KS3 · Chemistry

Compounds and their formation

Some substances act as if they've forgotten what they were made from. Learn two questions and you can read any atom diagram: element, compound or not a compound.

Chemistry · Reading particle diagrams

Element, compound, or neither?

Here's the code. In these particle diagrams, every colour is a different element, and atoms that overlap are chemically bonded: joined together by a strong force. Same element, same colour, every time. Pick a diagram, decide where it belongs, THEN read why.

Still to sort

Element (0)

Only one type of atom, so one colour.

Where the line is: Joined or not, one colour is still one type of atom, so it's an element.

Compound (0)

Two or more different atoms, chemically bonded.

Where the line is: It needs different colours AND overlapping. Different colours alone isn't enough.

Not a compound (0)

Different atoms that are NOT bonded.

Where the line is: Different colours, but with gaps between them: nothing is chemically bonded.

8 of 8 still to sort.

Two questions decide every diagram: how many colours? Are the atoms joined?

Watch out: Two traps. Joined atoms of ONE colour are still an element. Two colours with a gap between them are still not a compound.

Predict, then check

Iron is a grey solid and it's magnetic. Sulfur is a yellow solid and it isn't. Time to make a call.

Iron and sulfur are heated gently together and react to make iron sulfide. Will the iron sulfide be magnetic?

Chemistry · Properties

What does magnesium oxide share with its elements?

Hold magnesium in a flame and it burns with a bright white flame, reacting with oxygen to make magnesium oxide. For each property, tick every set that has it. Tick 'Elements' if magnesium OR oxygen has the property.

  • A Elements (magnesium or oxygen)
  • B Compound (magnesium oxide)
  1. Silver and shiny
  2. A solid
  3. A white, brittle powder
  4. Conducts electricity
  5. A colourless gas
  6. Does not conduct electricity

Before and after

The elements (before)vsThe compound (after)

Two reactions, each with the elements on the left and the compound they make on the right. Sodium reacts with chlorine with a bright yellow-orange flame.

Focus

Copper + chlorine: colour

The elements (before)

Copper is a reddish, copper-coloured metal. Chlorine is a dangerous greenish-yellow gas.

The compound (after)

Copper chloride is a greenish-blue powder.

The insight

Completely different to both of the elements it's made from.

Copper + chlorine: what you end up holding

The elements (before)

A metal and a gas

The compound (after)

A powder

Sodium + chlorine: colour

The elements (before)

Sodium is a shiny silver solid. Chlorine is a yellow-green gas.

The compound (after)

Sodium chloride is a white solid.

Sodium + chlorine: state

The elements (before)

A solid and a gas

The compound (after)

Just a solid

Chemistry · What do you think?

How does a compound form?

You've seen diagrams sorted and three reactions. Now be honest about what you actually think.

Which idea is closest to what you think right now?
How sure are you?

Chemistry · In your own words

Prove it's a compound

Iron and sulfur are heated and make a black solid that a magnet ignores. A particle diagram of the black solid shows two different colours of atom, overlapping. Explain why the black solid is a compound and a new substance. [3 marks]

0 words · your answer stays on this page and is not sent anywhere.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

When different elements react and bond, something brand new turns up.

What you need to know

  • An element is made up of one type of atom. Many substances are not elements but compounds.
  • A compound is made when two or more different elements react and chemically bond together. Just mixing them is not enough.
  • A chemical bond is a strong force holding atoms together in a compound, and it takes lots of energy to break.
  • In particle diagrams, different colours are different elements and overlapping atoms are chemically bonded.
  • A compound usually has completely different properties from its elements, so you test it and compare.

The big picture

An element is made of one type of atom. A compound is made when two or more different elements react and chemically bond, and it is a new substance, so its properties have to be tested and compared rather than guessed from its elements.

Key points

1One colour = one type of atom = element, even if two of its atoms are joined.
2Two or more different colours, overlapping = compound. Different colours with gaps = not a compound.
3Water, carbon dioxide, methane and ammonia are compounds: each is one type of atom bonded to others of a different type.
4A change of colour, a loss of magnetism or a white solid appearing is evidence that a new substance has formed.

Worked example

Problem

A diagram shows one purple atom overlapping one orange atom, and nothing else. Is it an element, a compound, or not a compound?

⚠ Watch out

Thinking one clue is enough. Joined atoms are not automatically a compound (two joined atoms of the same colour are an element), and two different colours are not automatically a compound (they must be bonded as well).

🧠

Memory hook

Two questions, in this order: 'How many colours?' then 'Are they joined?' One colour: element. Two or more colours, joined: compound. Two or more colours, not joined: not a compound.

✓

Check yourself

Cover the page. A diagram shows three atoms of one colour, all overlapping. Is it an element, a compound or not a compound? Say which question settled it.

Flashcards

(13)
What is an element?
A substance made up of one type of atom.
What is a chemical compound?
A substance made when two or more different elements react and chemically bond together.
Does mixing two elements together make a compound?
No. The elements have to react and chemically bond.
What is a chemical bond?
A strong force that holds atoms together in a compound. It takes lots of energy to overcome and break.
In a particle diagram, what do the colours show?
Different colours are different elements (different types of atom).
In a particle diagram, what does overlapping show?
Overlapping atoms are chemically bonded.
Two atoms of the same colour are joined. Element or compound?
An element: it has only one type of atom, even though two atoms are joined.
Two different atoms are drawn not touching. Is that a compound?
No. Different atoms that are not bonded do not make a compound.
Which atoms are in water and in carbon dioxide?
Water: one oxygen atom bonded to two hydrogen atoms. Carbon dioxide: one carbon atom bonded to two oxygen atoms.
Which atoms are in methane and in ammonia?
Methane: one carbon atom bonded to four hydrogen atoms. Ammonia: one nitrogen atom bonded to three hydrogen atoms.
How do you find out what a compound is like?
Test the compound and compare its properties with the elements it's made from.
What shows that magnesium burning makes a new substance?
White smoke and a white solid appearing.
Why put a magnet underneath the watch glass when testing for magnetism?
So the magnet isn't touching the substance on top.

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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