Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- In a metal, each atom releases its valence electrons into a shared sea of delocalized electrons.
- The rest of each atom is a positive ion core (nucleus + inner electrons). The cores sit in a regular, closely packed array.
- Metallic bonding = attraction between the cores and the electron sea, in every direction.
- An alloy is a mixture of a metal with other elements. It keeps metallic bonding and has a variable composition (no formula).
Key relationships
| Idea | Rule | Example |
|---|---|---|
| Charge balance in a model | electrons in sea = (number of cores) × (valence electrons per atom) | 16 Mg atoms: 16 Mg²⁺ cores, 32 electrons |
| Size comparison | percent difference = |r(host) − r(added)| ÷ r(host) × 100 | Cu/Zn 4.7 %; Fe/C 40.5 % |
| Interstitial alloy | added atoms much smaller; fill gaps; no host atom removed | steel (C in Fe) |
| Substitutional alloy | added atoms of comparable size; replace host atoms | brass (Zn for Cu) |
| Mass % to atom ratio | convert each mass to moles, then compare | 70 % Cu / 30 % Zn brass: 29 % of atoms are Zn |
Properties from the model
- Conducts electricity and heat: mobile delocalized electrons (alloys too).
- Malleable and ductile: layers of cores slide; the sea adjusts; no like-charge repulsion.
- Interstitial alloy: more rigid lattice, harder, less malleable and ductile.
- Substitutional alloy: uneven rows, layers slide less easily, usually harder than the pure metal.
Assumptions
- Atoms are treated as spheres of fixed radius; tables list slightly different radii.
- Models show a small 2-D section; the real array is 3-D and extends in every direction.
Mistakes to avoid
- Calling metallic bonding “ionic” because the cores are positive.
- Drawing electrons attached to particular cores.
- Saying interstitial atoms replace host atoms.
- Treating mass percent as atom percent when the molar masses are very different (C and Fe).
- Saying alloys stop conducting.
Quick self-check
- Zinc (134 pm) and copper (128 pm): which type of alloy? (Substitutional: 4.7 % difference.)
- How many delocalized electrons in a model of 12 Al atoms? (36)
- Why is steel less ductile than iron? (Carbon atoms in the gaps make the lattice more rigid, so layers cannot slide as easily.)
Next: practice questions.