KS3 · Chemistry

Naming compounds

Salt is made from a reactive metal and a toxic gas, yet its name, sodium chloride, still tells you exactly what's inside. Let's crack the code.

Crack the code

What's inside each compound?

A compound is made of two or more different elements, chemically joined, and its name tells you which ones. The clues: -ide usually means only the two elements named. -ate means oxygen is there too, as well as the elements named. Tick every circle each compound belongs in. The last three names give no clues, so use their formulas.

A Only two elements
B Contains oxygen

sodium chloride
iron sulfide
magnesium oxide
carbon dioxide
copper sulfate
calcium carbonate
potassium nitrate
water (H₂O)
ammonia (NH₃)
methane (CH₄)

Your turn to build one

From elements to name to formula

Magnesium joins with oxygen to make a compound. What is the compound called, and what is its formula?

Sort the two elements: magnesium is the metal, oxygen is the non-metal.
missing step

Which line is step 2?

The metal goes first, with its name unchanged: magnesium …The non-metal goes first: oxygen …The metal goes first, with an -ide ending: magneside …

Count the atoms

Carbon monoxide and carbon dioxide

Showing 2 layers: Names and formulas, Atoms

Layers

Explore

Tap a part to explore. Try hiding the atoms and counting from the formula alone.

Each circle is one atom. Hide them, read the formula, then bring them back to check your count.

Same two elements, different numbers. Tap each part to see how the name and the formula both count the oxygen atoms.

What do you think?

What should salt be like?

Sodium is a reactive metal. Chlorine is a toxic gas. When they join chemically, they make sodium chloride, which is common salt.

Which idea is closest to what you think right now?

Salt should be a bit reactive and a bit toxic. It's made from sodium and chlorine, so it keeps some of what they're like.Salt is just sodium and chlorine mixed together.Salt is so different from sodium and chlorine that its name can't really tell you what's in it.Salt is a new substance. Sodium and chlorine are joined in it, so it has its own, different properties.

How sure are you?

I'm sureI think soI'm guessing

Salt should be a bit reactive and a bit toxic. It's made from sodium and chlorine, so it keeps some of what they're like.

This is a common misconception

It's a natural guess, but a compound doesn't keep its elements' properties. Once sodium and chlorine are chemically joined, you have a new substance with different properties from both. Common salt is neither a reactive metal nor a toxic gas.

Salt is just sodium and chlorine mixed together.

This is a common misconception

In a mixture, the substances aren't chemically joined. In salt, the sodium and chlorine atoms are joined, and that's why salt has different properties from the elements it's made from. Different properties are one way to tell a compound from a mixture of its elements.

Salt is so different from sodium and chlorine that its name can't really tell you what's in it.

This is a common misconception

The properties change, but the elements are still there. The name is a record of what went in: sodium chloride means sodium and chlorine. Different behaviour, same elements.

Salt is a new substance. Sodium and chlorine are joined in it, so it has its own, different properties.

This matches the evidence

Exactly right. A compound has different properties from the elements it's made from, and its name still tells you which elements those are.

A compound is something new, with its own properties. Its name tells you which elements went into it, not how it behaves.

Prove it

Decode without looking back

Answer all three, then mark yourself. (a) Name the compound made when calcium joins with chlorine. (b) Which elements are in sodium carbonate? (c) The formula of ammonia is NH₃. How many atoms of each element does it show? [4 marks]

Your answer

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The mark scheme

(a) calcium chloride: calcium first, and chlorine changed to chloride (1 mark)
(b) sodium, carbon and oxygen: all three, including oxygen from the -ate ending (1 mark)
(c) one nitrogen atom (N has no number after it) (1 mark)
(c) three hydrogen atoms (the small 3 after H) (1 mark)

Model answer

(a) Calcium chloride. (b) Sodium, carbon and oxygen. (c) One nitrogen atom and three hydrogen atoms.

If you missed (b), check whether you left out the oxygen. The -ate ending is the only clue that it's there.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

The ending and the prefixes tell you which elements are inside, and sometimes how many atoms of each.

What you need to know

A compound is made of atoms of two or more different elements chemically joined together.
Metal + non-metal: the metal goes first, unchanged, and the non-metal goes second with its ending changed to -ide (oxygen → oxide, chlorine → chloride, sulfur → sulfide).
A name ending in -ide usually means only the two elements named.
A name ending in -ate means the compound contains oxygen as well as the elements named.
Prefixes count atoms: mono- = one, di- = two, tri- = three.
In a formula, a small number after a symbol counts atoms of that element; no number means one.
Water (H₂O), ammonia (NH₃) and methane (CH₄) have common names that don't follow the rules.
A compound has different properties from the elements it is made from.

The big picture

A compound is made of atoms of two or more different elements chemically joined, and its name tells you which elements it contains. For a metal and a non-metal, the metal comes first, unchanged, and the non-metal comes second with an -ide ending. An -ide name usually means only the two elements named. An -ate name means oxygen is there too. Prefixes such as mono- and di- count atoms, just as the small numbers in a formula do. A few common compounds, such as water, ammonia and methane, have names that break the rules.

Key points

1Read the ending first: -ide usually means just the two elements named; -ate adds oxygen.

2Oxides end in -ide but still contain oxygen, as one of their two elements (magnesium oxide, carbon dioxide).

3To name a metal + non-metal compound: metal unchanged, then non-metal + -ide.

4The prefix in a name and the small number in a formula can show the same count: carbon dioxide, CO₂.

5Some names, like water, ammonia and methane, have to be learned, because they give no clues.

6Sodium chloride is made from a reactive metal and a toxic gas, but it has its own, different properties.

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Worked example

Problem

Iron joins with sulfur to make a compound. What is it called, and which elements does it contain?

⚠ Watch out

Thinking only -ate compounds contain oxygen. Oxides end in -ide and still contain oxygen: magnesium oxide is made of magnesium and oxygen, with nothing else.

🧠

Memory hook

IDE: I'm Done at two (usually). ATE: oxygen's in the crate. Then mono, di, tri: 1, 2, 3.

✓

Check yourself

Without scrolling up: which elements are in calcium carbonate, and which part of the name tells you about the one that isn't named?

Flashcards

(12)

What is a compound?

A substance made of atoms of two or more different elements, chemically joined together.

Naming a metal + non-metal compound: which element comes first?

The metal, with its name unchanged.

What happens to the non-metal's name?

It goes second and its ending changes to -ide: oxygen → oxide, chlorine → chloride, sulfur → sulfide.

What does an -ide ending usually tell you?

The compound contains only the two elements named.

What does an -ate ending tell you?

The compound contains oxygen as well as the elements named. Copper sulfate = copper, sulfur and oxygen.

Does an -ide name mean there's no oxygen?

No. Oxides end in -ide and contain oxygen as one of their two elements, like magnesium oxide.

What do mono-, di- and tri- mean?

One, two and three atoms of that element.

What does a small number after a symbol show?

How many atoms of that element there are. No number means one atom.

Formula for carbon dioxide, and what it shows

CO₂: one carbon atom and two oxygen atoms.

Formula for magnesium oxide

MgO: magnesium and oxygen, one atom of each.

Three common compounds whose names break the rules

Water (H₂O), ammonia (NH₃) and methane (CH₄).

Sodium is reactive and chlorine is toxic. What about sodium chloride?

It has different properties from both. A compound doesn't keep its elements' properties, and that's one way it differs from a mixture of them.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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What's inside each compound?

From elements to name to formula

What should salt be like?

Decode without looking back

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