Topic 3.2 Notes – Properties of Solids

1. How Particle Structure Determines Solid Properties

In a solid, particles are fixed in place and only vibrate. What those particles are, and how strongly they attract each other, controls the properties you measure in lab.

Here’s the core relationship you should always think about:

• Stronger attractions → higher melting point
• Stronger attractions → higher boiling point
• Stronger attractions → lower vapor pressure

Why vapor pressure and boiling point track so cleanly with force strength:
When a substance vaporizes, particles must completely overcome attractions. Stronger forces make that harder.

Melting is slightly subtler. During melting, particles are rearranged but not fully separated, so melting points correlate with force strength, but trends can be less perfectly ordered.

Crystalline Solids

Most solids you’ll see on the AP exam are crystalline solids:

• Particles arranged in a repeating 3D pattern
• The smallest repeating piece is a unit cell
• Definite melting point

Particulate diagrams are huge on tests. You might see a diagram and have to identify the solid type based on:

• What the particles are (ions? molecules? atoms?)
• How they’re arranged
• What forces connect them

Here’s a visual comparison to anchor the four structures. Notice how the top row shows alternating charged particles (ionic), the next row shows discrete molecules packed together (molecular), the third row shows extended covalent networks, and the bottom row shows metal atoms in a uniform array:

Arrangements of particles in ionic, molecular, network covalent, and metallic solids

2. The Four Types of Crystalline Solids

Ionic Solids

Particles: Cations and anions
Force: Electrostatic (Coulombic) attraction

Properties

• High melting and boiling points
• Low vapor pressure
• Hard but brittle
• Conduct electricity only when molten or dissolved

Why brittle?
If layers shift, like charges line up → strong repulsion → the crystal fractures.

Strength increases with:

• Larger ionic charge
• Smaller ionic radius

That’s straight Coulomb’s law logic.

A classic trap on tests: solid ionic compounds do not conduct. Ions must be mobile.

Covalent Network Solids

Particles: Atoms connected in an extended network
Force: Covalent bonds throughout

Only formed from nonmetals or metalloids (like C, Si).

Properties

• Extremely high melting points
• Extremely low vapor pressure
• Very hard and rigid (3D networks)
• Usually poor conductors

Two important structures:

• 3D networks (diamond, SiO₂)
  • Fixed bond angles
  • Very hard
  • Nonconductive
• 2D layered networks (graphite)
  • Strong bonds within layers
  • Weak forces between layers
  • Layers slide → soft
  • Delocalized electrons → conducts

Graphite is a favorite AP example because it breaks the “covalent = nonconductive” assumption.

Molecular Solids

Particles: Discrete molecules
Force: Intermolecular forces (LDF, dipole-dipole, hydrogen bonding)

Properties

• Low melting and boiling points
• Higher vapor pressure
• Soft or brittle
• Do not conduct electricity

Electrons are localized in covalent bonds, so there are no mobile charge carriers.

Melting point depends on:

• Strength of IMFs
• Molecular size and shape (packing efficiency)

These can include very large molecules like polymers.

Metallic Solids

Particles: Metal cations
Force: Metallic bonding (delocalized valence electrons)

Think of positive metal ions arranged in a lattice, surrounded by a sea of mobile valence electrons:

Metallic bonding: lattice of metal cations in a sea of delocalized electrons

Properties

• Conduct electricity and heat (in solid state)
• Malleable
• Ductile
• Variable melting points
• Shiny

Layers can shift without breaking attraction because the electron sea adjusts and continues to hold the metal cations together.

Alloys

Mixtures of metals that retain conductivity.

• Substitutional: similar-sized atoms replace host atoms
• Interstitial: small atoms fill gaps
  → Lattice becomes more rigid
  → Decreased malleability and ductility

Steel is a classic interstitial example.

3. Comparing the Four Solid Types

General bonding strength trend:
Covalent network ≈ Ionic > Metallic > Molecular

Vapor pressure trend goes in the opposite direction.

4. Large Biomolecules and Polymers

Large molecules form molecular solids, but their properties depend heavily on noncovalent interactions.

These interactions can be:

• Intermolecular (between different molecules)
• Intramolecular (within one large molecule)

Forces involved:

• Hydrogen bonding
• Dipole-dipole
• London dispersion

These interactions determine:

• 3D shape
• Flexibility
• Mechanical strength
• Biological function

Examples:

• Protein folding depends on intramolecular hydrogen bonding.
• DNA’s double helix is stabilized by hydrogen bonds between base pairs.
• Polymer strength depends on interactions between chains.

On free-response questions, they often expect you to link shape → noncovalent interactions → macroscopic property.