Properties of Solids: Study Guide (Chemistry 3.2)

From particles to properties

In Topic 3.1 you learned the forces that act between particles. This topic uses them to explain what you can see and measure: why some solids melt in your hand and others survive a furnace, why some conduct electricity, and why some bend while others shatter.

Every explanation in this topic has the same three parts:

1. Particles: what the solid is made of (ions, atoms, molecules, metal cations and electrons).
2. Interactions: what holds those particles together (ionic attractions, covalent bonds, intermolecular forces, metallic bonding).
3. Property: how that structure produces the observation.

An answer that names the property and the force but skips the particles usually loses marks. Link all three.

Boiling point and vapor pressure

When a liquid vaporizes, its particles move far apart. The interactions between them are overcome completely. So the energy needed to vaporize, and the temperature at which a liquid boils, depend directly on how strong those interactions are.

Vapor pressure is the pressure of the vapor above a liquid (or solid) in a closed container, once the amount of vapor has stopped changing, at a given temperature. At any moment, only some molecules at the surface have enough energy to escape. The weaker the forces holding them, the more molecules escape, and the higher the vapor pressure.

Liquid at 20 °C	Forces between molecules	Vapor pressure	Boiling point
Diethyl ether, (C₂H₅)₂O	dispersion, dipole–dipole	58.7 kPa	35 °C
Ethanol, C₂H₅OH	dispersion, dipole–dipole, hydrogen bonding	5.95 kPa	78 °C
Water, H₂O	dispersion, dipole–dipole, extensive hydrogen bonding	2.34 kPa	100 °C

The pattern runs both ways: stronger forces → lower vapor pressure → higher boiling point. Notice that diethyl ether has the most electrons (42, against 26 for ethanol and 10 for water) but cannot hydrogen bond with itself. Its vapor pressure is about ten times that of ethanol.

Melting point is more subtle

Melting does not separate the particles. It changes a regular, packed arrangement into a disordered liquid in which the particles still touch. The interactions are rearranged, not removed.

Melting points still tend to rise with the strength of the interactions, so they are useful evidence. But they also depend on how well the particles fit together in the solid. Compact, symmetrical molecules pack neatly into a crystal and can have surprisingly high melting points. So if you need to compare the strength of intermolecular forces, prefer boiling points or vapor pressures. Worked example 3 shows a case where melting and boiling points disagree.

Four kinds of solid

Solids are grouped by the particles they contain and the interactions that hold them together.

Ionic solids

Particles: cations and anions in a lattice (Topic 2.3). The attractions between ions are strong, so ionic solids have low vapor pressures and high melting and boiling points. They are brittle: if a sharp blow slides one layer of ions along the next, ions of like charge end up side by side, they repel, and the crystal splits. They conduct electricity only when the ions can move: when molten or dissolved in water or another solvent. In the solid, the ions are fixed in place.

Covalent network solids

Particles: atoms, every one covalently bonded to its neighbours, so the whole crystal is one giant structure. There are no separate molecules. Network solids form only from nonmetals and metalloids, either as elements (diamond and graphite, both carbon) or as binary compounds (silicon dioxide, SiO₂, and silicon carbide, SiC).

• High melting points. To melt a network solid you must break covalent bonds, not just intermolecular forces.
• 3D networks are rigid and hard. In diamond, each carbon atom is bonded to four others in a fixed tetrahedral arrangement. The bond angles cannot change without breaking bonds, so the atoms cannot slide.
• Graphite is soft. Its carbon atoms form flat sheets of hexagons. Within a sheet, the bonds are strong. Between sheets there are only weak dispersion forces, so the sheets slide past each other easily.

Molecular solids

Particles: separate molecules. Inside each molecule the atoms are joined by covalent bonds, but the molecules are held to each other only by relatively weak intermolecular forces. So:

• Low melting points. Melting overcomes only intermolecular forces; no covalent bonds break.
• No electrical conductivity. The valence electrons are held in the covalent bonds and lone pairs of each molecule. There are no ions and no mobile electrons.

Some molecular solids are made of very large molecules or polymers, such as waxes and plastics.

Metallic solids

Particles: metal cations in a sea of delocalized valence electrons (Topic 2.4). The free electrons make metals good conductors of electricity and heat. The cations can shift into new positions without breaking the electron “glue”, so metals are malleable (can be hammered into sheets) and ductile (can be drawn into wires). In an interstitial alloy, small atoms in the gaps make the lattice more rigid, so the alloy is less malleable and ductile. Alloys keep their sea of mobile electrons, so they still conduct.

Summary table

Solid	Particles	Held by	Melting point	Conducts?	Mechanical
Ionic	cations and anions	ionic attractions	high	only molten or dissolved	brittle
Covalent network	atoms	covalent bonds throughout	very high	no (graphite is an exception)	3D: hard, rigid; graphite: soft
Molecular	molecules	intermolecular forces	low	no	often soft
Metallic	cations + electrons	metallic bonding	varies, often high	yes, as solid and liquid	malleable, ductile

Drawing interacting particles

Particulate diagrams are a strong way to show why a property arises. A good diagram for this topic:

• shows several particles, so the interactions between them can be seen;
• uses a key that separates covalent bonds (solid lines) from intermolecular forces (dashed lines);
• keeps molecules intact in a molecular solid, and shows no separate molecules in a network solid;
• labels charges (+, −, δ+, δ−) in words or symbols, not by colour alone.

For example, to explain why an ionic solid shatters, draw two rows of alternating ions, then the same rows shifted by one ion so that + faces + and − faces −.

Large molecules and polymers

In proteins, nucleic acids and many polymers, noncovalent interactions act between different molecules and between different parts of the same long molecule. These interactions largely decide the shape of the molecule, and the shape decides how it behaves. A protein that loses its folded shape usually stops working, even though its covalent backbone is unchanged. In a polymer, chains that can hydrogen bond to their neighbours are held together more strongly than chains with dispersion forces alone, which changes properties such as strength and softening temperature.

Worked example 1: identifying four unknown solids

Question. The data below are invented for this example. Classify each solid as ionic, covalent network, molecular or metallic, and justify each claim.

Solid	Melting point (°C)	Conducts as solid?	Conducts when molten?	In water	Other observation
W	1850	no	no	insoluble	scratches glass
X	80	no	no	insoluble	soft, waxy, has an odour
Y	770	no	yes	dissolves; solution conducts	shatters when struck
Z	1085	yes	yes	insoluble	can be drawn into a wire

1. W: covalent network. A very high melting point means strong bonds must break, and it never conducts, so there are no mobile ions or electrons. Its hardness fits a rigid 3D network with fixed bond angles.
2. X: molecular. The low melting point shows only weak intermolecular forces hold the particles. The odour shows molecules escape easily from the solid, which means a noticeable vapor pressure. No conduction: electrons are held in bonds and lone pairs.
3. Y: ionic. It conducts only when molten or dissolved, when ions are free to move. Brittleness fits like-charge repulsion when layers shift.
4. Z: metallic. It conducts as a solid, which needs mobile electrons; ductility fits cations that can rearrange in the electron sea.

Why W is not ionic. Both W and Y melt at high temperatures, but W does not conduct even when molten. A molten ionic compound always conducts.

Worked example 2: two dioxides, two kinds of solid

Question. Silicon dioxide, SiO₂, melts above 1600 °C. Solid carbon dioxide, CO₂, turns directly to gas at −78 °C at 1 atm. Silicon and carbon are in the same group. Explain the huge difference.

1. CO₂ is a molecular solid. Each carbon atom forms double bonds to two oxygen atoms, making separate linear O=C=O molecules. The molecules are nonpolar, so only dispersion forces hold them together in the solid (22 electrons each). Little energy is needed to separate them.
2. SiO₂ is a covalent network solid. Each silicon atom is single-bonded to four oxygen atoms, and each oxygen atom bridges two silicon atoms. There are no separate SiO₂ molecules: the formula is only the ratio of atoms.
3. To melt SiO₂ you must break strong Si–O covalent bonds. To vaporize CO₂ you overcome only weak intermolecular forces; the C=O bonds stay intact.

Answer. The difference comes from the type of solid, not from the strength of the C=O and Si–O bonds themselves.

Worked example 3: when melting and boiling points disagree

Question. Pentane, CH₃(CH₂)₃CH₃, and neopentane, C(CH₃)₄, are isomers (C₅H₁₂, 42 electrons). Data: pentane melts at −130 °C and boils at 36 °C; neopentane melts at −16.5 °C and boils at 9.5 °C. (a) Which has the stronger intermolecular forces? Use the data. (b) Explain why the melting points suggest the opposite.

1. (a) Use the boiling points, because boiling overcomes all the intermolecular forces. Pentane boils 26.5 °C higher, so it has the stronger forces. This fits the structures: both have dispersion forces only, with the same electron count, and the chain shape of pentane gives more contact area than the compact, near-spherical neopentane.
2. (b) Melting only rearranges molecules. Neopentane’s compact, highly symmetrical molecules pack neatly into a regular crystal, which makes the solid more stable compared with the liquid, so it melts at a much higher temperature. Floppy pentane chains pack less neatly.

Takeaway. Use boiling points or vapor pressures, not melting points, to compare the strength of intermolecular forces.

Common misconceptions

• “Melting a molecular solid breaks covalent bonds.” No. Only intermolecular forces are overcome; the molecules stay whole.
• “A high melting point means the solid is ionic.” Covalent network solids and many metals also melt at high temperatures. Use conductivity to tell them apart.
• “Ionic solids conduct electricity.” Not as solids. The ions must be free to move.
• “Diamond is hard because of strong intermolecular forces.” Diamond has no molecules. Its hardness comes from covalent bonds in a rigid 3D network.
• “All network solids are hard.” Graphite is soft, because its layers are held together only by weak forces.
• “Alloys stop conducting.” Alloys keep a sea of mobile electrons and still conduct.
• “A higher melting point always means stronger intermolecular forces.” Packing also matters; check boiling points.

Where this leads

Next, Topic 3.3, Solids, Liquids and Gases, compares the arrangement and motion of particles in the three states. Vapor pressure returns in phase changes and in solutions. Try the practice questions now, then use the revision notes and the checklist to consolidate.