# Molecular and Ionic Compound Structure and Properties Overview

> AP Chemistry · Structure and properties of molecular and ionic compounds
> Source: https://www.owlsprep.com/study/ap-chemistry-u2-overview/
> Weight: 7-9% of overall AP Chemistry exam score

This unit explores how atoms bond to form ionic, molecular, and metallic compounds, and how resulting structure determines bulk properties. It is a foundational topic for understanding chemical reactivity and intermolecular forces later in the course.

**Prerequisites:** [AP Chemistry Unit 1: Atomic Structure and Properties](https://www.owlsprep.com/study/ap-chemistry-u1-overview/)

## Learning objectives

- Distinguish between ionic, covalent, and metallic bonding based on electron behavior and electronegativity
- Draw and interpret Lewis diagrams, including resonance structures and formal charge assignments
- Predict molecular geometry and bond hybridization using VSEPR theory
- Relate bulk properties of ionic, metallic, and alloy materials to their microscopic structure
- Explain the relationship between potential energy, bond length, and bond strength

## Unit at a Glance

This unit builds on the atomic structure concepts you learned in Unit 1, connecting electron behavior to the formation of stable chemical bonds. We progress from classifying bond types, to analyzing molecular bonding and geometry, to connecting microscopic structure to the bulk properties of ionic and metallic materials.

A core theme of this unit is structure-property relationships: how the arrangement of atoms and electrons in a compound directly determines its observable physical and chemical behavior. This is one of the most heavily tested themes on the AP Chemistry exam.

The following sub-topics are covered in this unit:
- [AP Chemistry Intramolecular force and potential energy](https://www.owlsprep.com/study/ap-chemistry-u2-intramolecular-force-and-potential-energy/) — Relate bond length, bond strength, and potential energy on an atomic interaction energy curve.
- [AP Chemistry Lewis diagrams](https://www.owlsprep.com/study/ap-chemistry-u2-lewis-diagrams/) — Learn to draw Lewis dot structures for atoms, ions, and neutral covalent molecules.
- [AP Chemistry Resonance and formal charge](https://www.owlsprep.com/study/ap-chemistry-u2-resonance-and-formal-charge/) — Calculate formal charge and identify resonance structures to describe delocalized bonding.
- [AP Chemistry Structure of ionic solids](https://www.owlsprep.com/study/ap-chemistry-u2-structure-of-ionic-solids/) — Connect the crystal lattice structure of ionic solids to their bulk physical properties.
- [AP Chemistry Structure of metals and alloys](https://www.owlsprep.com/study/ap-chemistry-u2-structure-of-metals-and-alloys/) — Explain the structure of pure metals and how alloying alters metallic properties.
- [AP Chemistry Types of chemical bonds](https://www.owlsprep.com/study/ap-chemistry-u2-types-of-chemical-bonds/) — Distinguish between ionic, covalent, and metallic bonding based on electronegativity and electron behavior.
- [AP Chemistry VSEPR and bond hybridization](https://www.owlsprep.com/study/ap-chemistry-u2-vsepr-and-bond-hybridization/) — Predict molecular geometry, bond angles, and orbital hybridization from Lewis structures.

## Common pitfalls

- **Wrong:** Confusing intramolecular bonds with intermolecular forces
  - Why it fails: This unit covers intramolecular bonds that hold atoms together within a compound; intermolecular forces are covered in Unit 3.
  - Correct: Always confirm if a question refers to forces within a compound or between separate molecules before answering.
- **Wrong:** Incorrectly counting valence electrons for Lewis diagrams
  - Why it fails: Forgetting to add electrons for negative anions or subtract electrons for positive cations leads to invalid structures.
  - Correct: Always calculate total valence electrons first, adjusting for ionic charge, before drawing your structure.
- **Wrong:** Ignoring lone pairs when calculating hybridization
  - Why it fails: Hybridization depends on total electron domains, not just the number of bonded atoms.
  - Correct: Count both bonding domains and lone pair domains to find the total number of electron domains for hybridization.

## Cheatsheet

| Concept / Key Formula | Description |
| --- | --- |
| Bond classification by $\Delta EN$ | $\Delta EN < 0.4$ = nonpolar covalent; $0.4 \leq \Delta EN < 1.8$ = polar covalent; $\Delta EN \geq 1.8$ = ionic |
| Formal Charge | $FC = \text{Valence e}^- - \text{Nonbonding e}^- - \frac{\text{Bonding e}^-}{2}$ |
| VSEPR Electron Domains | 2 = linear, 3 = trigonal planar, 4 = tetrahedral, 5 = trigonal bipyramidal, 6 = octahedral |
| Hybridization by Electron Domains | 2 domains = $sp$, 3 domains = $sp^2$, 4 domains = $sp^3$ |
| Ionic Solid Properties | High melting point, brittle, low conductivity as solid, high conductivity when molten/dissociated |
| Metallic Bonding Model | Delocalized sea of valence electrons around metal cations, creates conductivity and malleability |
| Stable Bond Energy | Minimum potential energy on an interaction curve equals the most stable bond length |

## What's next

Begin your learning with the foundational first sub-topic on classifying chemical bonds. Once you complete all sub-topics in this unit, you will move on to Unit 3, which extends these bonding concepts to intermolecular forces between molecules.

- [AP Chemistry Types of chemical bonds](https://www.owlsprep.com/study/ap-chemistry-u2-types-of-chemical-bonds/)
- [AP Chemistry Unit 3: Intermolecular Forces and Properties Overview](https://www.owlsprep.com/study/ap-chemistry-u3-overview/)
- [Intramolecular Force and Potential Energy](https://www.owlsprep.com/study/ap-chemistry-u2-intramolecular-force-and-potential-energy/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ap-chemistry-u2-overview/