What you'll be able to do: Classify a solid from its composition and predict melting point, conductivity and hardness.
Best after: Hydrogen Bonding
Two white solids can look identical and behave nothing alike. Sugar melts in a pan; salt does not budge. The difference is what holds each lattice together.
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Ionic solids contain cations and anions held by electrostatic attraction. Metallic solids contain positive atomic cores and delocalized electrons. Covalent network solids contain atoms joined in an extended covalent framework, rather than separate small molecules. Molecular solids contain discrete molecules held together by intermolecular forces.
| Type | Structural units and cohesion | Typical properties |
|---|---|---|
| Ionic | Cations and anions in an extended lattice | Often high melting point; brittle; conducts when mobile ions are present |
| Metallic | Positive atomic cores with delocalized electrons | Conducts as a solid; often malleable; melting points vary widely |
| Covalent network | Atoms joined by an extended covalent framework | Often hard and high melting; generally poor conduction with important exceptions |
| Molecular | Discrete molecules held together by IMFs | Often lower melting and softer; generally poor conduction |
These are tendencies, not a universal melting-point ladder. Tungsten melts above many ionic solids, and network solids differ in structure. Diamond does not have an ordinary atmospheric-pressure melting point; its thermal behavior depends on pressure. A molecule can contain strong covalent bonds yet form a low-melting solid because melting separates molecules without breaking those internal bonds.
A metal has mobile electrons even while its positive atomic cores remain in a solid lattice. A solid salt has charged ions, but they cannot migrate through the fixed lattice, so ordinary ionic solids are poor conductors. Melting frees the ions to move; dissolving a soluble salt in water also supplies mobile hydrated ions. An insoluble salt does not make a strongly conducting solution simply because it is ionic.
Diamond has valence electrons localized in an extended bonding framework and is generally an electrical insulator. Graphite has delocalized electrons within its carbon sheets, allowing conduction mainly along them; weaker interlayer attractions allow the sheets to slide. Do not place the conducting electrons “between the layers.” Molecular solids generally lack mobile charged particles, although dissolved molecular acids can react with water to generate ions.
For a pair of oppositely charged ions, electrostatic potential energy is proportional to −|q₁q₂|/r. Larger charge magnitudes and smaller center-to-center distance make the attraction more stabilizing. Force magnitude instead scales as |q₁q₂|/r²; force and energy are related but not the same quantity. A full lattice involves many neighbors and its geometry as well as one ion pair.
MgO combines 2+ and 2− ions, while NaCl combines 1+ and 1− ions. The larger charge product and smaller ionic separations support much stronger lattice attraction for MgO. For two salts with the same ion charges and similar structures, compare radii as the next factor. Do not infer an exact fourfold melting point from a fourfold charge product: melting involves the whole solid and the change to the liquid.
Attraction energy magnitude ∝ |q₁q₂|/r; force magnitude ∝ |q₁q₂|/r²
Classifying a solid starts with what occupies its lattice: ions, atoms linked by a covalent network, metal atoms, or molecules. The strongest connection within a molecular unit is not necessarily the force holding separate units together. Graphite is a useful boundary case: strong covalent bonds form sheets, while electrons delocalized within those sheets allow conduction; weak interactions between sheets allow sliding.
Brittleness also follows from structure. Shifting layers of an ionic lattice can bring like charges alongside one another, promoting fracture. Metallic bonding can tolerate layer shifts while delocalized electrons maintain cohesion. When classifying an unknown, combine observations rather than using a melting point alone: no solid conduction but conduction after melting strongly supports mobile ions in the liquid.
Metal plus nonmetal, hard and brittle, high melting, conducts only when molten or aqueous.
Positive atomic cores with delocalized electrons; typically conducts as a solid and liquid, with variable melting point and mechanical properties.
An extended covalent framework, often high melting and hard; graphite shows that conductivity and hardness depend on the particular network.
Discrete molecules held by IMFs, soft, low melting, non-conducting.
F ~ (q₁ * q₂) / r²
Classify SiO₂, Cu, CaF₂ and I₂. Using the approximate melting points SiO₂ 1710 °C, CaF₂ 1418 °C, Cu 1085 °C and I₂ 114 °C, rank these substances.
Try it first: Name the particles at the lattice points for each one.
0 of 3 steps revealed.
Explain why NaCl does not conduct as a solid but does when molten or dissolved.
Try it first: Ask what charge carriers exist and whether they can move.
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Why is MgO melting point 2852 °C while NaCl is 801?
Try it first: Write the charges on each ion.
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Why does graphite conduct along its sheets while diamond does not conduct in ordinary conditions?
Try it first: Both are carbon networks; compare bonding and available electrons.
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| Mistake | Why it's wrong | Check instead |
|---|---|---|
| Saying ionic solids conduct electricity. | Ions are immobile in the solid lattice. | Specify molten or aqueous whenever you claim an ionic compound conducts. |
| Describing melting of a molecular solid as breaking covalent bonds. | Only intermolecular forces are overcome; the molecules survive intact. | Name the specific IMF being broken. |
| Treating all network solids as insulators. | Graphite conducts along its layers because of delocalised pi electrons. | Note graphite as the standard exception. |
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After the lesson · Question bank
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Classify solids by ions, metallic cores and mobile electrons, extended covalent networks, or discrete molecules. Connect properties to the attractions and mobile charge carriers present. Melting points vary within categories; graphite conducts within layers, while ionic conduction requires mobile ions.
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