Ch 2 · Bonding and structure · AS & A Level Chemistry 9701

Topic 2 of 2 in States of matter — AS & A Level Chemistry 9701 — 67 lessons.

Introduction

Every solid can be classified by its structure and bonding into one of four families — giant ionic, simple molecular, giant molecular (giant covalent), or giant metallic — and each family has a characteristic pattern of melting point, conductivity and solubility. Cambridge questions frequently give unfamiliar data and ask you to deduce the structure type, so the link between bonding and property must be understood, not memorised as isolated facts.

Key ideas — Giant ionic lattices

In sodium chloride and magnesium oxide, ions are arranged in a regular, repeating three-dimensional giant ionic lattice, with each ion surrounded by, and electrostatically attracted to, several oppositely charged neighbours throughout the structure. Magnesium oxide, with doubly charged ions of smaller radius than the singly charged ions in sodium chloride, has much stronger ionic bonding and a correspondingly higher melting point.

🧑‍🏫 Teacher's explanation

Imagine a massive, perfectly organized dance floor where every person is wearing a bright red shirt and every second person is wearing a bright blue shirt. Everyone is holding hands with everyone of the opposite color. This is a giant ionic lattice. Because everyone is locked in this intense, mutual grip, it is incredibly hard to pull anyone out of the crowd.

📖 Definitions

🔍 Worked example — Easy

Calculate the formula mass (M_r) of magnesium oxide (MgO).

🔍 Worked example — Standard

Explain why MgO has a higher melting point than NaCl.

🔍 Worked example — Exam-style

A substance has a high melting point and conducts electricity when molten but not when solid. Identify the type of bonding and structure.

⚠️ Common errors

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🎯 Exam question

(a) Explain this difference in terms of the structure and bonding present. [3] (b) Predict whether NaCl is more or less soluble in water than MgO and give a reason. [1] [4 marks]

Key ideas — Simple molecular lattices

In iodine, buckminsterfullerene (mathrmC_60) and ice, discrete covalently bonded molecules occupy fixed positions in a regular lattice, but are held to one another only by weak intermolecular forces — van der Waals' forces in iodine and buckminsterfullerene, and hydrogen bonding (in addition to van der Waals' forces) in ice. Within each molecule the covalent bonds are strong, but only the weak intermolecular forces between molecules need to be overcome on melting or subliming.

🧑‍🏫 Teacher's explanation

Imagine a large group of people standing in a room. Each person is holding onto their own heavy backpack very tightly (this is the strong covalent bond within a molecule). However, the people are only standing near each other because they are lightly brushing shoulders (this is the weak intermolecular force). To move the group to a different room, you don't need to force the people to let go of their backpacks; you only need to provide enough energy to make them step away from each other.

📖 Definitions

🔍 Worked example — Easy

Calculate the number of moles in 4.98 g of iodine (I_2) at room temperature.

🔍 Worked example — Standard

Explain why iodine (I_2) has a much lower melting point than silicon(IV) oxide (SiO_2).

🔍 Worked example — Exam-style

A substance consists of discrete molecules held together by hydrogen bonding and van der Waals' forces. It has a high boiling point for a molecular substance and is soluble in water. Identify the most likely substance from: CH_4, NH_3, CO_2, CH_4.

⚠️ Common errors

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🎯 Exam question

Key ideas — Giant molecular (giant covalent) lattices

In silicon(IV) oxide, diamond and graphite, atoms are joined throughout the entire structure by a continuous network of strong covalent bonds, forming one giant molecule. In silicon(IV) oxide and diamond, each atom is bonded tetrahedrally to four neighbours, giving a rigid three-dimensional network. In graphite, each carbon atom is bonded to three others in flat hexagonal layers, with one delocalised electron per atom free to move within each layer; the layers themselves are held together only by weak van der Waals' forces, allowing them to slide over one another.

🧑‍🏫 Teacher's explanation

Imagine a giant, interconnected web of strong steel cables. In a simple molecular substance like water, you have small, independent groups of molecules that are like tiny, separate balls held together by weak static cling. But in a giant molecular (or giant covalent) lattice, the entire crystal is essentially one single, massive molecule. To melt it, you aren't just separating molecules; you are physically breaking the strong covalent bonds that hold the entire structure together.

📖 Definitions

🔍 Worked example — Easy

Explain why diamond has a much higher melting point than iodine (I_2).

🔍 Worked example — Standard

A substance consists of carbon atoms arranged in flat layers. Each carbon is bonded to three other carbons in a hexagonal pattern. Explain why this substance can conduct electricity in one direction but not in another.

🔍 Worked example — Exam-style

Calculate the molar mass (M_r) of silicon(IV) oxide (SiO_2) given A_r(Si) = 28 and A_r(O) = 16.

⚠️ Common errors

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🎯 Exam question

(a) Suggest the type of structure present in substance X. [1] (b) Explain why substance X conducts electricity when molten. [2] [3 marks]

Key ideas — Giant metallic lattices

In copper, positive metal ions are arranged in a regular giant metallic lattice, held together by electrostatic attraction to a sea of delocalised electrons that is free to move throughout the entire structure, as described for metallic bonding generally.

🧑‍🏫 Teacher's explanation

Imagine a crowded ballroom where everyone is holding hands in a very strict, organized grid pattern. Now, imagine that instead of people, we have positive metal ions, and instead of hands, there is a 'sea' of tiny, fast-moving electrons flowing between them. Because these electrons aren't stuck to any one person, they can flow anywhere in the room.

📖 Definitions

🔍 Worked example — Easy

Explain why copper has a high melting point.

🔍 Worked example — Standard

A sample of pure copper is heated. Predict and explain its electrical conductivity.

🔍 Worked example — Exam-style

Compare the structures of copper and iodine. Why does copper conduct electricity while iodine does not?

⚠️ Common errors

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🎯 Exam question

Key ideas — Effect of structure and bonding on physical properties

🧑‍🏫 Teacher's explanation

Think of a substance's structure like how people are organized in a building. A 'giant' structure is like a massive skyscraper where everyone is welded together; to change the building, you have to break those heavy welds (strong bonds). A 'simple molecular' structure is like a crowd of people standing near each other; they are close, but they aren't holding hands tightly. To move the crowd, you just need a little nudge (weak intermolecular forces), because the people themselves (the molecules) are still perfectly fine.

📖 Definitions

🔍 Worked example — Easy

Explain why NaCl has a high melting point, whereas I_2 has a low melting point.

🔍 Worked example — Standard

A substance X is a solid at room temperature. It conducts electricity when molten but does not conduct when solid. What type of structure does X have?

⚠️ Common errors

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🎯 Exam question

(a) State the type of structure present in Y. (b) Explain why Y does not conduct electricity when it is in the solid state. [3 marks]

Key ideas — Deducing structure and bonding from data

Given a substance's melting point, electrical conductivity (as a solid and when molten) and solubility, its structure can be deduced systematically: a high melting point with conductivity only when molten indicates giant ionic; a high melting point with conductivity as a solid indicates giant metallic (or graphite); a high melting point with no conductivity at all indicates giant covalent (other than graphite); and a low melting point with no conductivity indicates simple molecular.

🧑‍🏫 Teacher's explanation

Think of substances as different types of social gatherings. A giant ionic lattice is like a highly organized military parade where everyone is locked in a strict formation; it takes a massive amount of energy to break that formation, and they only move (conduct electricity) if you melt the whole group so they can slide around. A giant covalent structure is like a massive, interconnected web of people holding hands tightly; it is incredibly strong and hard to break. A simple molecular substance is like a group of friends standing near each other but not actually touching; they are easy to separate (low melting point) and don't conduct anything because there are no free charges or electrons moving between them.

Ch 2 · Bonding and structure · AS & A Level Chemistry 9701

All chapters in States of matter