S4.6.8 Chapter 8

Metals and alloys

Why can a metal wire be bent, when a lump of salt shatters?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A The layers of a metal can slide over each other, and the bonding still holds.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The bonds in a metal are very weak.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Metal atoms are not joined to one another.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how metallic bonding lets metals conduct and bend
Explain why alloys are harder than pure metals

Think of copper wire, an iron nail and a gold ring. Most metals share some properties. They are shiny when polished and strong. They conduct heat and electricity. They bend and can be hammered into shape.

A metal that can be hammered or bent into shape is malleable. Most metals also have high melting points. A few are exceptions, such as mercury, which is a liquid at room temperature.

In Chapter 6 a metal atom lost its outer electrons to become a positive ion. In a lump of metal this happens to all the atoms. Their outer electrons come free. They no longer belong to one atom. We say they are delocalised.

The positive ions sit in a regular pattern, in layers. The delocalised electrons move freely through all the spaces between them, like a sea. The attraction between the ions and the sea of electrons is the metallic bond.

A metal. The ions are in layers. The free electrons move between them.

The metal as a whole has no charge. There are as many free electrons as there are charges on the ions. The sea of negative electrons balances the positive ions.

Metals conduct electricity because the free electrons can move. When a metal is part of a circuit, the electrons are pushed along it and carry charge. Salt in the solid state cannot do this, because its ions are fixed.

Metals conduct heat well for the same reason. The free electrons carry energy quickly from the hot end of a metal to the cold end. This is why a metal spoon in a hot drink soon feels hot.

Draw it first

Why a metal bends

1 Layers of positive ions are held in place by the sea of free electrons.
2 A push slides one layer over the next. The ions are still surrounded by the sea of electrons, so the metal holds together and just changes shape.

In salt the layers cannot slide in the same way. A slide puts like charges side by side, and they repel. The crystal splits. In a metal the electrons hold the layers whatever the position, so a metal bends instead of breaking.

Pure metals are often soft. In a pure metal all the atoms are the same size, so the layers slide easily. Pure gold and pure iron are examples. A soft metal is no use for a bridge or a tool.

An alloy is a mixture of a metal with one or more other elements. Brass is copper with zinc. Bronze is copper with tin. Steel is iron with a small amount of carbon. The other atoms are a different size.

An alloy. The larger atoms disturb the layers, so they cannot slide easily.

The odd sized atoms get in the way when a layer tries to slide. The layers slide less, so an alloy is harder and stronger than the pure metal. Alloys are still held by metallic bonding, so they still conduct.

The mass of each part of an alloy is often given as a percentage by mass. A percentage tells us how many parts out of 100.

A common kind of brass is 70% copper and 30% zinc.

Worked example

A common kind of brass is 70% copper by mass. How many grams of copper are in a brass piece of mass 250 g?

70% means 70 parts in every 100.
Find 70% of 250 g: 70 ÷ 100 × 250 g.
70 ÷ 100 × 250 g = 175 g.

Answer: 175 g

A common kind of bronze is 88% copper and 12% tin.

Worked example

A small statue is made of bronze with a mass of 500 g. This bronze is 88% copper by mass and the rest is tin. How many grams of tin are in the statue?

The tin is the rest: 100% − 88% = 12%.
Find 12% of 500 g: 12 ÷ 100 × 500 g.
12 ÷ 100 × 500 g = 60 g.

Answer: 60 g

Try it

Make layers slide, then stop them

You need: a shallow tray or box lid with a rim, about 30 round things of one size: dried peas, marbles or small round stones, a few bigger things, such as large beans or bigger stones.

Be safe: Marbles and small stones can be swallowed by small children, and they roll. Do the activity on a table, and pick up every piece when you finish.

Fill the tray with rows of the small round things, touching, in neat rows.
Push the top row sideways with a finger. Watch how easily it slides over the row below.
Take out about six of the small ones, spread through the rows. Put the bigger things in their places.
Push the top row again. Does it slide as freely?

What you will see: With rows of one size the top row slides easily. With bigger pieces mixed in, the rows catch on them and slide much less. This is how an alloy is harder than a pure metal.

If you cannot try it, this is what happens.

A wrong idea many people have

A metal has a negative charge, because it has a sea of electrons.

Why it feels right: The electrons are negative, and there are very many of them.

The test that breaks it: Count the charges. Every free electron came from an atom that became a positive ion. Each ion has a charge that one electron balances.

What is true: A metal is neutral. The negative sea of electrons exactly balances the positive ions.

Key words

malleable: able to be hammered or bent into shape without breaking.
delocalised: not attached to one atom, and free to move.
metallic bond: the attraction between the positive ions of a metal and its sea of free electrons.
alloy: a mixture of a metal with one or more other elements.
percentage by mass: how many grams out of every 100 g of a mixture are one part of it.

Back to your guess

The answer is A: The layers of a metal can slide over each other, and the bonding still holds.

A metal is positive ions in a sea of free electrons. The layers of ions slide, and the electrons still hold them together. In salt a slide puts like charges side by side, and the crystal splits.

Keep it

Copy this into your exercise book.

A metal is positive ions in a sea of free electrons. The electrons carry charge and heat. Layers slide, so metals bend. In an alloy, atoms of other sizes block the layers, so it is harder.

Draw: Rows of positive ions with dots between them for electrons, and a second picture with two larger atoms in the rows.