to reason from electronegativity, not just the periodic table positions.
Polar water molecules form ion-dipole attractions with the cations and anions, pulling the lattice apart. This is the LO 3.10.A logic of "like dissolves like," and it's also why ionic compounds won't dissolve in nonpolar solvents like hexane.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.
Put the full course together before test day.