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AP Chemistry / Unit 2 / Topic 2.3
LESSON 06 / 20 · TOPIC 2.3

Why is a salt crystal an extended 3D structure?

You will be able to: Represent an alternating-ion solid and explain brittleness with charge relationships.

Bonding, geometry and chemical reasoningFree study resourceReview editionTeacher review pending

Why is a salt crystal an extended 3D structure?

A grain of salt looks like one object, but its ion arrangement repeats in three dimensions. One sodium ion is attracted to several chloride neighbors; it is not paired with just one private partner.

A useful starting point: How do ionic charge and size affect attraction? →

Words and symbols before equations

Lattice
A repeating spatial arrangement in a crystal.
Formula unit
The simplest whole-number ratio of ions.
Nearest neighbor
One of the closest particles to a chosen particle.
Brittleness
A tendency to fracture rather than deform extensively.
NaCl-type cutout · 32 Na⁺ and 32 Cl⁻ · spatial modelNa⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Na⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Schematic perspective; sizes are not atomic radii.
Read this model snapshot. NaCl-type cutout · 32 Na⁺ and 32 Cl⁻. Equal ion counts give zero net charge.
What this picture assumes

Finite 4 × 4 × 4 NaCl-type cutout, equal counts of ±1 ions. Every bulk site would have six opposite-charge nearest neighbors; boundaries omit neighbors. Atom sizes and gaps are schematic. No memorization of this crystal type is required.

Read the picture in three steps

  1. 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.
  2. NaCl-type cutout · 32 Na⁺ and 32 Cl⁻. Equal ion counts give zero net charge.
  3. Check what the picture assumes below. Use the Explore task to predict one change before moving a control.

Connect the picture to the chemistry

A useful ionic model places opposite charges nearby and avoids putting like charges at the closest sites. The solid’s stability reflects the balance of all attractions and repulsions.

The NaCl-type block shown is an illustrative local model, not a requirement to memorize a named crystal structure. The outer faces truncate a lattice that continues beyond the picture; coordination at an edge is not bulk coordination.

If a layer shifts so like charges approach each other, repulsion can promote fracture. This contrasts with the more nondirectional bonding in many metals. Not every ionic compound has the same geometry or ion ratio.

A worked example, step by step

A fragment contains 32 Na⁺ ions and 32 Cl⁻ ions. Interpret its formula and charge.

  1. Reduce the count ratio 32:32 to 1:1.
  2. Write NaCl as the formula unit; do not label each adjacent pair a molecule.
  3. Total charge is 32(+1) + 32(−1) = 0 in units of e.
  4. The arrangement continues through space, with multiple neighboring ions contributing to each ion’s environment.
Common mix-up

An ionic formula gives a ratio, not the size of a molecule or the entire crystal.

CHECK THE IDEA

Does a corner ion in a finite drawing have the full bulk environment?

Compare with an explanation

No. The drawing cuts away neighbors that exist beyond the shown boundary.

Now investigate one change Explore →

Predict. Change one thing. Explain.

Inspect one layer, then load the optional 3D block and rotate it. Identify opposite-charge nearest neighbors along three directions; camera rotation must not change the charges.

On narrow screens, swipe or scroll diagrams sideways to read all labels.

NaCl-type cutout · 32 Na⁺ and 32 Cl⁻ · spatial modelNa⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Na⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Cl⁻Na⁺Cl⁻Na⁺Na⁺Cl⁻Na⁺Cl⁻Schematic perspective; sizes are not atomic radii.

NaCl-type cutout · 32 Na⁺ and 32 Cl⁻. Equal ion counts give zero net charge.

Finite 4 × 4 × 4 NaCl-type cutout, equal counts of ±1 ions. Every bulk site would have six opposite-charge nearest neighbors; boundaries omit neighbors. Atom sizes and gaps are schematic. No memorization of this crystal type is required.

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.

1. What does NaCl specify?

Show answer and reasoning

A 1:1 ion ratio. The formula specifies stoichiometry, not an isolated molecule or the crystal’s total particle count.

2. Why can shifting an ionic layer promote fracture?

Show answer and reasoning

Like charges can become adjacent. Repulsion between newly adjacent like charges can destabilize the shifted arrangement.

Original written challenge

4 points · self-check · not an official AP question

Sketch two alternating rows of + and − ions. Explain attraction, neutrality, and why a shifted row can be unfavorable. Identify one limitation of a flat sketch.

This response is not submitted or saved. Copy it before leaving.

Compare with the answer and four-point rubric
  1. 1 point: The unshifted drawing places unlike charges near one another.
  2. 1 point: Equal numbers of +1 and −1 ions give zero net charge.
  3. 1 point: A shift can align like charges and increase repulsion.
  4. 1 point: A 2D sketch omits neighbors above and below the plane.

Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.

Recall the ideas without notes Review →

Retrieve it before you reveal it.

RECALL 1Why are ionic solids extended?

Each ion interacts with many surrounding ions.

RECALL 2What does rotating a model change?

The view, not its ion identities, charges or relationships.

RECALL 3Must every salt use the pictured lattice?

No. It is one illustrative arrangement.

Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.

Why is a salt crystal an extended 3D structure?

  • Neutral crystal: sum of all ionic charges = 0.
  • A finite block is a cutout of an extended arrangement.

Remember: An ionic formula gives a ratio, not the size of a molecule or the entire crystal.

Conditions: Finite 4 × 4 × 4 NaCl-type cutout, equal counts of ±1 ions. Every bulk site would have six opposite-charge nearest neighbors; boundaries omit neighbors. Atom sizes and gaps are schematic. No memorization of this crystal type is required.

Refresh Kid · AP Chemistry Unit 2 · Objectives 2.3.A · Review edition

Framework, scope and review status

Mapped to College Board CED, Topic 2.3, objectives 2.3.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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