Properties of Solids
Link solid structure, particle mobility and disrupted contacts to observed material properties.
AP Chemistry Topic 3.2 · CED printed p. 60, 61 / PDF p. 67, 68. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Ionic and metallic structures
- Intermolecular interactions
The structure tells you which contacts must change
Vaporizing a molecular liquid separates its molecules almost completely, so stronger attractions usually mean lower vapor pressure at the same temperature and a higher boiling point at the same external pressure. Compare at matched conditions. Melting is subtler: particles rearrange while many contacts remain. Crystal packing and symmetry can therefore influence melting points alongside interaction strength. Do not use one simple force ranking to predict all melting points exactly.
Draw several interacting particles when explaining a bulk property. One molecule may show polarity, but it cannot by itself show contacts between molecules, a lattice, charge mobility or layers sliding. A particle model should identify the units: molecules, ions, metal cores or network atoms.
Four useful solid models
| Solid model | Particle structure | Typical property explanation |
|---|---|---|
| Ionic | Extended cation/anion array | Strong attractions; high melting, low volatility; brittle; conduction only when ions become mobile. |
| Molecular | Distinct covalent molecules with noncovalent contacts | Often lower melting; ordinarily nonconducting because charges/electrons are not freely mobile. Large molecules and polymers can be exceptions to simple melting expectations. |
| Covalent network | Covalent connections through 3-D network or 2-D layers | Strong covalent structure; high melting or decomposition temperatures. 3-D networks rigid/hard; layer behavior differs. |
| Metallic | Positive cores and delocalized electrons | Conduct heat/electricity; often malleable and ductile; alloy guest atoms can hinder rearrangement. |
Diamond’s three-dimensional covalent framework resists deformation. Graphite has covalent networks within layers, but weaker contacts between layers let them slide, so it is soft. Delocalized electronic structure within graphite layers also allows electrical conduction; “every covalent solid is a nonconductor” is too broad. Network solids here involve nonmetals/metalloids, as elemental carbon or binary SiO2/SiC, rather than separate small molecules.
Ionic solid charges cannot carry current until mobile; a molten salt or a suitably dissolved electrolyte can conduct. Metallic electrons are already mobile in the solid. Interstitial alloy guests hinder layer rearrangement while retaining metallic electrons. Always name the actual carrier and its mobility.
Polymers and biomolecules connect shape to properties
A large covalent chain still contains noncovalent contacts between regions or between neighboring chains. Those contacts can affect flexibility, folding, association and function. Changes in temperature or surroundings can disrupt a stable conformation without necessarily severing its covalent backbone. A molecular-solid model of large chains may need information on cross-linking and packing before an exact melting or mechanical prediction is possible.
Let's work through it
Work it through
A solid A is brittle, does not conduct as a solid but conducts when molten; B is soft, consists of carbon and conducts in layers; C is easily drawn into wire and conducts as a solid. Explain consistent models.
- A supports an ionic lattice: fixed solid ions versus mobile molten ions, plus repulsive like-charge contacts after layer displacement.
- B supports layered graphite: strong covalent bonding within layers, weaker interlayer contacts for sliding and delocalized electrons for conduction.
- C supports metallic structure: delocalized electrons carry current while metal cores can rearrange without losing cohesion.
- The combined observations support the assignments. A single property, such as solid conductivity, would not uniquely distinguish graphite from a metal.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Why is a boiling-point comparison most meaningful at equal external pressure?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Why is a boiling-point comparison most meaningful at equal external pressure?
Worked answer and reason
Boiling occurs when vapor pressure reaches external pressure, so changing that pressure changes the boiling temperature.
Conditions must be controlled when relating a property to interactions.
Retrieval 2
Why is graphite softer than diamond?
Worked answer and reason
Graphite’s covalent layers can slide across weaker interlayer contacts; diamond has a rigid 3-D covalent framework.
Both contain carbon, but their connectivity differs.
Retrieval 3
Can changing noncovalent contacts change protein function without breaking the backbone?
Worked answer and reason
Yes. Shape depends strongly on these contacts, and function can depend on shape.
Conformation and covalent connectivity are distinct structural properties.
Catch the tempting mistake
- Every covalent material melts at a low temperature.
- Molecular solids and extended covalent networks are different structural classes.
- Every covalent solid fails to conduct.
- Graphite is a layered-network exception with mobile delocalized electrons.
- A high melting point proves an ionic lattice.
- Covalent networks can also resist melting; use additional structural and mobility evidence.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 3 problem 4 · ap application · macroscopic
- Unit 3 problem 5 · ap synthesis · multiple
- Unit 3 problem 6 · ap synthesis · particle
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For the teacher
- Require two independent pieces of evidence before classifying an unknown solid.
- Ask students to specify the contacts disrupted in melting versus vaporization.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 3.2.A
- Explain the relationship among the macroscopic properties of a substance, the particulate-level structure of the substance, and the interactions between these particles.
- 3.2.A.1
- Many properties of liquids and solids are determined by the strengths and types of intermolecular forces present. Because intermolecular interactions are overcome completely when a substance vaporizes, the vapor pressure and boiling point are directly related to the strength of those interactions. Melting points also tend to correlate with interaction strength, but because the interactions are only rearranged, in melting, the relations can be more subtle.
- 3.2.A.2
- Particulate-level representations, showing multiple interacting chemical species, are a useful means to communicate or understand how intermolecular interactions help to establish macroscopic properties.
- 3.2.A.3
- Due to strong interactions between ions, ionic solids tend to have low vapor pressures, high melting points, and high boiling points. They tend to be brittle due to the repulsion of like charges caused when one layer slides across another layer. They conduct electricity only when the ions are mobile, as when the ionic solid is melted (i.e., in a molten state) or dissolved in water or another solvent.
- 3.2.A.4
- In covalent network solids, the atoms are covalently bonded together into a three-dimensional network (e.g., diamond) or layers of two-dimensional networks (e.g., graphite). These are only formed from nonmetals and metalloids: elemental (e.g., diamond, graphite) or binary compounds (e.g., silicon dioxide and silicon carbide). Due to the strong covalent interactions, covalent solids have high melting points. Three-dimensional network solids are also rigid and hard, because the covalent bond angles are fixed. However, graphite is soft because adjacent layers can slide past each other relatively easily.
- 3.2.A.5
- Molecular solids are composed of distinct, individual units of covalently-bonded molecules attracted to each other through relatively weak intermolecular forces. Molecular solids generally have a low melting point because of the relatively weak intermolecular forces present between the molecules. They do not conduct electricity because their valence electrons are tightly held within the covalent bonds and the lone pairs of each constituent molecule. Molecular solids are sometimes composed of very large molecules or polymers.
- 3.2.A.6
- Metallic solids are good conductors of electricity and heat, due to the presence of free valence electrons. They also tend to be malleable and ductile, due to the ease with which the metal cores can rearrange their structure. In an interstitial alloy, interstitial atoms tend to make the lattice more rigid, decreasing malleability and ductility. Alloys typically retain a sea of mobile electrons and so remain conducting.
- 3.2.A.7
- In large biomolecules or polymers, noncovalent interactions may occur between different molecules or between different regions of the same large biomolecule. The functionality and properties of such molecules depend strongly on the shape of the molecule, which is largely dictated by noncovalent interactions.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §10.1, Intermolecular Forces — Dispersion, dipole and hydrogen-bonding comparisons and vapor-pressure reasoning.
- OpenStax Chemistry 2e §10.5, The Solid State of Matter — Solid classifications and network/layer structure distinctions.
Glossary and deeper reading · 7 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Vaporization
Vaporization is a phase change from liquid to gas. It requires energy to separate particles against their attractive interactions without ordinarily breaking the molecules' internal bonds. For example, boiling liquid water produces water vapor while preserving the water molecules.
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Crystal lattice
A crystal lattice describes the repeating arrangement of structural sites in a crystalline solid. The occupants can be ions, atoms, or molecules depending on the material. For example, an ionic lattice places opposite charges near one another throughout a sodium chloride crystal.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Electron delocalization
Electron delocalization means that an electronic description extends across multiple atoms rather than being confined to one localized bond or atom. It can affect bond equivalence and charge distribution. For example, nitrate's bonding electrons are described across several nitrogen–oxygen positions.
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Covalent network solid
A covalent network solid contains atoms joined by covalent bonds throughout an extended structure. Changing or melting that structure requires more than separating intact small molecules. For example, diamond contains a three-dimensional network of carbon–carbon bonds.
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Electrolyte
An electrolyte forms mobile ions when dissolved and can allow a solution to conduct electric current. Strong and weak electrolytes differ in the extent to which dissolved species form ions under the stated conditions. For example, dissolved sodium chloride supplies sodium and chloride ions to water.
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Alloy
An alloy is a metallic material containing more than one element. Added atoms can replace host atoms or occupy spaces between them, changing properties such as hardness. For example, carbon atoms in steel affect how easily iron atoms can move relative to one another.