Non-metals, metalloids and the exceptions

The physical properties of non-metals

Non-metals have properties that are mostly the opposite of those of metals.

• Lustre. They are usually dull. Sulphur, carbon (as coal) and phosphorus are dull.
• State. Non-metals are solids (carbon, sulphur, phosphorus), gases (hydrogen, oxygen, nitrogen) or, in one case, a liquid (bromine).
• Brittle. A solid non-metal breaks into pieces when it is hit with a hammer. It does not flatten.
• Not malleable or ductile. They cannot be beaten into sheets or drawn into wires.
• Not sonorous. They do not ring.
• Poor conductors of heat and electricity.
• Melting and boiling points are usually low, and the densities are low.

The exceptions among non-metals

• Iodine is a non-metal, but it is shiny.
• Graphite, a form of carbon, conducts electricity. It is used for electrodes in batteries and in the electrolysis of salt.
• Diamond, another form of carbon, is the hardest natural substance, and has a very high melting point. It is a non-metal, too.

Carbon in different forms, such as diamond and graphite, is an example of allotropes: the same element, in different forms, with different properties.

Metalloids

Some elements have properties of both metals and non-metals. They are the metalloids. Examples are silicon, germanium, boron and arsenic. Silicon is shiny like a metal but brittle like a non-metal, and it is a semiconductor: it conducts better than a non-metal but worse than a metal. This makes silicon the basis of computer chips.

You should find a temperature, near $400\ \text{K}$, at which two metals and two non-metals have already melted.

A quick comparison

Practise with new numbers

Try it in Python

This program classifies a solid as a metal or a non-metal from four tests, and it shows the cases where the tests alone give a wrong or unclear answer.

Remember these

• Non-metals are mostly dull, brittle and poor conductors, with low melting points.
• Exceptions: iodine is shiny, graphite conducts, diamond is very hard.
• Metalloids, such as silicon, have properties of both. Silicon is a semiconductor.
• One test is not enough. We use many properties together.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What happens when a lump of sulphur is hit with a hammer?

2. Name the only non-metal that is a liquid at room temperature.

3. Diamond and graphite are both carbon. What are such forms of one element called?

4. Why is one property not enough to decide whether an element is a metal?