# Substitutional alloys

> Atoms of the same size take each other's places


## Objectives

- Describe a substitutional alloy: atoms of another metal replace some atoms of the pure metal.
- State the three conditions: the same atomic radius, crystal lattice and chemical properties.
- Explain why metals such as Fe, Cr and Ni form substitutional alloys easily.
- Classify the book's examples: ferrochrome, copper-gold, ferronickel.

## 1. Alloys and their types




## 2. Where does chromium go?

Predict: where do the chromium atoms sit?

- Nowhere: atoms of two metals cannot mix
- In the small spaces between iron atoms
- In the places of some iron atoms ✓

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.

[ar-EG] ذرات الـ chromium بتقعد في أماكن بعض ذرات الـ iron. ذرة الـ Cr (1.17) تقريبًا في نفس حجم ذرة الـ Fe (1.16)، فتقدر تاخد مكان ذرة iron في الـ lattice بدل ما تدخل في الفراغات الصغيرة.
## 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 Any two metals can form a substitutional alloy.


## 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) It replaces a host atom. If the lattice and chemical properties also match, the alloy is substitutional. It goes into the spaces instead. That is an interstitial alloy, like carbon in steel. It cannot simply take a host atom's place. The radius condition fails.


## 6. Why Fe, Cr and Ni swap easily

Recall: from Cr to Cu, the atomic radius is nearly constant. Book table, p.12 (for illustration) 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. Which type is each alloy?


## 8. Check yourself

Copper (radius 1.17) and nickel (1.15) have the same crystal lattice and similar chemical properties. Which type of alloy do they form? Which of these is NOT one of the book's conditions for a substitutional alloy? In stainless steel, chromium atoms sit in the spaces between iron atoms.


## 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. The book's examples (p.19-20) Book question (p.25): "Chromium and iron form ... alloy." Answer: substitutional. Can you explain why Cr atoms take iron's places, while C atoms go into the spaces?



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-alloy.substitutional