Compound Structure and Properties
Ionic Crystals & Metals — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 8 parts Lesson 1 of 13 Unit quiz
Lesson summary
Ionic Crystals & Metals — AP CHEMISTRY notes: Explain the structure and properties of ionic crystals, metallic bonding and alloys.Ionic crystals are lattices of positive and negative ions in a three-dimensional, regular repeating pattern.The arrangement maximises Coulombic attrac…
- Coulomb's law
- Higher charge
- Smaller radius
- Delocalised electrons
- Interstitial alloy
- Substitutional alloy
Section 1 of 8Ionic crystals and lattices
Ionic crystals and lattices
Ionic crystals are lattices of positive and negative ions in a three-dimensional, regular repeating pattern.
The arrangement maximises Coulombic attraction between opposite charges and minimises repulsion between like charges.
Lattices are close-packed and symmetric, with an overall neutral charge.
Section 2 of 8Strength of ionic bonding
Coulomb's law
- Coulomb's law attraction increases with ion charge and decreases with ion radius.
Higher charge
- Higher charge the attraction in MgCl2 is stronger than in NaCl because Mg2+ is 2+ and Na+ is 1+.
Smaller radius
- Smaller radius the attraction in NaCl is stronger than in KCl because Na+ is smaller than K+.
The distance r is the sum of the cation and anion radii.
Section 3 of 8Properties of ionic crystals
| Property | Explanation |
|---|---|
| Brittle | Shifting layers bring like charges together, so the crystal cleaves |
| High melting and boiling points | Strong Coulombic attraction in the lattice |
| Generally soluble in water | Ion-dipole interactions with water compensate for breaking the lattice |
| Conduct when molten or dissolved | Ions are free to move; in the solid they are fixed |
Section 4 of 8Metallic bonding
1
Metal cations sit in a fixed lattice surrounded by delocalised valence electrons, a sea of electrons.
2
Metals are good conductors of electricity and heat, malleable and ductile.
3
Strong attraction between cations and the electron sea gives most metals high melting points.
4
Melting points vary, for example magnesium 650 °C, copper 1085 °C, gallium 30 °C and platinum 1768 °C.
5
Network covalent solids also have high melting points, but most (excluding graphite) do not conduct electricity because their electrons are localised.
Section 5 of 8Alloys
Alloy
Alloy a homogeneous mixture made by melting and mixing metals (or metals with nonmetals such as carbon) and cooling.
The properties of an alloy differ from those of its components.
| Type | Structure | Example |
|---|---|---|
| Interstitial | Much smaller atoms occupy spaces between larger atoms in the lattice | Steel (carbon in iron) |
| Substitutional | Atoms of similar radius replace some atoms in the lattice | Brass (zinc in copper) |
- Alloys with elements from the first two periods, such as carbon, are typically interstitial.
Section 6 of 8More from the specification
- Physical property
- A property observed or measured without changing the material.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Lattice | Regular three-dimensional array of ions. |
| Delocalised electrons | Valence electrons free to move through a metal. |
| Alloy | Homogeneous mixture of metals or a metal with a nonmetal. |
| Interstitial alloy | Small atoms fill spaces in a lattice of larger atoms. |
| Substitutional alloy | Atoms of similar radius replace some lattice atoms. |
| Ion-dipole interaction | Attraction between an ion and a polar molecule. |
Section 8 of 8Exam technique
1
Saying ionic solids conduct as solids — ions are fixed, so they conduct only when molten or in solution
2
Ignoring ion size and discussing only charge
3
Calling the sea of electrons a set of bonds between individual atoms
4
Mixing up interstitial and substitutional alloys
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