Chemistry · 12th Grade · The Mathematics of Reactions · Weeks 10-18

Properties of Solids: Ionic, Molecular, Covalent Network, Metallic

Students will classify solids based on their bonding and predict their physical properties.

• Common Core State Standards
• HS-PS1-3

About This Topic

Solid-state chemistry connects the submicroscopic world of bonding to the macroscopic properties students observe in everyday materials. In the US high school curriculum, this topic typically follows intermolecular forces and acts as a synthesis unit where students apply their knowledge of bonding to predict and explain why salt is brittle, copper is conductive, diamond is hard, and ice floats. The four solid types, ionic, molecular, covalent network, and metallic, each represent a distinct bonding environment with predictable, testable properties.

Ionic solids consist of alternating cations and anions in a crystal lattice held by strong electrostatic forces, producing high melting points, brittleness, and conductivity only when melted or dissolved. Molecular solids are held together by intermolecular forces rather than bonds, giving them low melting points and non-conductivity. Covalent network solids like diamond are held by covalent bonds throughout, explaining extreme hardness. Metallic solids have mobile electrons and are characterized by malleability and conductivity.

Active learning is well-suited to this topic because students can handle real samples, table salt, ice, graphite pencils, copper wire, and directly link observable properties to bonding models. Data-driven prediction tasks, where students explain physical property trends using bonding theory, build the integrated understanding that AP Chemistry assessments consistently require.

Key Questions

1. Differentiate between ionic, molecular, covalent network, and metallic solids based on their bonding.
2. Predict the melting point, conductivity, and hardness of a solid given its bonding type.
3. Compare the macroscopic properties of different solid types to their underlying microscopic structures.

Learning Objectives

• Classify solids into ionic, molecular, covalent network, or metallic categories based on their bonding characteristics.
• Predict the relative melting points, hardness, and electrical conductivity of different solid types using their bonding models.
• Compare the macroscopic properties of common materials (e.g., salt, sugar, diamond, copper) to their underlying microscopic structures and bonding.
• Explain how the arrangement of particles and types of forces dictate the observable properties of solids.

Before You Start

Chemical Bonding: Ionic, Covalent, Metallic

Why: Students must understand the fundamental types of chemical bonds to classify solids based on their bonding.

Intermolecular Forces

Why: Knowledge of intermolecular forces is necessary to differentiate the bonding in molecular solids from stronger chemical bonds.

Structure and Properties of Matter

Why: A foundational understanding of how the arrangement of particles influences macroscopic properties is essential.

Key Vocabulary

Ionic Solid	A solid composed of ions held together by electrostatic attractions in a crystal lattice, typically exhibiting high melting points and brittleness.
Molecular Solid	A solid formed from discrete molecules held together by weaker intermolecular forces, characterized by low melting points and poor electrical conductivity.
Covalent Network Solid	A solid in which atoms are linked by a continuous network of covalent bonds, resulting in extreme hardness and very high melting points.
Metallic Solid	A solid consisting of metal atoms held together by metallic bonds, featuring a 'sea' of delocalized electrons that allows for good electrical conductivity and malleability.
Crystal Lattice	The regular, repeating three-dimensional arrangement of atoms, ions, or molecules in a crystalline solid.

Watch Out for These Misconceptions

Common Misconception

All solids are hard and have high melting points.

What to Teach Instead

Molecular solids often have very low melting points because they are held together by intermolecular forces, not bonds. Ice melts at 0°C; candle wax at roughly 50°C; dry ice sublimes at -78°C. Displaying melting point distributions across all four solid types makes clear that 'solid' does not imply high melting point, the bonding type is what determines thermal stability.

Common Misconception

Ionic compounds conduct electricity in solid form.

What to Teach Instead

In solid ionic compounds, ions are fixed in lattice positions and cannot carry charge. Conductivity requires mobile charge carriers, ions become mobile only when melted or dissolved in water. A simple conductivity tester applied to solid NaCl versus NaCl dissolved in water makes this distinction experimentally concrete and corrects a very common AP exam error.

Common Misconception

Diamond and graphite must have different chemical formulas since they behave so differently.

What to Teach Instead

Both are pure carbon. The difference is bonding geometry: diamond has sp3-hybridized carbon in a 3D tetrahedral network; graphite has sp2-hybridized carbon in layered hexagonal sheets with delocalized electrons between the sheets. Same atoms, different structure, completely different properties. This comparison reinforces the central AP Chemistry principle that structure determines function.

Active Learning Ideas

Data Analysis Lab: Classify Unknown Solids

Provide students with a table of physical property data (melting point, electrical conductivity in solid and molten forms, hardness, solubility in water) for eight unlabeled solids. Students build a bonding-type decision tree and classify each solid, writing a justification for every classification. Groups compare decisions and resolve disagreements with evidence from the data.

35 min·Small Groups

Gallery Walk: Four Solid Types Stations

Four stations each feature a physical sample (NaCl, candle wax, graphite rod, copper wire), a structural diagram, and a property data card. Students complete a comparison table, noting bonding type, representative particles, melting point range, conductivity, and hardness for each solid type. A synthesis question asks them to rank all four by melting point and explain the ranking.

30 min·Small Groups

Think-Pair-Share: Predicting Melting Points

Present six substances with only their formulas. Students independently rank them by predicted melting point using bonding type, then pair to compare reasoning. After checking against actual values, pairs write explanations for any incorrect predictions, focusing on what bonding evidence they misread.

20 min·Pairs

Socratic Seminar: Why Does Diamond Cut Glass?

After background reading on covalent network solids, students discuss why diamond's structure produces extreme hardness while graphite's nearly identical composition makes a good lubricant and electrical conductor. The discussion extends to why ionic solids shatter when struck (layer shift brings like charges together) while metals deform (electron sea accommodates layer movement).

25 min·Whole Class

Real-World Connections

• Materials scientists at Intel use their understanding of covalent network solids (like silicon dioxide) and metallic solids (like copper interconnects) to design and manufacture microprocessors, optimizing for conductivity and durability.
• Geologists classify minerals based on their crystalline structure and bonding, which helps predict properties like hardness and cleavage, crucial for identifying valuable ore deposits or understanding rock formation.
• Engineers in the automotive industry select materials for car bodies, considering the malleability of metallic alloys for shaping and the brittleness of ionic compounds used in certain ceramic components.

Assessment Ideas

Quick Check

Provide students with a list of common substances (e.g., NaCl, H2O(s), SiO2, Fe). Ask them to classify each substance into one of the four solid types and provide a one-sentence justification based on its bonding. Review responses to identify common misconceptions.

Exit Ticket

On an index card, have students draw a simplified model of one solid type (ionic, molecular, covalent network, or metallic). Below the drawing, they should list two predicted physical properties and explain how the bonding supports those properties.

Discussion Prompt

Pose the question: 'Why is diamond (a covalent network solid) used in cutting tools, while copper (a metallic solid) is used for electrical wiring?' Facilitate a class discussion where students compare and contrast the bonding and resulting properties of these two materials.

Frequently Asked Questions

What are the four types of solids in chemistry and how do they differ?

The four types are ionic (lattice of cations and anions, high melting points, brittle, conducts when dissolved or melted), molecular (held by IMFs, low melting points, non-conductive), covalent network (bonded throughout in 3D or layered covalent networks, extremely hard or unique conductivity), and metallic (mobile electrons, conductive, malleable and ductile). Each solid type's properties follow directly from its bonding.

Why can ionic solids conduct electricity when dissolved but not as solids?

Electrical conductivity requires mobile charge carriers. In a solid ionic compound, ions are locked into fixed lattice positions and cannot move to carry current. Dissolving in water or melting frees the ions to move through solution or liquid, allowing current flow. This explains why saltwater conducts but dry NaCl crystals do not, the ions are present in both cases, but only mobile in solution.

How do you predict the melting point of a solid from its bonding type?

Melting point reflects the energy needed to overcome the forces holding the solid together. Covalent network solids (covalent bonds throughout) and metals (strong metallic bonds with delocalized electrons) have the highest melting points. Ionic solids have moderate to high melting points depending on ion charge and size. Molecular solids have the lowest melting points because only intermolecular forces, not bonds, must be overcome.

What active learning strategies work well for teaching solid types?

Physical sample stations are highly effective, students observe, test, and measure properties of actual substances before connecting observations to bonding models. Data classification tasks, where students receive property tables for unlabeled solids and must identify the bonding type with written justification, build the analytical reasoning that AP free-response questions demand. Real samples make abstract bonding models tangible.