Lesson 2 of 6

Ionic and metallic lattices.

About 25 minutes · 7 practice questions

Inside a salt crystal and a lump of copper: a lattice of ions each pulled by all its neighbours, and a lattice of positive ions in a sea of electrons, and why salt cracks while a metal bends and conducts.

Step 1 of 7 · A giant lattice

Step 1 of 7

A giant lattice

A grain of table salt is a tiny cube holding billions of billions of ions. In the Scene Zoom to the atoms is at Atoms: small sodium ions, Na⁺, and large chloride ions, Cl⁻, alternating in three directions. This is an ionic lattice.

There are no NaCl molecules here: each ion is pulled not by one neighbour but equally by all the oppositely charged ions around it. Count the nearest neighbours of one sodium ion in the middle of the crystal.

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Step 2 of 7

Salt shatters

The pull between ions is strong, so ionic compounds are hard and melt only when very hot: table salt melts at 801 °C. But they are also brittle. Slide the top half moves the top half of the crystal along one plane. Predict what happens when the top half moves one whole step.

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Step 3 of 7

When salt conducts

An electric current is a flow of charged particles. Salt has charged particles, its ions, but in the solid each ion is locked in its place in the lattice, so solid salt does not conduct. When salt is melted by heat, or dissolved in water, the lattice comes apart and the ions are free to move: such a liquid conducts.

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Step 4 of 7

A sea of electrons

Now the Scene shows copper. A metal is a lattice too, but each of its atoms hands its outer electrons over to the whole crystal. What is left is a lattice of positive ions, with the electrons they gave up roaming freely among them, a sea of electrons. The pull between the ions and this sea is the metallic bond.

Metal atoms are packed very closely: in copper each atom touches a large number of neighbours. Count them.

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Step 5 of 7

Metals bend

Predict what happens to copper when you slide the top half one whole step, as you did with salt.

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Step 6 of 7

Why metals conduct

The same sea of electrons explains the familiar properties of metals. It carries a current, because its electrons are free to move through the whole crystal. It carries heat for the same reason. And it holds the crystal together even as the layers slide, which is why metals can be hammered flat and drawn into wires.

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Step 7 of 7

The copper cell

In copper the atoms are packed as tightly as they can be. Where in the cell do they touch?

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Interactive scene

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Each step sets up the Scene for you, and a control marked “fixed” stays put for that step.

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Practice · Calculate

How many chloride ions touch each sodium ion inside the salt crystal?

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Practice · Predict, then look

Slide the top half of the salt crystal one whole step along the plane. What happens?

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Practice · Choose

When does table salt conduct electricity? Select every right answer.

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Practice · Calculate

In copper the atoms are packed as tightly as they can be. How many touching neighbours does each copper atom inside the crystal have?

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Practice · Predict, then look

Slide the top half of the copper crystal one whole step, as you did with salt. What happens?

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Practice · Choose

A copper wire carries a current. Which particles move along it?

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Practice · Calculate

Bond type: Metallic: copper. Copper’s unit cell has an atom at each corner and in the middle of each face, and the atoms touch along each face’s diagonal. A copper atom’s radius is 128 pm. How long is the cell’s edge?