Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- Vaporizing overcomes interparticle forces completely, so vapor pressure and boiling point track force strength directly.
- Melting only rearranges particles. Melting points usually rise with force strength, but packing in the crystal also matters.
- Ionic solids: strong attractions between ions; low vapor pressure, high melting and boiling points; brittle (like charges meet when layers slide); conduct only when molten or dissolved.
- Covalent network solids: atoms of nonmetals or metalloids bonded covalently throughout (diamond, graphite, SiO₂, SiC). Very high melting points. 3D networks are hard and rigid because bond angles are fixed; graphite’s layers slide, so it is soft.
- Molecular solids: separate molecules held by weak intermolecular forces. Low melting points; no conductivity, since electrons stay in bonds and lone pairs. Can be very large molecules or polymers.
- Metallic solids: cations in a sea of mobile electrons. Conduct heat and electricity; malleable and ductile. Interstitial alloys are more rigid but still conduct.
- In large biomolecules and polymers, noncovalent interactions set the shape, and the shape sets the function.
Key relationships
| Observation | Explanation to give |
|---|---|
| High vapor pressure, low boiling point | weak forces between particles; many escape at a given temperature |
| Conducts as a solid | mobile electrons (metal; graphite is the network exception) |
| Conducts only when molten or dissolved | ions that are fixed in the solid become free to move |
| Never conducts, very high melting point | covalent network |
| Never conducts, low melting point | molecular solid |
| Shatters when struck | ionic: like charges forced together |
| Bends or flattens | metallic: cations rearrange in the electron sea |
Assumptions behind the comparisons
- Boiling points are compared at the same pressure; vapor pressures at the same temperature.
- The solid is pure; impurities change melting points.
- The classification uses several properties together, not one alone.
Mistakes to avoid
- Saying covalent bonds break when a molecular solid melts or boils.
- Classifying a solid as ionic just because it melts at a high temperature.
- Saying ionic solids conduct as solids.
- Explaining diamond’s hardness with intermolecular forces.
- Using melting points alone to rank intermolecular forces.
- Naming the property and the force but never describing the particles.
Quick self-check
- A solid melts at 2700 °C and never conducts. What kind of solid is it? (Covalent network)
- Why does solid CO₂ vaporize far more easily than SiO₂ melts? (CO₂: weak forces between molecules; SiO₂: covalent bonds must break)
- Why is graphite soft? (Layers held by weak dispersion forces slide past each other)
- Which has the higher vapor pressure at 20 °C, ethanol or diethyl ether? (Diethyl ether: no hydrogen bonding)
Next: practice questions.