GCSE · Chemistry · AQA · Spec 8462

Properties of metals and alloys

Hammer a lump of pure metal and it bends. Mix in a little of another metal and it gets harder. What changed? The way the atoms are stacked.

Look inside the metal

A pure metal and an alloy, side by side

View: Before the push. Showing 2 layers: Frame, Before the push

View

Explore

switch views, then tap each part

Two blocks, bottoms held still, and the same push about to hit the top two layers. Before anything moves, compare the rows: flat and neat on the left, bumpy on the right.

Same push on both blocks. Watch which layers slide.

What do you really think?

Why is an alloy harder?

Pure metals are too soft for many uses. So they're mixed with other metals to make alloys, which are harder. But where does that extra hardness actually come from?

Which is closest to what you think right now?

The metal that gets added is a harder metal, so the alloy takes on its hardness.Mixing the metals makes the metallic bonds stronger, so the atoms are held tighter.The added atoms are a different size, so they distort the layers and the layers can't slide as easily.An alloy has more atoms crammed into the same space, so it's more solid.

How sure are you?

I'm sureI think soI'm guessing

The metal that gets added is a harder metal, so the alloy takes on its hardness.

This is a common misconception

Really common, and it sounds sensible, but hardness isn't passed on like a flavour. The explanation doesn't need the added metal to be hard at all. What matters is that its atoms are a different size, which spoils the neat layers of the pure metal.

Mixing the metals makes the metallic bonds stronger, so the atoms are held tighter.

This is a common misconception

The atoms are still held together by metallic bonding, and 'stronger bonds' isn't the explanation you need here. The change that matters is in the layers: different-sized atoms distort them, so they can't slide over each other as easily.

The added atoms are a different size, so they distort the layers and the layers can't slide as easily.

This matches the evidence

Yes, that's the whole explanation. Different-sized atoms push the rows out of line. Distorted layers can't slide over each other as easily, so it takes more force to change the alloy's shape.

An alloy has more atoms crammed into the same space, so it's more solid.

This is a common misconception

Hardness here isn't about how much is squeezed in. It's about whether the layers can slide. Look again at the alloy in the diagram: the rows are bumpy, not more crowded, and it's the bumps that stop them sliding easily.

An alloy is harder because atoms of a different size distort the layers, so the layers can't slide over each other as easily. The structure changes; no hardness is borrowed.

Same structure, one more property

Reason it through

Why do most metals have high melting and boiling points?

Link 1 of 4

First link · your turn

First, what is a lump of metal actually made of?

Locked — reveal the link above first

Now write it yourself

Put the explanation into words

Explain why alloys are harder than pure metals. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

In a pure metal, the atoms are all the same size and arranged in layers. (1 mark)
In an alloy, atoms of a different size distort the layers. (1 mark)
So the layers cannot slide over each other as easily. (1 mark)

Model answer

In a pure metal all the atoms are the same size, so they are arranged in regular layers that can slide over each other. In an alloy, atoms of a different metal are mixed in, and they are a different size. These atoms distort the layers, so the layers cannot slide over each other as easily. This makes the alloy harder.

Build it in three links: same-size atoms in layers, then different-sized atoms distort the layers, then the layers can't slide as easily. Saying the alloy is 'stronger' or 'mixed' doesn't explain anything on its own.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why you can bend a pure metal, why most metals take so much heat to melt, and why mixing metals makes them harder.

What you need to know

Metals have giant structures of atoms with strong metallic bonding.
Because a lot of energy is needed to overcome all that strong bonding, most metals have high melting and boiling points.
In a pure metal the atoms are arranged in layers. The layers can slide over each other, so pure metals can be bent and shaped.
Pure metals are too soft for many uses, so they are mixed with other metals to make alloys, which are harder.
Alloys are harder because atoms of a different size distort the layers, so the layers can't slide over each other as easily.

The big picture

Metals have giant structures of atoms with strong metallic bonding, so a lot of energy is needed to overcome the bonds and most metals have high melting and boiling points. In a pure metal the atoms are arranged in layers that can slide over each other, which is why pure metals can be bent and shaped. Pure metals are too soft for many uses, so they are mixed with other metals to make alloys, which are harder. In an alloy, atoms of a different size distort the layers, so the layers can't slide over each other as easily.

Key points

1High melting point comes from the bonding: a giant structure full of strong metallic bonds takes lots of energy to overcome.

2Bending comes from the layers: in a pure metal they slide over each other.

3Hardness of an alloy comes from distortion: different-sized atoms knock the layers out of line.

4Pure metals are too soft for many uses, which is exactly why alloys are made.

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Worked example

Problem

A student writes: "Metals have high melting points because their atoms are arranged in layers." What's wrong with this, and how would you fix it?

⚠ Watch out

Writing that an alloy's layers "can't slide at all" or that alloys "can't bend". The layers can still slide, just not as easily, so more force is needed to change the shape. That's what harder means here.

🧠

Memory hook

Neat rows slide. Bumpy rows catch.

✓

Check yourself

Close the page. Sketch a pure metal and an alloy as two small grids of circles. Use them to explain, in two sentences, which one bends more easily and why.

Flashcards

(7)

What holds the atoms in a metal together?

Strong metallic bonding, reaching right through a giant structure.

Why do most metals have high melting and boiling points?

So many strong bonds must be overcome that it takes a huge amount of energy, which means a high temperature.

How are the atoms arranged in a pure metal?

In regular layers. All the atoms are the same size.

What lets a pure metal be bent into a new shape?

Its layers of atoms slide over each other. The atoms just end up in new positions.

Why are pure metals often mixed with other metals?

Pure metals are too soft for many uses. Mixing them with other metals makes alloys, which are harder.

Why is an alloy harder than a pure metal?

Atoms of a different size distort the layers, so the layers can't slide over each other as easily.

True or false: an alloy's layers can't slide at all.

False. They slide less easily, so more force is needed to change the alloy's shape.

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Why is an alloy harder?

Put the explanation into words

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