Ch 3 · Metallic bonding · AS & A Level Chemistry 9701

Topic 3 of 7 in Chemical bonding — AS & A Level Chemistry 9701 — 47 lessons.

Introduction

Metallic bonding accounts for properties that ionic and covalent bonding cannot: metals conduct electricity as solids, can be hammered into sheets without shattering, and often have very high melting points. Cambridge questions expect the bonding to be explained in terms of a specific model — positive ions in a lattice held by a "sea" of delocalised electrons — and that model to be linked directly to each property.

Key ideas — Defining metallic bonding

Metallic bonding is the electrostatic attraction between positive metal ions and delocalised electrons. Each metal atom loses its outer-shell electrons into a shared, mobile pool; these electrons no longer belong to any one ion, and the resulting positive ions are held together by their attraction to this negatively charged "sea".

🧑‍🏫 Teacher's explanation

Imagine a crowded ballroom where everyone is holding onto a long, flowing silk ribbon. The people are the positive metal ions, and the ribbon is the sea of delocalised electrons. Because the ribbon is long and flexible, everyone is connected to everyone else, but the ribbon can also slide and shift without the people losing their connection. This is why metals are so strong yet so flexible.

📖 Definitions

🔍 Worked example — Easy

Explain why magnesium (Mg) has a higher melting point than sodium (Na).

🔍 Worked example — Standard

Calculate the molar mass of a hypothetical metal lattice where each atom contributes 2 electrons, given the total mass of 0.5 moles of the metal is 24g. Find the M_r of the metal.

🔍 Worked example — Exam-style

A metal sample is described as being 'malleable'. Explain this property in terms of the metallic lattice structure.

⚠️ Common errors

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🎯 Exam question

Key ideas — The giant metallic lattice

Metal ions pack together in a regular, repeating three-dimensional arrangement called a giant metallic lattice. The delocalised electrons occupy the spaces between the ions and are free to move throughout the entire structure, rather than being associated with any particular ion.

🧑‍🏫 Teacher's explanation

Imagine a crowded dance floor where everyone is holding onto a long, continuous silk ribbon that weaves through the entire room. The dancers are the positive metal ions, fixed in their positions, and the ribbon is the sea of delocalised electrons. Because the ribbon is everywhere, it holds every dancer to every other dancer, creating a single, massive, interconnected structure.

📖 Definitions

🔍 Worked example — Easy

Explain why magnesium, which has 2 valence electrons, has a higher melting point than sodium, which has 1 valence electron.

🔍 Worked example — Standard

Calculate the molar mass of a hypothetical metal lattice where the formula is M_2X and the atomic mass of M is 24 and X is 32.

🔍 Worked example — Exam-style

A sample of copper metal is subjected to a force. Explain, in terms of the structure, why the metal deforms (malleability) rather than shattering like an ionic crystal.

⚠️ Common errors

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🎯 Exam question

Key ideas — Metallic bonding and physical properties

🧑‍🏫 Teacher's explanation

Imagine a crowded ballroom where everyone is holding hands in a rigid, fixed formation. If you try to push one person, the whole group stays locked together, and if you force them to move, they might break the formation. This is like an ionic lattice. Now, imagine a large group of people standing in a room, but instead of holding hands, everyone is tossing a bunch of tennis balls to each other constantly. The people (the positive ions) stay in their general spots, but the tennis balls (the delocalised electrons) are flying everywhere. Because the balls are moving freely, they can carry energy or charge across the room instantly.

📖 Definitions

🔍 Worked example — Easy

Explain why magnesium (Mg) has a higher melting point than sodium (Na).

🔍 Worked example — Exam-style

Compare the electrical conductivity of solid Sodium Chloride (NaCl) and solid Copper (Cu).

⚠️ Common errors

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🎯 Exam question

Key ideas — Effect of the number of delocalised electrons on bonding strength

The strength of metallic bonding increases with the number of delocalised electrons contributed per atom, and also with a smaller ionic radius and a greater ionic charge on the resulting metal ion. Across Period 3, sodium (Na^+, one delocalised electron per atom) has weaker metallic bonding than magnesium (mathrmMg^2+, two delocalised electrons per atom), which in turn is weaker than aluminium (mathrmAl^3+, three delocalised electrons per atom). More delocalised electrons per ion, combined with a higher ionic charge and generally smaller ionic radius, increase the electrostatic attraction holding the lattice together, which is reflected in rising melting points: Na, 98 °C; Mg, 650 °C; Al, 660 °C (aluminium's radius partly offsets its extra charge, but its bonding is still markedly stronger than sodium's).

🧑‍🏫 Teacher's explanation

Imagine a large crowd of people (the positive metal ions) standing in a grid, all trying to hold onto a group of energetic, fast-moving toddlers (the delocalised electrons) running between them. The strength of the 'bond' depends on how many toddlers are running around and how tightly the people are holding onto them. If each person has more toddlers to hold, the crowd becomes much harder to pull apart.

Ch 3 · Metallic bonding · AS & A Level Chemistry 9701

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