AP Chemistry Ionic vs Covalent Bonds
How to classify bonding from formulas and from properties, and how to rank ionic bond strength, with three worked examples.
Telling ionic, covalent, and metallic bonding apart is a two-step skill: a first guess from where the elements sit on the periodic table, then a check against the substance's observable properties. The College Board is explicit that properties are the better evidence, so both steps are worked below.
The Three Bond Types and How to Tell Them Apart
- Ionic: electrostatic attraction between cations and anions, generally a metal with a nonmetal. Forms a lattice, not separate molecules.
- Covalent: shared electron pairs, generally between nonmetals. Nonpolar when the atoms attract electrons about equally, polar when one atom is clearly more electronegative.
- Metallic: metal atoms whose valence electrons are delocalized across the whole solid.
The ionic/covalent distinction is a continuum rather than a sharp line, so the Unit 2 guidance is to confirm a formula-based guess against properties like melting point and electrical conductivity.
Worked Example 1: Classifying Bonds From Formulas
Classify the bonding in each substance.
| Substance | Elements | Bond type |
|---|---|---|
| NaCl | Metal + nonmetal | Ionic |
| MgO | Metal + nonmetal | Ionic |
| CO2 | Two nonmetals, oxygen more electronegative | Polar covalent bonds |
| Cl2 | Two identical atoms | Nonpolar covalent |
| CH4 | Two nonmetals, nearly equal electronegativity | Effectively nonpolar covalent |
| Cu | One metal | Metallic |
| NaNO3 | Metal + a polyatomic ion | Ionic between Na+ and NO3−; covalent inside the nitrate ion |
The last row is the one that catches people: one compound can contain both ionic and covalent bonding. The same split drives how ionic and molecular compounds are named in compound naming.
Worked Example 2: Identifying Bond Type From Properties
Three unlabeled solids are tested. Identify the bonding in each.
- Substance A: hard and brittle, melts near 800°C, does not conduct as a solid, conducts when molten.
- Substance B: soft, melts near 45°C, does not conduct as a solid or as a liquid.
- Substance C: shiny, malleable, conducts as a solid.
A is ionic. A high melting point shows strong attractions that need a lot of energy to
break, and conduction only after melting means ions are present but locked in place in the solid.
B is molecular covalent. A low melting point and no conduction in any state point to
neutral molecules held together by weak forces between molecules, with no mobile charges.
C is metallic. Conducting as a solid requires mobile charge in the solid itself, which
delocalized valence electrons provide, and malleability fits atoms sliding without breaking directional bonds.
Worked Example 3: Ranking Ionic Melting Points With Coulomb's Law
Rank NaCl, MgO, and NaBr from highest to lowest melting point.
Coulomb's law says the attraction increases with the product of the ion charges and decreases as the ions
are farther apart.
MgO vs. NaCl: Mg2+ and O2− carry charges of 2 and 2, a charge
product of 4, against 1 for Na+ and Cl−. The MgO ions are also smaller, so MgO
has the much stronger attraction.
NaCl vs. NaBr: the charges are identical, but Br− is a larger ion than
Cl−, so the ions sit farther apart in NaBr and attract less strongly.
Predicted order: MgO > NaCl > NaBr. This matches measured melting points of roughly 2850°C, 801°C, and 747°C.
Common Bond Type Mistakes
- Calling every compound containing a metal fully ionic and every compound containing nonmetals fully covalent. NaNO3 has both kinds of bonding.
- Relying on one electronegativity-difference cutoff. Bonding is a continuum; use properties to confirm.
- Saying ionic solids conduct electricity. They conduct only when molten or dissolved, because the ions in the solid cannot move.
- Comparing ionic bond strength by charge alone or size alone. Coulomb's law depends on both the charges and the distance.
This same classify-then-check reasoning applies across the rest of AP Chemistry too -- for every other free tool and guide on this site, start from the AP Chem Score Calculator.
Related Resources
- AP Chem Score Calculator
- Unit 2 Review: Compound Structure and Properties
- AP Chemistry Naming Compounds
- AP Chemistry Polyatomic Ions
- AP Chemistry VSEPR Chart
- AP Chemistry Periodic Table
- AP Chemistry Study Guide
Frequently Asked Questions
What is the difference between ionic and covalent bonds?
An ionic bond is the electrostatic attraction between oppositely charged ions and generally forms between a metal and a nonmetal. A covalent bond is a shared pair of electrons and generally forms between two nonmetals. In a polar covalent bond the sharing is unequal, so one atom carries a partial negative charge.
How can you tell whether a compound is ionic or covalent from its formula?
As a first pass, a metal combined with a nonmetal (NaCl, MgO) is ionic, and two nonmetals combined (CO2, CH4) is covalent. Ionic compounds that contain a polyatomic ion, such as NaNO3, still have covalent bonds inside the polyatomic ion.
Is there an electronegativity-difference cutoff between ionic and covalent?
Not one that the AP exam relies on. The ionic/covalent distinction is a continuum, and the best way to characterize the bonding in a real compound is to examine its properties, not to run a single electronegativity-difference number.
Why do ionic compounds conduct electricity only when melted or dissolved?
In the solid, the ions are locked in fixed positions in the lattice, so no charge can move. When the compound is melted or dissolved, the ions are free to move, and the liquid or solution conducts.
How does Coulomb's law help compare ionic compounds?
The attraction between ions grows with the product of their charges and shrinks as the distance between them grows. Ions with larger charges and smaller radii bond more strongly, which shows up as a higher melting point.
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