How do ionic charge and size affect attraction?
You will be able to: Compare ionic interactions using charge product and center-to-center distance.
How do ionic charge and size affect attraction?
Two positive and negative ion pairs can have the same separation but different charges. A +2/−2 pair attracts more strongly than a +1/−1 pair in the simple point-charge model.
A useful starting point: How do single, double and triple bonds compare? →
Words and symbols before equations
- Charge q
- Electric charge; model labels use multiples of the elementary charge e.
- Separation r
- Center-to-center distance, not the empty gap between ion surfaces.
- Coulomb model
- Point-charge attraction proportional to the charge product and inversely related to separation.
- Relative force
- A dimensionless ratio to a reference interaction.
What this picture assumes
Isolated point-charge comparison. Cation is +ze and anion is −ze. Distance factor multiplies an arbitrary fixed reference r₀. No lattice sum or short-range repulsion is included.
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.
- Force ratio = 1; attraction-energy magnitude ratio = 1. Both ions have charge magnitude 1e.
- 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 magnitudes, electric force scales as
q₊q₋
/r². Doubling both charge magnitudes multiplies the force by four at fixed separation.
The magnitude of pair potential energy scales as
q₊q₋
/r, with a negative sign for opposite charges when infinitely separated ions define zero. Do not confuse its inverse-distance dependence with the force’s inverse-square dependence.
Smaller ions often allow smaller center separations and stronger attractions for comparable structures. A crystal contains many interactions and repulsions, so this pair comparison is not an exact calculation of lattice energy or melting point.
A worked example, step by step
Compare +1/−1 ions at distance r with +2/−2 ions at distance 2r.
- The reference charge-product magnitude is 1 × 1 = 1.
- The second product is 2 × 2 = 4.
- Force ratio = 4/(2²) = 1: equal force magnitudes in this model.
- Potential-energy magnitude ratio = 4/2 = 2. Equal forces at these positions do not imply equal pair energies.
Force and potential energy have different distance dependences; a pair model is not a complete lattice calculation.
At double separation, does the force drop by one half?
Compare with an explanation
No. Its magnitude falls to one quarter; the pair potential-energy magnitude halves.
Predict. Change one thing. Explain.
First keep separation fixed and change charge product. Then hold charges fixed and double separation. Compare force and energy ratios separately.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Force ratio = 1; attraction-energy magnitude ratio = 1. Both ions have charge magnitude 1e.
Isolated point-charge comparison. Cation is +ze and anion is −ze. Distance factor multiplies an arbitrary fixed reference r₀. No lattice sum or short-range repulsion is included.
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 +1/−1 at r with +1/−2 at 2r. Give force and energy ratios, identify the direction of the interaction, and state a limitation.
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Compare with the answer and four-point rubric
- 1 point: Force ratio = 2/4 = 0.5.
- 1 point: Energy-magnitude ratio = 2/2 = 1.
- 1 point: Opposite charges attract.
- 1 point: An isolated point-charge pair does not account for the entire crystal or short-range repulsion.
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 distance belongs in the comparison?
Distance between ion centers.
RECALL 2What happens to force when r doubles?
Its magnitude becomes one quarter at fixed charges.
RECALL 3Why is melting point not an exact pair calculation?
The extended lattice and other material effects also contribute.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How do ionic charge and size affect attraction?
- Force magnitude ratio ∝
- q₊q₋
- /r².
- Pair attraction energy magnitude ∝
- q₊q₋
- /r.
Remember: Force and potential energy have different distance dependences; a pair model is not a complete lattice calculation.
Conditions: Isolated point-charge comparison. Cation is +ze and anion is −ze. Distance factor multiplies an arbitrary fixed reference r₀. No lattice sum or short-range repulsion is included.
Refresh Kid · AP Chemistry Unit 2 · Objectives 2.2.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 2.2, objectives 2.2.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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