Unit 3 · Topic 3.2 Beta

Properties of Solids

Solids are ionic, metallic, molecular or covalent network.

Practice 1: Models and RepresentationsPractice 4: Model AnalysisPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Salt melts at 801 °C, sugar at about 186 °C, sand above 1,700 °C. Copper bends, salt shatters, and graphite smears onto paper. Every one of these differences comes from two questions: what are the particles, and what holds them together?

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Why does a solid ionic compound fail to conduct electricity?

  1. Its ions are fixed in the lattice
  2. It has no charged particles
  3. Its electrons are delocalized
Show the answer

The ions carry charge but cannot move until the solid melts or dissolves.

  • Correct: Its ions are fixed in the lattice:
  • It has no charged particles:
  • Its electrons are delocalized:

2. What makes metals malleable?

  1. Layers of cations slide within the sea of electrons
  2. Their covalent bonds bend easily
  3. Their ions repel each other
Show the answer

Metallic bonding is not tied to particular atoms, so layers slide without breaking it.

  • Correct: Layers of cations slide within the sea of electrons:
  • Their covalent bonds bend easily:
  • Their ions repel each other:

3. Which attraction holds I₂ molecules to each other?

  1. London dispersion forces
  2. Hydrogen bonds
  3. Ionic bonds
Show the answer

I₂ is nonpolar, so its molecules attract only by dispersion forces.

  • Correct: London dispersion forces:
  • Hydrogen bonds:
  • Ionic bonds:

Part 4 · See it

See it first

Four kinds of solid drawn at the particle level: an ionic solid as a grid of alternating cations and anions; a molecular solid as separate two-atom molecules packed side by side; a covalent network solid as atoms joined by covalent bonds in one continuous web; and a metallic solid as metal cations in a sea of delocalized electrons.
Ionic, molecular, covalent network and metallic solids, drawn at the particle level. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Molecular solids are separate molecules held only by intermolecular forcesmelting overcomes weak attractions, so they melt low and are soft
  2. Molecules carry no net charge and have no free electronsmolecular solids do not conduct in any state
  3. In a covalent network every atom is covalently bonded to its neighborsmelting must break covalent bonds, so network solids melt very high and are very hard
  4. A network's valence electrons are locked in bonds (graphite's layers excepted)most network solids do not conduct

Part 6 · Key ideas

Key ideas

  • Ask what the particles are and what holds them: ions (ionic bonds), cations and electrons (metallic), molecules (IMFs) or atoms (covalent network).
  • Molecular solids melt low and never conduct; covalent network solids melt very high and are very hard.
  • Crystalline solids have repeating order and sharp melting points; amorphous solids soften over a range.

Part 7 · Misconception

A common mistake

The wrong idea: Molecular solids melt low because their covalent bonds are weak.

What actually happens: Melting a molecular solid breaks no covalent bonds. It only overcomes the intermolecular forces between molecules, which are much weaker than covalent bonds.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Data table

Four unknown solids

A student tests four white or gray solids, W to Z.

Properties of four solids
SolidMelting point (°C)Conducts as a solid?Conducts when melted?Hardness
W801NoYesHard, shatters when struck
X114NoNoSoft, crumbles
Y1,710NoNoVery hard
Z660YesYesBends without breaking

1. Which solid is most likely a molecular solid?

  1. W
  2. X
  3. Y
  4. Z
Show the answer

X melts low (114 °C), is soft and conducts in neither state. Low melting means only weak intermolecular forces hold the particles together, and no conduction means no ions or free electrons.

  • W: W conducts only when melted, the signature of an ionic solid whose ions become free to move.
  • Correct: X: Right: low melting point, soft, no conduction.
  • Y: Y melts at 1,710 °C, far too high for a solid held by intermolecular forces.
  • Z: Z conducts as a solid and bends, which marks a metal.

2. Which explanation accounts for solid Y's very high melting point and lack of conductivity?

  1. Its atoms form one covalent network, so melting breaks covalent bonds and no charges can move.
  2. Its molecules form many hydrogen bonds to each other, which are strong enough to keep it solid at 1,700 °C.
  3. Its ions are held by very strong ionic bonds, and ions do not carry a current even when the solid has melted into a liquid.
  4. It is a metal whose electrons are held so tightly that they stay put, which raises the melting point.
Show the answer

Only breaking covalent bonds explains a melting point near 1,710 °C in a nonconductor. In a covalent network, all valence electrons are in bonds, so nothing carries a charge.

  • Correct: Its atoms form one covalent network, so melting breaks covalent bonds and no charges can move.: Right: a covalent network solid such as quartz.
  • Its molecules form many hydrogen bonds to each other, which are strong enough to keep it solid at 1,700 °C.: Hydrogen bonds are intermolecular forces; no molecular solid stays solid anywhere near 1,700 °C.
  • Its ions are held by very strong ionic bonds, and ions do not carry a current even when the solid has melted into a liquid.: An ionic solid conducts once melted, and Y does not, so Y is not ionic.
  • It is a metal whose electrons are held so tightly that they stay put, which raises the melting point.: A metal conducts as a solid because of its delocalized electrons; Y does not.

3. A fifth solid, V, melts at 186 °C and does not conduct when melted. Which conclusions are supported? Select all that apply.

  1. V is a molecular solid.
  2. V's covalent bonds break when it melts.
  3. V contains no free ions or electrons.
  4. V is an ionic solid with small ions.
Show the answer

A low melting point means only intermolecular forces hold the particles, so V is molecular, and no conduction when melted means it has no ions or free electrons. (Sucrose fits.)

  • Correct: V is a molecular solid.: Right: a low melting point points to intermolecular forces between molecules.
  • V's covalent bonds break when it melts.: Melting a molecular solid overcomes intermolecular forces; its covalent bonds stay intact.
  • Correct: V contains no free ions or electrons.: Right: a melted ionic or metallic solid would conduct.
  • V is an ionic solid with small ions.: A melted ionic solid conducts, and V does not.

Particle view

Particle views of four solids

1234+−+−−+−++−+−−+−+++++++++++++++++

Key: blue +, cation; green −, anion; gold +, metal cation; small blue dot, electron; line, covalent bond; purple pair, a two-atom molecule.

4. Which box best represents solid iodine, I₂?

  1. Box 1
  2. Box 2
  3. Box 3
  4. Box 4
Show the answer

Iodine is made of separate I₂ molecules. Each pair is joined by a covalent bond, and the pairs are held to each other only by dispersion forces.

  • Correct: Box 1: Right: separate two-atom molecules.
  • Box 2: Iodine contains no ions; I₂ is a neutral molecule of one element.
  • Box 3: In a network every atom bonds to several neighbors; each iodine atom bonds to just one other.
  • Box 4: Iodine is a nonmetal, so it has no cations in a sea of electrons.

5. The substance in box 1 melts far lower than the substance in box 3. Which reasoning is correct?

  1. Melting box 1 overcomes intermolecular forces; melting box 3 breaks covalent bonds.
  2. The covalent bonds in box 1 are weaker than the covalent bonds in box 3, so they break at a lower temperature.
  3. Box 1 has fewer atoms in the drawing than box 3, so less energy is needed to melt each gram of it.
  4. Box 3 contains ions that attract each other strongly through ionic bonds, while box 1 contains neutral atoms that attract each other far less.
Show the answer

Melting a molecular solid pulls whole molecules apart; the bonds within each pair are untouched. Melting a network requires breaking covalent bonds, which takes far more energy.

  • Correct: Melting box 1 overcomes intermolecular forces; melting box 3 breaks covalent bonds.: Right: intermolecular forces versus covalent bonds.
  • The covalent bonds in box 1 are weaker than the covalent bonds in box 3, so they break at a lower temperature.: Melting box 1 does not break its covalent bonds at all.
  • Box 1 has fewer atoms in the drawing than box 3, so less energy is needed to melt each gram of it.: The number of atoms drawn is a sampling choice, not a cause of melting point.
  • Box 3 contains ions that attract each other strongly through ionic bonds, while box 1 contains neutral atoms that attract each other far less.: Box 3 contains neutral atoms joined by covalent bonds; there are no ions.

6. Dry ice, CO₂(s), turns to gas at −78 °C, while quartz, SiO₂(s), melts near 1,710 °C. Which statement explains the difference?

  1. Solid CO₂ is molecules held by dispersion forces; quartz is a Si–O covalent network.
  2. Si–O bonds are polar while C=O bonds are not, so quartz molecules attract each other more strongly.
  3. Quartz molecules are heavier than CO₂ molecules, so they need more energy to leave the solid.
  4. Quartz is an ionic solid made of Si⁴⁺ and O²⁻ ions, while CO₂ is made of neutral molecules.
Show the answer

The formulas look alike, but CO₂ forms small molecules and SiO₂ forms an endless network. Changing CO₂ to a gas pulls molecules apart; melting quartz breaks Si–O covalent bonds.

  • Correct: Solid CO₂ is molecules held by dispersion forces; quartz is a Si–O covalent network.: Right: molecular solid versus covalent network.
  • Si–O bonds are polar while C=O bonds are not, so quartz molecules attract each other more strongly.: C=O bonds are polar too; and quartz has no separate molecules to attract.
  • Quartz molecules are heavier than CO₂ molecules, so they need more energy to leave the solid.: Quartz has no separate molecules at all, so molecule mass is the wrong idea.
  • Quartz is an ionic solid made of Si⁴⁺ and O²⁻ ions, while CO₂ is made of neutral molecules.: Si–O bonds are polar covalent, and molten quartz does not conduct like an ionic liquid.

Part 9 · Summary

Summary

Solids are ionic, metallic, molecular or covalent network. Molecular solids are molecules held by intermolecular forces: soft, low-melting and nonconducting. Covalent network solids are one continuous web of covalent bonds: very hard, very high-melting and (except graphite) nonconducting.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections