Molecular and Ionic Compound Structure and Properties

What this unit covers

The topics below follow the published Chemistry course framework for Unit 2. This unit is worth 7–9% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Lewis Structures & Formal Charge14 min · 3 objectivesDraw valid Lewis (electron-dot) structures for molecules and polyatomic ions · Assign formal charges to choose the best of several resonance structures · Recognize exceptions to the octet rule (incomplete and expanded octets)
• Ionic, Covalent & Metallic Bonding13 min · 3 objectivesClassify a bond as ionic, covalent, or metallic from the atoms involved · Explain the sea-of-electrons model of metallic bonding · Predict conductivity, melting point, and malleability from bonding type
• VSEPR Molecular Geometry14 min · 3 objectivesCount electron domains around a central atom to find its geometry · Distinguish electron-domain geometry from molecular (atom) geometry · Predict bond angles and how lone pairs distort them
• Hybridization & Polarity13 min · 3 objectivesAssign sp, sp², or sp³ hybridization from the number of electron domains · Distinguish sigma (σ) from pi (π) bonds in single, double, and triple bonds · Determine molecular polarity by combining bond dipoles with geometry
• Resonance, Formal Charge & Bond Order14 min · 3 objectivesDraw competing resonance structures and use formal charge to select the most plausible one · Calculate the average bond order of a delocalized (resonance) system · Predict relative bond length and bond strength from bond order and delocalization
• Bonding Models: Sigma/Pi & Band Theory15 min · 3 objectivesDistinguish σ from π bonds and count each in single, double, and triple bonds · Relate the σ-bond framework and π bonds of a molecule to its hybridization · Use the electron-sea/band model and solid-type classification to explain conductivity, hardness, and melting point

Formulas in Unit 2

Every term in Unit 2

All 20 terms we publish for Molecular and Ionic Compound Structure and Properties, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Ionic bonding

Electrostatic attraction between oppositely charged ions formed by electron transfer, generally between a metal and a nonmetal.

Lattice energy

Energy released forming an ionic solid from gaseous ions. Follows Coulomb's law, so it rises with charge magnitude and falls with ion size.

Covalent bonding

Shared electron pairs between nonmetals. Bond order rises from single to triple, with shorter and stronger bonds at higher order.

Metallic bonding

Cations in a delocalized sea of electrons — which is why metals conduct, and why they deform rather than shatter.

Lewis structures

Show bonding and lone pairs. Assign total valence electrons, give the central atom the lowest electronegativity, then complete octets and check formal charge.

Formal charge

Valence electrons minus lone-pair electrons minus half the bonding electrons. The best structure minimizes formal charges and puts negatives on electronegative atoms.

Resonance

Two or more valid Lewis structures for one molecule. The real structure is an average, giving bond lengths between those of single and double bonds.

VSEPR theory

Electron domains arrange to minimize repulsion. Count domains for the electron geometry, then ignore lone pairs to name the molecular shape.

Molecular geometries

2 domains linear; 3 trigonal planar (bent with one lone pair); 4 tetrahedral (trigonal pyramidal with one, bent with two).

Bond polarity vs molecular polarity

A molecule with polar bonds is nonpolar if symmetry cancels the dipoles — CO₂ is linear and nonpolar, H₂O is bent and polar.

Hybridization

sp for 2 domains, sp² for 3, sp³ for 4. Determined by domain count, not by the shape name.

Sigma vs pi bonds

A single bond is one sigma; a double is one sigma and one pi; a triple is one sigma and two pi. Pi bonds prevent rotation.

Bond energy and length

Shorter bonds are stronger. Breaking bonds absorbs energy and forming them releases it — the basis of the ΔH from bond enthalpies calculation.

Alloys

Substitutional alloys replace atoms with similar-sized ones; interstitial alloys fit small atoms into gaps, as carbon does in steel, blocking layers from sliding.

Why ionic solids are brittle

Displacing one layer brings like charges into contact and the crystal repels itself apart, unlike a metal's delocalized electrons which tolerate sliding.

Conductivity of ionic compounds

They do not conduct as solids because ions are fixed, but do when molten or dissolved because ions become mobile.

Coulombic reasoning for melting point

Higher ion charges and smaller ionic radii give a larger lattice energy and therefore a higher melting point. Cite both charge and distance to earn the justification.

Expanded octet

Elements in period 3 and beyond can hold more than eight valence electrons because empty d orbitals are energetically accessible.

Network covalent solids

Diamond, silicon dioxide and silicon carbide are held by a continuous lattice of covalent bonds, giving very high melting points and hardness.

Bond dipole

A separation of charge caused by an electronegativity difference. Drawn as an arrow pointing toward the more electronegative atom.

What examiners penalize here

• On free-response Lewis questions, always tally your electrons against your step-1 total before moving on — an off-by-two count is the single most common way to lose the point. Then verify formal charges sum to the overall charge of the species.
• The AP exam constantly links a property to a structural cause. Never just name the bond type — state the mechanism: "conducts when molten because the ions become mobile" or "malleable because the nondirectional electron sea allows cores to slide."
• Keep two questions separate on the exam: "How many total domains?" fixes the electron-domain geometry and ideal angle; "How many of those are lone pairs?" fixes the named molecular shape and how far the real angle is squeezed below ideal.
• The exam's favorite two-step: first use domain count for both geometry AND hybridization (they come from the same number), then judge polarity by asking whether the shape lets the bond dipoles cancel. Symmetry with identical outer atoms → nonpolar; asymmetry or lone pairs → polar.
• On free response, never justify equal bond lengths by saying the molecule "switches" between structures — that earns no credit. State that the bonding is delocalized into one resonance hybrid with a single fractional bond order. And always confirm your chosen Lewis structure by showing formal charges that sum to the species' charge.
• Two exam reflexes: (1) σ bonds equal the number of bonded pairs, so σ + π together must equal the total bond count you drew — use that to self-check. (2) When asked to explain a solid's property, name the model explicitly — "delocalized electron sea/band" for metals, "continuous covalent network" for diamond, "mobile ions only when molten" for ionic — and connect it directly to the observed property.

Practice Chemistry

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 9 units of AP Chemistry

1. Unit 1 · Atomic Structure & Properties
2. Unit 2 · Molecular and Ionic Compound Structure and Properties
3. Unit 3 · Intermolecular Forces and Properties
4. Unit 4 · Chemical Reactions
5. Unit 5 · Kinetics
6. Unit 6 · Thermodynamics
7. Unit 7 · Equilibrium
8. Unit 8 · Acids & Bases
9. Unit 9 · Applications of Thermodynamics