Substitutional alloys
Atoms of the same size take each other's places
1. Alloys and their types
2. Where does chromium go?
Stainless steel has chromium mixed into iron. In the book's table, the atomic radius of Cr is 1.17 and of Fe is 1.16.
Pick the right answer
Predict: where do the chromium atoms sit?
- A. In the small spaces between iron atoms
- B. In the places of some iron atoms
- C. Nowhere: atoms of two metals cannot mix
Explanation
The chromium atoms sit in the places of some iron atoms. A Cr atom (1.17) is about the same size as an Fe atom (1.16), so it can take an iron atom's place in the lattice instead of fitting into the small spaces.
3. Taking each other's places
Substitutional alloy
An alloy in which some of the atoms of the pure metal are replaced by atoms of another metal.
The lattice keeps its shape. Only some of its atoms are different.
4. Three conditions
The book's conditions: the other metal must have the same
- atomic radius
- crystal lattice
- chemical properties
True or false?
Any two metals can form a substitutional alloy.
Explanation
The statement is false. A substitutional alloy needs the other metal to have the same atomic radius, crystal lattice and chemical properties as the pure metal; being metals is not enough.
5. The fit test
Try three atoms in a host metal. Watch where each one goes.
Atom tried (1 = same size, 2 = much smaller, 3 = much bigger) 1
It replaces a host atom. If the lattice and chemical properties also match, the alloy is substitutional.
6. Why Fe, Cr and Ni swap easily
Recall: from Cr to Cu, the atomic radius is nearly constant.
| Element | Atomic radius |
|---|---|
| Cr | 1.17 |
| Mn | 1.17 |
| Fe | 1.16 |
| Co | 1.16 |
| Ni | 1.15 |
| Cu | 1.17 |
The book says this explains their use in making alloys. An Fe atom's place fits a Cr or Ni atom.
7. Sort the book's examples
Use the size of the added atom to decide.
Sort into groups
Which type is each alloy?
Ferrochrome (Fe-Cr), in stainless steelCopper-gold alloyFerronickel (Fe-Ni)Steel (Fe + C)
8. Check yourself
Pick the right answer
Copper (radius 1.17) and nickel (1.15) have the same crystal lattice and similar chemical properties. Which type of alloy do they form?
- A. Substitutional
- B. Interstitial
- C. None: two metals cannot mix
Explanation
They form a substitutional alloy. Cu (1.17) and Ni (1.15) are nearly the same size, with the same crystal lattice and similar chemical properties, so Ni atoms can take the places of some Cu atoms.
Pick the right answer
Which of these is NOT one of the book's conditions for a substitutional alloy?
- A. Both elements are metals
- B. The same atomic radius
- C. The same crystal lattice
- D. The same chemical properties
Explanation
"Both elements are metals" is not one of the book's conditions. The three conditions are the same atomic radius, the same crystal lattice and the same chemical properties.
True or false?
In stainless steel, chromium atoms sit in the spaces between iron atoms.
Explanation
The statement is false. Cr (1.17) and Fe (1.16) atoms are nearly the same size, so in stainless steel the Cr atoms replace some Fe atoms (a substitutional alloy) rather than sitting in the spaces.
9. Summary
- Substitutional alloy: atoms of another metal replace some atoms of the pure metal.
- Conditions: the same atomic radius, crystal lattice and chemical properties.
- Book examples: ferrochrome (in stainless steel), copper-gold, ferronickel.
- From Cr to Cu the atomic radius is nearly constant, so these metals form such alloys.
| Alloy | Elements | Type |
|---|---|---|
| Ferrochrome (in stainless steel) | Fe, Cr | substitutional |
| Copper-gold alloy | Cu, Au | substitutional |
| Ferronickel | Fe, Ni | substitutional |
| Steel | Fe, C | interstitial |
Can you explain why Cr atoms take iron's places, while C atoms go into the spaces?