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C2.5 · Predict solid type and properties from bonding

Learn to predict solid type and properties from bonding through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Structure and Properties of Matter

Use particle structure to explain how solids behave

A salt crystal can break when struck, while copper can be bent into a new shape. Candle wax softens at a much lower temperature than quartz. These visible differences connect to the particles in each solid and the way those particles are held together. In this lesson, you will identify a solid’s structure first, then use it to predict properties. The predictions are common patterns, not claims that every material in a category behaves exactly alike.

What you will learn

Distinguish molecular, ionic, metallic, and covalent network solids by their particles and bonding.
Use a solid’s structure to make qualitative predictions about conductivity, hardness, brittleness, and melting behaviour.
Explain why predictions are useful patterns rather than guarantees of exact properties.

1. Begin with particles and bonding

In SCH3U, you learned that covalent bonds hold atoms together by sharing electrons, and that positive and negative ions attract. You also learned that metals conduct electricity and can be shaped. For this topic, ask two questions: what particles make up the solid, and what holds them together?

In a solid, particles stay close together and vibrate near fixed positions. The four useful solid types are molecular, ionic, metallic, and covalent network. Their names describe different particle arrangements and bonding models.

A molecule is a group of atoms joined by covalent bonds. In a molecular solid, separate molecules are held near one another by attractions between molecules. These attractions are different from the covalent bonds within each molecule. Water ice and solid carbon dioxide are examples of molecular solids.

An ionic solid contains positive and negative ions arranged in a repeating structure. The attraction between oppositely charged ions holds the structure together. Sodium chloride is one example. Its formula,

ext{NaCl}

, gives the simplest ratio of sodium ions to chloride ions; it does not represent a separate molecule in the crystal.

A metallic solid consists of metal particles held together by metallic bonding. A useful course-level model is positive metal particles surrounded by mobile electrons. A covalent network solid has covalent bonds connecting atoms throughout an extended structure, rather than forming separate molecules. Diamond and quartz are examples.

Identify the particles and structure before predicting properties.
Molecular solids contain separate molecules; covalent network solids have covalent bonds extending through the structure.
An ionic formula gives an ion ratio, not a separate molecule in the crystal.

2. Connect structure to observable properties

Melting changes a solid into a liquid. During melting, particles gain enough energy to leave their ordered positions and move past one another. How much energy is needed depends on the structure and the attractions or bonds holding it together. Solid type helps with qualitative predictions, but does not provide an exact melting point.

Molecular solids often have relatively low melting points compared with many ionic and covalent network solids. Their molecules generally remain intact as the solid melts; attractions between molecules are overcome. Many molecular solids are soft or easily crushed. They do not conduct electricity as solids because they lack mobile charged particles. Solubility varies: some molecular substances dissolve in water, and others do not.

Ionic solids commonly have high melting points because oppositely charged ions attract throughout the structure. They are often brittle, meaning they tend to crack rather than bend. If layers shift, ions with the same charge can become neighbours and repel, helping the crystal break. In a solid ionic compound, ions are fixed in place, so they cannot carry charge through the solid. When molten or dissolved in water, ions can move and the substance can conduct.

Metallic solids conduct electricity as solids because mobile electrons can move through the structure. Metals are often malleable, meaning they can be hammered or shaped, and ductile, meaning they can be drawn into wires. Layers can shift without the solid breaking easily. Melting points vary among metals, so the metallic type alone does not determine an exact melting point.

Covalent network solids are often hard and have high melting points because covalent bonds connect atoms throughout the solid. Most do not conduct electricity. Graphite, a form of carbon, is a known exception that conducts along its layers. When a known structure is provided, use it rather than relying only on a general pattern.

Conductivity depends on whether charged particles can move through the material.
Solid ionic compounds do not conduct; molten or dissolved ionic compounds can conduct because their ions can move.
Property patterns are useful, but a solid’s specific structure matters.

3. Make a justified prediction

Start with composition. A pure metal element is a strong clue for a metallic solid. A compound made from a metal and a non-metal is often ionic. A substance made only of non-metals could be molecular or covalent network, so composition alone may not be enough.

Next, identify whether the substance has separate molecules or an extended structure. If the solid is made of familiar molecules, such as water molecules in ice, classify it as molecular. If it is identified as a network-forming material such as diamond or quartz, classify it as covalent network. Do not assume that every substance made of non-metals is molecular.

Then predict one property at a time and give the reason. For example, ‘The solid should not conduct because its ions are fixed in place’ connects the property to the particle model. If predicting conductivity, state whether the sample is solid, molten, or dissolved. The ability of ions to move depends on the form.

Keep the terms bond and solid type distinct. ‘Covalent’ describes bonding between atoms. It does not tell you by itself whether the solid contains separate molecules or an extended network. The structure decides which classification is appropriate.

Use composition as an initial clue, then identify the structure.
Connect each property prediction to a feature of the particle model.
State the form of the sample when predicting conductivity.

4. Know the limits of the model

The four solid types describe common patterns, not exact measurements. Substances within the same type can differ in melting point, hardness, or other properties. Use reference data when an exact value is needed.

A high melting point is a clue that substantial energy is needed to disrupt the solid structure. It does not mean every solid in that category has the same melting point. Likewise, solid type alone cannot reliably predict whether a substance will dissolve in water.

If the information does not reveal whether a non-metal substance has a molecular or network structure, do not guess. State what is known and identify the structural information needed to make a stronger prediction.

Use solid type for qualitative predictions, not unsupported exact values.
Recognize when a prediction needs more structural information or reference data.

Worked example

Predicting properties of quartz

Quartz is a solid made of silicon and oxygen atoms in an extended structure. Classify the solid and predict its likely melting behaviour, hardness, and electrical conductivity as a solid. Explain each prediction using its bonding.

Classify the structure
The description says the atoms form an extended structure. Covalent bonds connect atoms through the solid, so quartz is a covalent network solid, not a collection of separate molecules.
Predict melting behaviour and hardness
Covalent bonds extend throughout the network. The solid is therefore expected to have a high melting point and be hard. These are qualitative predictions, not exact measurements.
Predict conductivity
The description does not identify mobile charged particles. Following the common pattern for covalent network solids, predict that solid quartz does not conduct electricity.

Answer: Quartz is a covalent network solid. It is expected to have a high melting point and be hard because covalent bonds connect atoms throughout its structure. It is expected not to conduct electricity as a solid because the description provides no mobile charged particles.

Check: The classification follows from the stated extended covalent structure. The prediction does not assume that all substances made of non-metals consist of separate molecules.

Common mistakes and how to avoid them

Calling any solid with covalent bonds a molecular solid.

Correction: Molecular solids contain separate molecules. In a covalent network solid, covalent bonds extend through the structure.

Predicting that solid sodium chloride conducts because it contains ions.

Correction: Its ions are fixed in the solid structure. They can carry charge when mobile, such as in a molten sample or a solution.

Claiming that every ionic solid melts at a higher temperature than every molecular solid.

Correction: These are broad patterns, not a universal ranking. Melting points vary among substances, so avoid categorical comparisons.

Using the same conductivity prediction for a solid and its solution.

Correction: State the sample’s form. Ions can move in a solution or molten substance, but not through the solid structure.

Lesson summary

Classify a solid by its particles and the bonding that holds its structure together.
Molecular solids contain separate molecules; ionic solids contain fixed ions; metallic solids have mobile electrons; covalent network solids have extended covalent bonds.
Use structure to predict properties such as melting behaviour, hardness, brittleness, and conductivity.
Treat these as qualitative patterns and use more information or data when needed.

Check your understanding

Question 1

A solid sample of magnesium is metallic. Which model best explains its electrical conductivity as a solid?

Mobile electrons can move through the solid.
Positive and negative ions can move freely through the solid.
Separate molecules carry charge through the solid.
Covalent bonds connect every magnesium atom into a network.

Show answer and explanation

Mobile electrons can move through the solid.

The metallic bonding model includes mobile electrons that move through the solid. The other options do not describe the given metallic model.

Question 2

An ionic compound is molten. Why can it conduct electricity in this state when its solid form does not?

Its ions can move in the molten substance.
Its ions become neutral atoms when heated.
Its covalent molecules begin to move.
Its positive and negative charges disappear.

Show answer and explanation

Its ions can move in the molten substance.

In the solid, ions are fixed in place. In the molten state, ions can move and carry charge; their charges remain.

Question 3

A substance is described as having separate molecules in its solid form. Which classification fits?

Molecular solid
Metallic solid
Ionic solid
Covalent network solid

Show answer and explanation

Molecular solid

A molecular solid consists of separate molecules held near one another by attractions between molecules.

Key terms

Molecular solid: A solid made of separate molecules held near one another by attractions between molecules.
Ionic solid: A solid made of positive and negative ions arranged in a repeating structure.
Metallic solid: A solid of metal particles held together by metallic bonding, represented with mobile electrons in the course-level model.
Covalent network solid: A solid in which covalent bonds connect atoms in an extended structure.
Brittle: Likely to crack or break rather than bend when struck or stressed.
Malleable: Able to be hammered or shaped without breaking easily.
Ductile: Able to be drawn into wires.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation C2.5. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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