9.07.2 Properties of Metals and Alloys

Syllabus

- Metals have giant structures of atoms with strong metallic bonding. This means that most metals have high melting and boiling points.
- Metals are good conductors of electricity because the delocalised electrons in the metal carry electrical charge through the metal. Metals are good conductors of thermal energy because energy is transferred by the delocalised electrons.
- In pure metals, atoms are arranged in layers, which allows metals to be bent and shaped. Pure metals are too soft for many uses and so are mixed with other metals to make alloys which are harder.

Students should be able to explain why alloys are harder than pure metals in terms of distortion of the layers of atoms in the structure of a pure metal.

What does this mean?

Properties of metals

#1 Melting points

Like ionic substances, metals form giant structures.

So they share some properties with Ionic substances - notably they have High Melting and Boiling points due to there being being many strong bonds in a metallic lattice, all of which need to be broken to melt/boil.

Breaking lots of strong bonds takes a lot of energy and so a High Temperature - just like Ionic compounds.

In the graph we see that the melting points of all metals are high, and most non-metals are quite low.

#2 Electrical and Thermal conductivity

We've seen that metal structures have delocalised or free electrons

Because these electrons can move they can drift towards the positive end of a battery.

This is an electric current.

Generally, the more delocalised electrons the better the conductor.

Because theelectrons can move they can carry thermal energy (heat) along a piece of metal.

Making metals thermal conductors too.

Again, the more delocalised electrons the better the conductor.

#3 Hardness.

Pure metals are rather soft and very malleable.

This is because the metal ions are arranged in rows, and when all the ions are the same size the layers can slide over each other.

So pure metals are useless for most jobs.

Pure iron is too soft to make a bridge or a car.

We usually add Carbon to make Steel - which is harder

24 carat Gold is too soft to make hard-wearing jewelry and we usually add small amounts of silver to harden it.

Mixing other atoms that are either much larger or much softer the pattern is disrupted - we call a mixture of a metal and another element (often another metal) an alloy.

There are no longer layers that can slide.

So, alloys are harder but more brittle

The graph shows that pure Silver and pure Copper are both quite soft.

But a 50:50 mixture is twice as hard.

People have been making alloys for thousands of years for this reason.

Ancient people mixed Tin with Copper to make Bronze because pure Copper tools are rather soft but Bronze is almost as hard as steel.

And alloys can have other useful properties - making Steel into Stainless Steel stops it rusting and makes it useful for cutlery.

We add Chromium for this.