9.2 · Properties & polymers

Types of solids

What the particles are, and what holds them together.

By the end you should be able to:

Classify solids as molecular, ionic, metallic or covalent-network from composition and properties

Key idea

Four types of solid

Type	Particles	Held together by	Typical composition and examples
Ionic	cations and anions	ionic bonds (electrostatic attraction)	metal + nonmetal, or polyatomic ions: $\ce{NaCl}$ , $\ce{MgO}$ , $\ce{CaCO3}$ , $\ce{NH4NO3}$
Molecular	discrete molecules	intermolecular forces	nonmetals: $\ce{I2}$ , dry ice ( $\ce{CO2}$ ), ice, sucrose, $\ce{S8}$ , $\ce{P4}$
Metallic	metal atoms (cations) in a sea of delocalized electrons	metallic bonding	metals: $\ce{Cu}$ , $\ce{Fe}$ , $\ce{Na}$ , $\ce{Al}$ , $\ce{Au}$
Covalent network	atoms	a continuous network of covalent bonds	diamond and graphite ( $\ce{C}$ ), $\ce{Si}$ , quartz ( $\ce{SiO2}$ ), $\ce{SiC}$ , $\ce{BN}$

Key idea

Properties follow from the forces

Type	Hardness	Melting point	Conducts electricity?
Ionic	hard but brittle	high	not as a solid; yes when molten or dissolved in water
Molecular	soft	low; often volatile	no, in any state
Metallic	malleable, ductile, lustrous	low (Hg, Na) to very high (W)	yes, as a solid (heat too)
Covalent network	very hard	very high	usually not (graphite is the exception)

Ionic crystals are brittle: shifting one layer brings like charges next to each other, and they repel and split the crystal.
Metals bend without breaking: the delocalized electrons keep bonding the atoms as layers slide.
Melting a molecular solid overcomes only intermolecular forces; melting a network solid means breaking covalent bonds.
Ionic attraction grows with ion charge: MgO (Mg²⁺ and O²⁻) melts at 2852 °C, NaCl at 801 °C.
Graphite conducts along its layers (delocalized π electrons) and is soft because its layers, held only by dispersion forces, slide over each other.

Method

Classifying a solid

From the formula:

Only metal atoms: metallic.
Metal + nonmetal, or a polyatomic ion such as $\ce{NH4+}$ : ionic.
Diamond or graphite (C), Si, $\ce{SiO2}$ , SiC or BN: covalent network (learn this short list).
Any other nonmetal element or compound: molecular.

From the properties:

Conducts as a solid and is malleable: metallic.
Conducts only when molten or dissolved; brittle; high melting point: ionic.
Very hard, very high melting point, never conducts, insoluble: covalent network.
Soft, low melting point, never conducts: molecular.

Common mistake

Solid-classification traps

Wrong: $\ce{SiO2}$ is molecular, like $\ce{CO2}$ . Right: quartz is a covalent network (each Si bonded to four O; it melts near 1700 °C), whereas $\ce{CO2}$ is made of separate molecules (dry ice sublimes at −78 °C).
Wrong: ionic solids conduct because they contain ions. Right: the ions are locked in place in the solid; they carry a current only when they are free to move (molten or in solution).
Wrong: $\ce{NH4NO3}$ is molecular because it contains only nonmetals. Right: it is built from $\ce{NH4+}$ and $\ce{NO3-}$ ions, so it is ionic.
Wrong: molecular solids melt easily because their covalent bonds are weak. Right: their covalent bonds are strong; melting only overcomes the weak forces between molecules.

Worked example

Worked example: classify these solids

Substance	Type	Reason
$\ce{CaF2}$	ionic	metal + nonmetal: Ca²⁺ and F⁻ ions
$\ce{Au}$	metallic	metal atoms only
$\ce{SiC}$	covalent network	Si–C bonds extend through the whole crystal
$\ce{S8}$	molecular	discrete S₈ rings held by dispersion forces
sucrose, $\ce{C12H22O11}$	molecular	molecules held by hydrogen bonds
$\ce{CaCO3}$	ionic	Ca²⁺ and CO₃²⁻ ions

From properties: a white solid is very hard, melts above 1700 °C, is insoluble in water and conducts neither as a solid nor as a liquid. It is a covalent-network solid (quartz fits).

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