1.6.5Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Metallic Bonding and Lattices — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 21 of 36 Unit quiz
Part 1 of 5The Metallic Bond
Metallic bonding explains why metals conduct, bend and shine.
Structure
- A regular lattice of positive metal ions is surrounded by delocalised electrons from the outer shells of the atoms.
Bonding force
- The electrostatic attraction between the positive ions and the sea of electrons acts in all directions and holds the lattice together.
Strength
- It increases with the number of delocalised electrons per atom and with a smaller ion radius, so Mg is stronger than Na.
Periodic trend
- Melting points rise across Na, Mg and Al as charge increases from 1+ to 3+, and fall down Group 1 as the ions get larger.
Part 2 of 5Properties Explained
Properties Explained
Electrical conductivity
- Delocalised electrons are free to move through the lattice and carry charge when a voltage is applied.
Thermal conductivity
- Moving electrons and vibrating ions pass kinetic energy quickly through the metal.
Malleable and ductile
- Layers of ions can slide over each other and the electron sea readjusts, so the bonding is not broken.
High melting point
- Strong attraction between ions and electrons needs a large amount of energy to overcome, though some metals melt easily.
Insoluble
- Metals do not dissolve in water or similar solvents because the bonding is too strong to be offset by any new attractions.
Part 3 of 5Metal Lattice Types
1
In close packing, each atom is treated as a sphere, with layers stacked so that spheres sit in the dips of the layer below.
2
Hexagonal close packed has layers repeating as ABAB, giving a coordination number of 12; magnesium and zinc use it.
3
Cubic close packed has layers repeating as ABCABC, also with coordination number 12; copper, aluminium and silver use it.
4
Both close-packed forms fill about 74% of the space with spheres.
5
Body centred cubic is less efficient with a coordination number of 8 and about 68% filled; sodium and iron at room temperature use it.
Part 4 of 5Alloys and Further Points
Alloys and Further Points
Alloys
- A mixture of metals, such as brass, contains atoms of different sizes which disrupt the layers, making sliding harder and the metal stronger.
Why no molecules
- Metals do not contain separate molecules, so the formula is simply the symbol of the element.
Comparison with ionic
- Metals conduct as solids because the electrons move, while ionic solids need molten or dissolved ions.
Coordination number
- The number of nearest neighbours of an atom, found by counting touching atoms in the lattice.
| Lattice | Coordination number | Example |
|---|---|---|
| Hexagonal close packed | 12 | Magnesium |
| Cubic close packed | 12 | Copper |
| Body centred cubic | 8 | Sodium |
Part 5 of 5Exam Checklist & Tips
1.6.5 – Key Points for the Exam
- Metallic bond The attraction between positive metal ions and delocalised electrons.
- Delocalised electrons Outer electrons that are not tied to one atom and can move through the lattice.
- Malleable Able to be hammered or bent into shape without breaking.
- Coordination number The number of nearest neighbours surrounding an atom in a lattice.
- Alloy A mixture of a metal with other elements, usually harder than the pure metal.
Common Mistakes
- Describing a metal as containing ions and electrons moving freely, forgetting that the ions stay fixed in the lattice.
- Saying metals conduct because their ions move.
- Thinking metallic bonding is an attraction between metal atoms rather than ions and electrons.
- Mixing up the coordination numbers of the close-packed and body-centred lattices.
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