3.2 Properties of Solids

Solids fall into four main types: ionic, metallic, covalent network, and molecular. The way particles are arranged and the strength of the forces holding them together explain macroscopic properties like melting point, hardness, brittleness, and whether the solid conducts electricity. For AP Chemistry, connect each observed property to particle-level structure and forces.

Properties of Solids AP Chemistry Summary

AP Chemistry 3.2 is about connecting a solid's particulate-level structure to its observable properties. Ionic, metallic, covalent network, and molecular solids differ because the particles and forces holding them together are different.

On the exam, look for property clues. Ionic solids are brittle and conduct only when ions are mobile, metallic solids conduct because of delocalized electrons, covalent network solids have very high melting points, and molecular solids usually have low melting points because their intermolecular forces are weaker.

Why This Matters for the AP Chemistry Exam

This topic is built around one skill: connecting what happens at the particle level to properties you can actually observe. On the AP Chemistry exam, both multiple-choice and free-response questions ask you to compare physical properties of substances and tie them back to the forces between particles. You might be asked to explain why one solid melts at a higher temperature, why ionic solids shatter, or why metals conduct electricity while molecular solids do not.

Particulate-level drawings show up often here. Being able to read and sketch how particles are arranged, and explain how that arrangement produces a property, is exactly the kind of reasoning that earns points. The clearer you connect structure to behavior, the stronger your answers will be.

Key Takeaways

• The four solid types to know are ionic, metallic, covalent network, and molecular.
• Vapor pressure and boiling point are directly tied to the strength of the interactions holding particles together. Stronger interactions mean lower vapor pressure and higher boiling point. Melting point usually follows the same trend but can be less predictable.
• Ionic solids have high melting points, are brittle, and conduct electricity only when melted or dissolved.
• Covalent network solids are very hard with very high melting points; diamond is a rigid 3D network, while graphite is soft because its layers slide.
• Molecular solids have low melting points and do not conduct electricity because their electrons are locked in covalent bonds and lone pairs.
• Metallic solids conduct heat and electricity, and are malleable and ductile because of their mobile, delocalized electrons.

Properties of Solids by Type

Many properties of solids come from the strength and type of forces holding particles together. Since these interactions are completely broken when a substance vaporizes, vapor pressure and boiling point are directly related to interaction strength. Stronger forces mean lower vapor pressure and higher boiling point. Melting points usually follow the same pattern, but the relationship can be more subtle because melting only rearranges the interactions instead of fully breaking them.

You can classify solids by the forces that hold their particles together:

• Ionic solids are held together by electrostatic forces between cations and anions (Coulomb's law).
• Metallic solids are held together by metallic bonding.
• Covalent network solids are held together by an extended network of covalent bonds (think diamond).
• Molecular solids are held together by relatively weak intermolecular forces.

Particulate-level representations (drawings showing individual atoms, molecules, or ions) are useful for connecting microscopic structure to macroscopic properties. When you see these diagrams, look at how the particles are arranged and what forces hold them together, because that directly explains the solid's properties.

Metallic Solids

Metallic solids are made of metal atoms held together by metallic bonding. When metals form these solids, their valence electrons become delocalized and are visualized as a "sea" of electrons free to move throughout the structure, with metal cations arranged in a lattice.

Because of those free valence electrons, metallic solids tend to be:

• Good conductors of electricity and heat
• Malleable (can be hammered into sheets)
• Ductile (can be drawn into wires)

Malleability and ductility come from how easily the metal cores rearrange. When force is applied, layers of metal atoms can slide past each other while the electron sea adjusts to keep the bonding intact. This is very different from ionic solids, which shatter when layers shift.

Alloys

Alloys are mixtures of two or more elements that keep metallic properties. The way the atoms are arranged changes the material's behavior.

• In a substitutional alloy, atoms of one element take the place of atoms in the host metal's lattice.
• In an interstitial alloy, smaller atoms fit into the spaces (holes) between the larger metal atoms.

Interstitial atoms tend to make the lattice more rigid, which decreases malleability and ductility compared to the pure metal, because the small atoms keep the layers from sliding as easily. Alloys still keep a sea of mobile electrons, so they remain good conductors. Steel (iron with carbon in interstitial positions) is harder and less malleable than pure iron, which is why it works well for construction. (That specific example is an application, not required content.)

Ionic Solids

Ionic solids are made of cations and anions held in a lattice by electrostatic forces. Because these attractions are strong, ionic solids tend to have low vapor pressures, high melting points, and high boiling points.

The attractions get stronger as ion charges increase and ion sizes decrease (Coulomb's law again).

Ionic solids are brittle because of how attractions and repulsions interact. When force shifts one layer across another, ions of the same charge can end up next to each other. Like charges repel, and that repulsion fractures the crystal. So ionic solids are strong but not flexible.

They conduct electricity only when the ions are mobile, which happens when the solid is melted (molten) or dissolved in water or another solvent.

Covalent Network Solids

Covalent network solids are made of atoms connected by covalent bonds into large networks. These are formed only from nonmetals and metalloids, either as elements (diamond, graphite) or binary compounds (silicon dioxide, silicon carbide). Because the covalent bonding extends throughout the whole structure, these solids have very high melting points.

The network can be a 3D structure (diamond) or 2D layers (graphite).

Diamond

• 3D covalent network
• Rigid and extremely hard, because the covalent bond angles are fixed
• Electrical insulator

Graphite

• Layers of carbon atoms bonded in rings
• Strong bonding within each layer, but the layers are only weakly attracted to each other
• Soft, because the layers can slide past each other
• Conducts electricity due to delocalized electrons in the layers

Molecular Solids

Molecular solids are made of individual covalently-bonded molecules attracted to each other through relatively weak intermolecular forces. They have strong bonds inside each molecule (intramolecular) but weak forces between molecules (intermolecular).

General properties of molecular solids:

• Low melting and boiling points: The weak intermolecular forces are easy to overcome. Melting point depends on the strength of those forces and how efficiently the molecules pack together. Ice and sucrose are good examples.
• Do not conduct electricity: Their valence electrons are tightly held within covalent bonds and lone pairs, so there are no free charges to flow.

Molecular solids can also be made of very large molecules or polymers.

Crystalline vs. Amorphous Solids

The four solid types above are crystalline solids, meaning their particles are arranged in a regularly repeating pattern. Crystalline solids tend to have a definite melting point.

Amorphous solids do not have a long-range, repeating structure. Their particles are arranged with much more disorder, often because the material cooled quickly. Glass, rubber, and gum are common examples. You will not study amorphous solids in depth in AP Chemistry, but it helps to know that not all solids are crystalline.

In a crystalline solid, the repeating pattern of points is called a crystal lattice, and the smallest repeating unit that builds the lattice is a unit cell. You do not need to know the specific types of unit cells for the exam, but the basic idea helps: one unit cell repeated many times forms the full lattice. For ionic solids, the ions sit at the lattice points of the structure.

Biomolecules and Polymers

In large biomolecules and polymers, noncovalent interactions strongly affect properties and function. These interactions can happen between separate molecules or between different regions of the same large molecule.

• Between different molecules: for example, hydrogen bonds linking separate protein molecules
• Within one large molecule: for example, hydrogen bonds between different parts of the same protein chain

The shape of these molecules is largely set by noncovalent interactions like hydrogen bonding, London dispersion forces, and dipole-dipole interactions, and that shape strongly controls how the molecule functions. Proteins fold into specific 3D shapes, and DNA holds its double helix through hydrogen bonding between base pairs. These are useful applications of the concept rather than separate required facts.

Comparing Solids

Use this chart to compare properties across solid types:

Remember: vapor pressure is inversely related to the strength of intermolecular forces. Molecular solids have the weakest forces, so they have the highest vapor pressures and evaporate most easily.

How to Use This on the AP Chemistry Exam

MCQ

• Match a property to a solid type. If a question says a solid is hard, has a very high melting point, and does not conduct as a solid, think covalent network. If it conducts only when molten or dissolved, think ionic.
• Watch for conductivity clues. Metals conduct as solids; ionic solids conduct only when their ions are free to move.
• Compare melting points by reasoning about the strength of the forces being overcome, not just by memorizing.

Free Response

• When you explain a property, name the specific force (London dispersion, dipole-dipole, hydrogen bonding, ionic attraction, metallic bonding, or covalent bonding) and connect it to the observation.
• Avoid vague words like "strong" or "weak" by themselves. Say which force is present and compare its strength to the other forces at play.
• If a particulate diagram is provided or requested, use the arrangement of particles in your explanation rather than describing it in general terms.

Common Trap

• Do not mix up intramolecular and intermolecular forces. Boiling and melting a molecular solid breaks intermolecular forces, not the covalent bonds inside the molecules.

Common Misconceptions

• "Melting a molecular solid breaks covalent bonds." It breaks the weak intermolecular forces between molecules. The covalent bonds inside each molecule stay intact.
• "Ionic solids conduct electricity." They only conduct when the ions can move, which means melted or dissolved. As a solid, the ions are locked in place.
• "Higher melting point always means higher than every weaker-force substance, in a simple line." Melting point usually tracks with interaction strength, but it can be subtle because melting only rearranges interactions instead of fully breaking them. Packing efficiency also matters.
• "Diamond and graphite behave the same because both are carbon." Their structures differ. Diamond is a rigid 3D network and is an insulator, while graphite has layers that slide and conducts due to delocalized electrons.
• "Strong" or "weak" alone explains a property. Always identify the actual force present and compare its strength to the other forces involved.
• "Alloys stop conducting because they are mixtures." Alloys keep a sea of mobile electrons, so they still conduct. Interstitial atoms mainly reduce malleability and ductility.

Related AP Chemistry Guides

• 3.1 Intermolecular and Interparticle Forces
• Unit 3 Overview: Properties of Substances and Mixtures
• 3.4 Ideal Gas Law
• 3.5 Kinetic Molecular Theory
• 3.3 Solids, Liquids, and Gases
• 3.7 Solutions and Mixtures