AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/2/2-4)
Unit 2 · Topic 2.4
2.4 Structure of Metals and Alloys
A metal can be modeled as a lattice of positive ions in a sea of delocalized valence electrons. Mixing in a second element makes an alloy: atoms of similar size swap into the lattice (a substitutional alloy, like brass), while much smaller atoms squeeze into the gaps (an interstitial alloy, like steel).
Key terms
- metallic bonding
- sea of electrons
- delocalized electrons
- alloy
- substitutional alloy
- interstitial alloy
The electron-sea model
In a metal, each atom gives up its valence electrons to a shared pool. What's left is a regular array of positive metal ions (the nuclei plus core electrons), surrounded by valence electrons that move freely through the whole solid. These are called delocalized electrons. The attraction between the positive ions and the electron sea holds the metal together.
This model explains the classic properties of metals.
- Electrical conductivity: the delocalized electrons can flow when a voltage is applied.
- Thermal conductivity: mobile electrons carry energy through the metal quickly.
- Malleability (can be hammered into sheets) and ductility (can be drawn into wires): layers of metal ions can slide past each other, and the electron sea moves with them, so the metal bends instead of shattering.
Substitutional alloys
An alloy is a mixture of a metal with one or more other elements that keeps metallic properties. In a substitutional alloy, the added atoms are about the same size as the host atoms, so they take the place of some host atoms in the lattice.
Brass is the standard example: zinc atoms (radius about 134 pm) replace some copper atoms (about 128 pm). In a diagram, the lattice looks like pure copper's, but some circles are a different element of nearly the same size.
Interstitial alloys
In an interstitial alloy, the added atoms are much smaller than the host atoms, so they fit into the spaces (interstices) between them. Steel is the standard example: small carbon atoms sit in the gaps between larger iron atoms.
The small atoms in the gaps make it harder for layers of host atoms to slide past one another. So interstitial alloys are more rigid and harder than the pure metal, and less malleable and ductile.
Drawing metals and alloys
- Pure metal: identical circles in neat, closely packed rows. Label them as positive ions and indicate the delocalized electrons around them.
- Substitutional alloy: the same neat rows, with some circles replaced by a second kind of circle of nearly equal size.
- Interstitial alloy: rows of large host atoms with much smaller atoms sitting in the gaps between them, not replacing any host atoms.
- In every case, keep the ratio of atoms reasonable for the alloy described; steel has only a small fraction of carbon.
What stays the same
Both kinds of alloys still contain a sea of delocalized electrons, so they still conduct electricity and heat. When you explain an alloy's properties, name the type of alloy, describe where the added atoms sit and connect that to how easily layers can move.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying an alloy from atomic radii
Metal A has an atomic radius of 144 pm. Element B has a radius of 146 pm and element C has a radius of 77 pm. Predict the type of alloy A would form with B, and with C.
Show the solutionHide the solution
- Step 1: B's radius is almost the same as A's (within about 2%), so B atoms can replace A atoms in the lattice: a substitutional alloy.
- Step 2: C's radius is about half of A's, so C atoms are small enough to fit in the gaps between A atoms: an interstitial alloy.
Answer: A with B: substitutional alloy. A with C: interstitial alloy.
- Example 2
Explaining a property change (classic trap)
Pure iron is fairly easy to bend, but steel (iron with a small amount of carbon) is much harder to bend. A student says steel is harder because carbon forms ionic bonds with iron. Give a better explanation.
Show the solutionHide the solution
- Step 1: Steel is an interstitial alloy. Carbon atoms are much smaller than iron atoms and sit in the gaps between them.
- Step 2: Bending a metal requires layers of atoms to slide past each other. The carbon atoms in the gaps get in the way, so the layers can't slide as easily.
- Step 3: There's no ionic bonding; steel still has delocalized electrons and conducts electricity.
Answer: Carbon atoms occupy the interstitial spaces in the iron lattice and block layers of iron atoms from sliding, making steel more rigid and less malleable.
Common mistakes
- Mixing up the two alloy types. Substitutional means similar sizes swapping places; interstitial means small atoms in the gaps.
- Saying alloys stop conducting electricity. Both types keep their sea of mobile electrons.
- Drawing a metal as neutral atoms sharing pairs of electrons. Show positive ions in a sea of delocalized electrons.
- Explaining malleability without mentioning that layers of ions can slide while the electron sea keeps holding them together.
On the exam
- Expect to identify or draw a particle diagram of an alloy and explain a property such as malleability or conductivity. A strong answer describes the particles' sizes and positions, then connects to the property.
Connected topics
Videos
Check yourself
4 questions on 2.4 Structure of Metals and Alloys. Pick an answer to see if you got it, and why.
| Element | Atomic radius (pm) |
|---|---|
| C | 77 |
| Fe | 126 |
| Cu | 128 |
| Zn | 134 |
Approximate atomic radii of four elements used in common alloys
Based on the data, which pair of elements is most likely to form an interstitial alloy?
Brass is an alloy in which some of the copper atoms in a copper lattice are replaced by zinc atoms. Which feature of the data best supports this model?
Steel made by adding a small amount of carbon to iron is harder and less malleable than pure iron. Which of the following best explains this?
A copper wire can be bent and drawn into thinner wire without breaking. Which of the following best explains this property?
0 of 4 answered