Where do added atoms fit in an alloy?
You will be able to: Distinguish substitutional and interstitial alloy structures from particle size and position.
Where do added atoms fit in an alloy?
Imagine a row of large beads. You can replace a bead with another similar-sized one, or place a much smaller bead in a gap. These are useful starting pictures for two alloy arrangements.
A useful starting point: How can a metal conduct and bend? →
Words and symbols before equations
- Alloy
- A metallic material containing more than one element.
- Substitutional
- Guest atoms replace host atoms at lattice sites.
- Interstitial
- Small guest atoms occupy spaces between host atoms.
What this picture assumes
Eight host sites. Substitution replaces two hosts; interstitial addition puts two small atoms in gaps. Positions and sizes are illustrative, not a crystal-specific atomic model.
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.
- Substitution: 6 hosts + 2 guests. Guest fraction: 25% of atoms.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Comparable atom sizes can allow substitution at regular host sites. A simple copper–zinc picture illustrates this type of arrangement.
Much smaller atoms can occupy gaps between larger host atoms; carbon in iron is a common interstitial example. The actual phase structure can be more complicated than the sketch.
Added atoms can impede the movement of defects that permit layers to slip, often strengthening the material. Do not conclude that every alloy is harder or more conductive than every pure metal; composition and processing matter.
| Property | Substitutional | Interstitial |
|---|---|---|
| Guest position | Replaces a host | Occupies a gap |
| Typical relative size | Comparable to host | Much smaller than host |
| Model count | 6 hosts + 2 guests | 8 hosts + 2 guests |
A worked example, step by step
Model A replaces 2 of 8 host-site atoms with similar-sized guests. Model B adds 2 small guests between 8 host atoms. Compare their counts.
- A is substitutional: 6 hosts + 2 guests = 8 total atoms.
- Guest atom fraction in A is 2/8 = 25%.
- B is interstitial: 8 hosts + 2 guests = 10 total atoms.
- Guest atom fraction in B is 2/10 = 20%; atom fraction is not mass fraction.
A guest in a gap adds an atom; a guest replacing a host does not add a lattice site.
If 2 guests enter gaps among 8 host atoms, is the guest fraction 2/8?
Compare with an explanation
No. The total atom count is 10, so the guest fraction is 2/10.
Predict. Change one thing. Explain.
Switch between pure, substitutional and interstitial models. Keep eight host sites fixed and inspect whether guests replace hosts or occupy gaps. Rotate the optional model to see gaps in depth.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Substitution: 6 hosts + 2 guests. Guest fraction: 25% of atoms.
Eight host sites. Substitution replaces two hosts; interstitial addition puts two small atoms in gaps. Positions and sizes are illustrative, not a crystal-specific atomic model.
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 questionFor ten host sites, compare replacing two hosts with guests versus adding two guests in gaps. Name both arrangements, compute guest atom fractions, and explain one possible effect on deformation.
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Compare with the answer and four-point rubric
- 1 point: Replacement is substitutional: 2/10 = 20% guests.
- 1 point: Gap filling is interstitial: 2/12 ≈ 16.7% guests.
- 1 point: Both introduce differences that can hinder defect motion and slip.
- 1 point: This can strengthen a material, but a universal hardness ranking needs more evidence.
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 determines which simple alloy picture is plausible?
Relative sizes and where guests occupy the structure.
RECALL 2Does substitution increase the number of occupied host sites?
No; a guest replaces a host.
RECALL 3Why can alloying change strength?
Guests can impede defect motion associated with deformation.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Where do added atoms fit in an alloy?
- Guest atom fraction = guest count / total atom count.
- Atom fraction and mass fraction differ when atomic masses differ.
Remember: A guest in a gap adds an atom; a guest replacing a host does not add a lattice site.
Conditions: Eight host sites. Substitution replaces two hosts; interstitial addition puts two small atoms in gaps. Positions and sizes are illustrative, not a crystal-specific atomic model.
Refresh Kid · AP Chemistry Unit 2 · Objectives 2.4.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 2.4, objectives 2.4.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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