Why do salt, water and copper behave differently?
You will be able to: Connect a bonding model to the kinds of atoms and mobile charges present.
Why do salt, water and copper behave differently?
A copper wire carries current as a solid. A dry salt crystal does not, although melted salt can. Water consists of separate molecules. The different arrangements of electrons and nuclei help explain these observations.
A useful starting point: Valence patterns help predict common ion charges →
Words and symbols before equations
- Valence electrons
- Outer electrons involved in bonding.
- Covalent bond
- An attraction involving shared electron density between atoms.
- Ionic bonding
- Electrostatic attraction among oppositely charged ions.
- Delocalized electrons
- Electrons spread across many atoms rather than assigned to one bond.
What this picture assumes
Particle cartoons, not atomic sizes or measured electron densities. Water is molecular; a salt crystal is an extended ion array.
Read the picture in three steps
- Read the species and labels first. A Lewis line represents two electrons; a spatial stick indicates connectivity. Use the stated quantities and units for numerical comparisons.
- Ions exist but cannot carry current through an intact solid lattice. Melting allows ion motion.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
For two nonmetals, attraction between nuclei and shared electrons often gives covalent bonding. Sharing does not mean electrons stop moving or sit at one fixed point.
A metal and a nonmetal commonly form an ionic solid: an extended arrangement of cations and anions. A formula such as NaCl records a ratio, not an isolated two-atom molecule.
In a metal, mobile valence electrons extend throughout the material. An ionic solid has charges too, but its ions cannot travel through the intact lattice. Melting frees ions to carry charge. Use composition and observed properties together; one property alone is rarely conclusive.
A worked example, step by step
A substance contains sodium and chlorine. It is brittle and conducts when molten but not as a solid. Select and justify a model.
- Sodium is a metal; chlorine is a nonmetal. Ionic bonding is a reasonable first hypothesis.
- Represent Na⁺ and Cl⁻ in an extended neutral array, with a 1:1 ratio.
- In the solid the ions are constrained near lattice positions, so there is no mobile charge carrier through the sample.
- Molten ions can move. This explains the conductivity change without claiming that melting releases metallic electrons.
Having charged particles is insufficient for conduction; charges must be able to move through the sample.
Is an ionic solid a collection of separate NaCl molecules?
Compare with an explanation
No. NaCl describes the ion ratio in an extended array.
Predict. Change one thing. Explain.
Switch between salt, molecular water and copper. Keep the question fixed: what carries charge, and can that carrier move in the solid?
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Ions exist but cannot carry current through an intact solid lattice. Melting allows ion motion.
Particle cartoons, not atomic sizes or measured electron densities. Water is molecular; a salt crystal is an extended ion array.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using electron accounting, electrostatic interactions or spatial geometry. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionCompare solid copper and solid NaCl. Identify their charge carriers, explain the conductivity difference, and predict the effect of melting NaCl.
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Compare with the answer and four-point rubric
- 1 point: Copper has mobile delocalized electrons.
- 1 point: NaCl has cations and anions confined to lattice sites in the solid.
- 1 point: Copper conducts because its carriers can move; solid NaCl lacks mobile ions.
- 1 point: Molten NaCl conducts as ions become mobile.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What distinguishes a molecular substance from an ionic lattice?
Discrete bonded molecules versus an extended ion arrangement.
RECALL 2Why do metals conduct?
Delocalized valence electrons can move through the solid.
RECALL 3How should a bonding claim be supported?
Use atom identities together with relevant observed properties.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why do salt, water and copper behave differently?
- Covalent: shared electron density.
- Ionic solid: extended cation–anion arrangement.
- Metal: positive cores and delocalized valence electrons.
Remember: Having charged particles is insufficient for conduction; charges must be able to move through the sample.
Conditions: Particle cartoons, not atomic sizes or measured electron densities. Water is molecular; a salt crystal is an extended ion array.
Refresh Kid · AP Chemistry Unit 2 · Objectives 2.1.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 2.1, objectives 2.1.A. CED effective Fall 2024, current official file checked September 16, 2026, together with the published clarifications. This is Unit 2: Compound Structure and Properties, Topics 2.1–2.7. The focused lesson breakdown is Refresh Kid’s editorial sequence. Models and original practice are teaching materials, not official AP questions. Numerical potential curves, ion comparisons and orbital-alignment indices state their approximations. Five- and six-domain shapes are included; d-orbital hybridization and molecular-orbital diagrams are not required here. GitHub’s 3D website examples, including the Three.js Mars camera-control example, informed the use of rotatable scenes. Our scientific geometry and viewer code are original; no repository artwork or tutorial code was copied. The self-hosted Three.js library retains its MIT license. Camera rotation does not alter chemistry. See also the official clarifications.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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