Atoms, elements and compounds · Classify
Elements
Everything there has ever been is built from about a hundred kinds of atom. So how do you tell whether the thing in front of you is one of them?
Start here
A phone contains about sixty of them. There are only about a hundred to choose from.
Every substance in the world — every rock, gas, liquid, plastic, medicine and living thing — is built from the same short list. Not a long list. About a hundred entries, and roughly sixty of them are in the phone in your pocket.
If you had a sample of something, how would you find out whether it was on that list?
Only one of those four actually settles it, and it is not the one that tells you the most about the substance. An element is a substance made of only one kind of atom — so the test is whether anything simpler can be got out of it.
An element is a substance made of one kind of atom and nothing else. Copper is copper atoms; sulfur is sulfur atoms. That is the whole definition, and none of it is about how the substance looks.
Your turn · six unlabelled samples
Eight tests. Six samples. Choose carefully.
8 of 8 tests left · 0 of 6 decided
Each test costs one of your eight. You need a verdict on all six samples, so you cannot run everything on everything — and one of the four tests is worth far more than the others.
Sample 1
A dull orange solid that can be bent without snapping.
Look at it closely
Orange, metallic, bendable. Copper looks like this; so does brass, and so does bronze.
Test whether it conducts
It conducts electricity well. So does every other metal on this bench, and so does salt water.
Look for it on the periodic table
There is an entry: copper, Cu. That settles it, though it is looking the answer up rather than finding it out.
Try to break it down
Heated, electrolysed and treated with acid, nothing simpler ever comes out of it.
Your verdict on this sample
Copper — an element.
Nothing simpler can be got out of it, and it has its own entry on the periodic table. Made of copper atoms only.
Sample 2
A colourless liquid with no smell.
Look at it closely
Clear, colourless, runny. Indistinguishable by eye from a dozen other liquids.
Test whether it conducts
Pure water barely conducts at all. Tap water does, because of what is dissolved in it.
Look for it on the periodic table
No entry called water anywhere on the table.
Try to break it down
Pass electricity through it and two different gases come off, in a fixed ratio of two to one by volume.
Your verdict on this sample
Water — not an element. It is a compound.
Two different gases came out of it, so it was made of more than one kind of atom all along.
Sample 3
A bright yellow powder that smells faintly of nothing much.
Look at it closely
Yellow, powdery, does not look remotely like a metal.
Test whether it conducts
It does not conduct at all. Neither does sugar, and sugar is not an element.
Look for it on the periodic table
There is an entry: sulfur, S.
Try to break it down
Nothing simpler can be got out of it by heating, dissolving or electrolysis.
Your verdict on this sample
Sulfur — an element.
Not everything on the list is a metal. Yellow powder, one kind of atom.
Sample 4
A hard, shiny, gold-coloured metal that rings when tapped.
Look at it closely
Shiny, gold-coloured, hard. The most convincing-looking sample on the bench.
Test whether it conducts
It conducts well, like every metal here.
Look for it on the periodic table
No entry called brass. There is copper, and there is zinc.
Try to break it down
Its composition can be varied — 70% copper and 30% zinc, or 60/40 — and it can be separated. Nothing fixed about it.
Your verdict on this sample
Brass — not an element. It is a mixture of copper and zinc.
The most metal-looking sample here is not an element at all. Metal describes how it behaves; element describes what it is made of.
Sample 5
A colourless gas that does nothing obvious.
Look at it closely
Invisible. Looking at it tells you precisely nothing.
Test whether it conducts
It does not conduct under ordinary conditions.
Look for it on the periodic table
No entry called air. There is nitrogen, oxygen, argon.
Try to break it down
Cooled until it liquefies and then warmed slowly, it separates into several different gases with different boiling points.
Your verdict on this sample
Air — not an element. It is a mixture of several gases.
It separates into several gases, and the proportions vary from place to place. Nothing about being invisible makes something simple.
Sample 6
A soft, silvery metal that has to be kept under oil.
Look at it closely
Silvery when freshly cut, and it dulls over within seconds in air.
Test whether it conducts
It conducts, like every metal here.
Look for it on the periodic table
There is an entry: sodium, Na.
Try to break it down
Nothing simpler comes out of it. It reacts violently with water, which makes something more complicated, not something simpler.
Your verdict on this sample
Sodium — an element.
Reacting violently is not the same as breaking down. A reaction with water builds a compound out of it; it does not take it apart.
Six verdicts. Every one of them turned on the same question — can anything simpler be got out of it — and none of them turned on how the sample looked. Shine, colour and conducting were the three most interesting results you could buy, and all three were worthless.
Key fact
An element is made of one kind of atom, and cannot be broken down into anything simpler by chemistry. Nothing about how it looks will tell you.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Brass is a metal, and metals are elements, so brass is an element.”
Brass is hard, shiny, gold-coloured, conducts electricity and rings when you hit it. Commit before you read on.
Brass is copper and zinc, mixed while both are molten. There is no such thing as a brass atom, and brass has no entry on the periodic table. Every metallic thing in the sentence is true and none of it settles anything: metal describes how a substance behaves, and element describes what it is made of. Steel, bronze and solder are all in the same position.
This is the useful thing about brass: it is more useful than either of the metals in it, and that is why it exists. Behaving like a metal is not evidence of being an element. It is evidence of having metal atoms in it, and mixtures have those too.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What makes a substance an element?
Rung 2 · The one that catches people
A student has an unknown silvery solid. Which single result would settle whether it is an element?
Rung 3 · Explain
A student says "gold is an element because it is shiny and expensive, and brass is not because it is cheap". Their conclusion about both is right and their reasoning is wrong. Explain why, and give the reasoning that works.
Rung 4 · Take it somewhere new
Before electricity was available, chemists thought several substances were elements that turned out not to be — including one they called "soda". Explain how a new technique can move something off the list, and whether that means the earlier chemists were careless.
Key note
An element is a substance made of only one kind of atom. There are about a hundred, they are the entries on the periodic table, and the test is always the same: can anything simpler be got out of it? Looking like a metal proves nothing.
Going further
Ninety-nine per cent of your body is six elements — oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus — and almost all of the oxygen and hydrogen is water. The remaining one per cent includes iron you cannot carry oxygen without, and iodine you cannot grow properly without. Fewer than thirty of the hundred are known to be needed by living things at all, and every one of the atoms in you — except the hydrogen, which is as old as the universe itself — was made inside a star or in the collision of two of them. There is no such thing as a fresh atom.
Before this lesson
Next in this unit
At GCSE this becomes
- The periodic table as an organised list, and why the elements fall into groups at all.
Where to next
Ask Mr Badmus AI
Not sure why brass is not an element?
Lesson content © MrBadmusAI.