Worked example 1
Classify SiO₂, Cu, CaF₂ and I₂ and rank their melting points.
Try it first: Name the particles at the lattice points for each one.
0 of 3 steps revealed.
Types of Solids and IMFSSolidsContent level: Core 20 min
Ionic, Metallic, Covalent Network and Molecular Solids
What you'll be able to do: Classify a solid from its composition and predict melting point, conductivity and hardness.
Best after: Hydrogen Bonding
Introduction
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.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Identify the particles at the lattice points of each solid type
- Predict melting point, hardness and conductivity from solid type
- Explain why ionic solids conduct only when molten or aqueous
- Use Coulomb reasoning to compare ionic lattice strengths
Lesson
Four classes, four kinds of glue
Ionic solids hold cations and anions with electrostatic attraction. Metallic solids hold cations in a sea of delocalised electrons. Covalent network solids are one giant molecule of covalently bonded atoms. Molecular solids hold discrete molecules together with intermolecular forces only.
Melting points follow the glue
Covalent network solids melt highest because covalent bonds must break: diamond exceeds 3500 °C. Ionic solids follow, with NaCl at 801. Metals vary widely from mercury as a liquid to tungsten at 3422. Molecular solids melt lowest, since only weak intermolecular forces break.
Melting a molecular solid never breaks a covalent bond.
Conductivity distinguishes them
Metals conduct as solids and as liquids because electrons are mobile. Ionic solids do not conduct as solids, because the ions are locked in place, but conduct when molten or dissolved. Covalent network and molecular solids do not conduct, with graphite the famous exception thanks to delocalised electrons between its layers.
Solid salt is an insulator. Molten salt is an excellent conductor.
Comparing ionic lattices with Coulomb
Lattice attraction scales with the product of the charges divided by the distance between ion centres. MgO with 2+ and 2- ions melts at 2852 °C while NaCl with 1+ and 1- melts at 801. Higher charge dominates; smaller ions raise it further.
F ~ (q₁ * q₂) / r²
Key ideas
Rule
Ionic solid
Metal plus nonmetal, hard and brittle, high melting, conducts only when molten or aqueous.
Rule
Metallic solid
Metal atoms in an electron sea, malleable, conducts in every state.
Rule
Covalent network solid
Continuous covalent bonding, extremely hard, very high melting, usually non-conducting.
Rule
Molecular solid
Discrete molecules held by IMFs, soft, low melting, non-conducting.
Equation
Coulomb attraction
F ~ (q₁ * q₂) / r²
- q₁ = charge on the cation
- q₂ = charge on the anion
- r = distance between ion centres
Worked examples
Worked example 2
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.
0 of 3 steps revealed.
Worked example 3
Why is MgO melting point 2852 °C while NaCl is 801?
Try it first: Write the charges on each ion.
0 of 3 steps revealed.
Common mistakes
Saying ionic solids conduct electricity.
Why it's wrong: Ions are immobile in the solid lattice.
Check instead: Specify molten or aqueous whenever you claim an ionic compound conducts.
Describing melting of a molecular solid as breaking covalent bonds.
Why it's wrong: Only intermolecular forces are overcome; the molecules survive intact.
Check instead: Name the specific IMF being broken.
Treating all network solids as insulators.
Why it's wrong: Graphite conducts along its layers because of delocalised pi electrons.
Check instead: Note graphite as the standard exception.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Solids fall into four classes based on the particles at the lattice points and the attractions between them. Ionic solids are hard, high melting and conduct only when molten or dissolved; metallic solids conduct in all states and deform; covalent network solids are extremely hard and high melting; molecular solids are soft, low melting and non-conducting.
- Four solid types differ in the particles and attractions at the lattice points
- Melting point tracks the strength of the attraction broken
- Ionic solids conduct only when molten or dissolved
- Coulomb reasoning ranks ionic lattice strengths by charge and size
Sources and further reading
- Chemistry 2e, Section 10.5: The Solid State of MatterOpenStax · Rice University · 10.5View source
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