Chapter 1 of 8

All 8 chapters
  1. 1Elements and compounds, readunderstood
  2. 2Pure substances and mixtures, readunderstood
  3. 3Separating mixtures, readunderstood
  4. 4Physical and chemical changes, readunderstood
  5. 5Rusting and other reactions, readunderstood
  6. 6Burning and fire safety, readunderstood
  7. 7Acids and alkalis, readunderstood
  8. 8Neutralisation, readunderstood

S3.6.1 Chapter 1

Elements and compounds

Gold, water and air are all made of tiny particles. Are the particles inside them all the same kind?

Your guess

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

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain that an element has only one kind of atom
  • Name common elements, such as oxygen, carbon, iron and gold
  • Explain that a compound has elements joined together, such as water

A gold ring, a drink of water and the air you breathe are very different. Yet all three are made of tiny particles. In this chapter you look at the particles themselves.

The smallest bits of most materials are called atoms. There are only about a hundred kinds of atom. Everything around you is built from them, a little like words built from letters.

Draw it first

Draw an element and a compound

  1. Two boxes of circles side by side, with a key under them. In the box labelled gold, every circle is a black circle with the letters Au, and the circles touch in neat rows. In the box labelled water, each group is a large circle with the letter O joined to two small circles with the letter H, and the groups touch and are jumbled at the bottom under a flat top. Under the boxes, a key names the circles: gold, oxygen and hydrogen. Each circle has its letter inside.AuAuAuAuAuAuAuAuAuAuAuAugoldAugoldOoxygenHhydrogen

    1 Draw circles that are all the same. Write Au inside each one. This is gold. It has one kind of atom.

  2. Two boxes of circles side by side, with a key under them. In the box labelled gold, every circle is a black circle with the letters Au, and the circles touch in neat rows. In the box labelled water, each group is a large circle with the letter O joined to two small circles with the letter H, and the groups touch and are jumbled at the bottom under a flat top. Under the boxes, a key names the circles: gold, oxygen and hydrogen. Each circle has its letter inside.AuAuAuAuAuAuAuAuAuAuAuAugoldOOOOHHHHHHHHwaterAugoldOoxygenHhydrogen

    2 Draw a big circle with O joined to two small circles with H. Draw more groups like it. This is water. It has two kinds of atom.

Gold is an element. An element has only one kind of atom. Every atom in a gold ring is a gold atom.

Two boxes of circles side by side, with a key under them. In the box labelled iron, every circle has the letters Fe, and the circles touch in neat rows. In the box labelled oxygen, the circles are joined in pairs, the pairs are far apart in a closed box, and every circle has the letter O. The key names the circles: iron and oxygen. Each circle has its letter inside.FeFeFeFeFeFeFeFeFeFeFeFeironOOOOOOoxygenFeironOoxygen
Iron has one kind of atom. Oxygen gas also has one kind of atom, though they are joined in pairs. Both are elements.

Look at oxygen. Its atoms are joined in pairs. But every atom is still an oxygen atom. So oxygen is an element. Joined atoms do not make a compound. Different kinds of atom joined together do.

Each element has a symbol, one or two letters. Oxygen is O and carbon is C. Iron is Fe and gold is Au. Those two come from old Latin names. A symbol is a short way to write the name.

A table with the columns element, symbol and found in. Oxygen has the symbol O and is found in the air. Carbon has the symbol C and is found in charcoal. Iron has the symbol Fe and is found in a nail. Gold has the symbol Au and is found in a ring.elementsymbolfound inoxygenOthe aircarbonCcharcoalironFea nailgoldAua ring
Four elements you meet every day.

Now think about water. Its atoms are not all alike. Each small group has one oxygen atom joined to two hydrogen atoms. Water is a compound. A compound has atoms of two or more elements joined together.

Two boxes of joined groups side by side, with a key under them. In the box labelled water, each group is one large circle with the letter O joined to two small circles with the letter H, and the groups touch under a flat top. In the box labelled carbon dioxide, each group is a circle with the letter C in the middle, joined to a circle with the letter O on each side, in a straight line, and the groups are far apart in a closed box. The key names the circles: hydrogen, oxygen and carbon. Each circle has its letter inside.OOOOHHHHHHHHwaterOOOOCCcarbon dioxideHhydrogenOoxygenCcarbon
Two compounds. Water is a liquid and carbon dioxide is a gas. Each is made of small groups that are all alike. Some compounds, such as salt, are one big block of joined atoms instead.
A table with the columns compound and formula. Water has the formula H₂O. Carbon dioxide has the formula CO₂.compoundformulawaterH₂Ocarbon dioxideCO₂
A formula shows the atoms in one group. The small number says how many.

We write water as H₂O. The H is hydrogen and the small 2 means two atoms. The O is oxygen, one atom. Carbon dioxide is CO₂: one carbon atom and two oxygen atoms.

A compound is not like the elements inside it. Hydrogen is a gas that burns. Oxygen is a gas that helps things burn. Water, made from the two, is a liquid that puts fires out.

See it three ways

With things

Think of table salt. It is made from sodium, a metal that reacts fiercely with water, and chlorine, a poisonous gas. Yet you sprinkle it on your food. A compound can be very different from its elements.

As a picture

A table with the columns substance and what it does. Hydrogen burns. Oxygen helps burning. Water puts out fire.substancewhat it doeshydrogenburnsoxygenhelps burningwaterputs out fire

In words

Water is made of hydrogen and oxygen. Yet it is nothing like either. When atoms join, they make a new substance with its own properties.

Try it

Build water from paper circles

You need: paper, scissors, a pencil.

Be safe: Take care with scissors. Ask a grown up to cut if you are unsure.

  1. Cut out 2 big circles and 4 small circles from paper. Write O on each big one and H on each small one.
  2. Make one water group. Lay 1 big circle on the table. Put 2 small circles touching it.
  3. Make a second group the same way. Count the circles of each kind. Then count all the atoms.

What you will see: Each group has 1 oxygen and 2 hydrogen, which is 3 atoms. Two groups use 2 oxygen and 4 hydrogen, which is 6 atoms. Every group is the same.

If you cannot try it, this is what happens.
A box with two water groups, with a key under it. Each group is one large circle with the letter O joined to two small circles with the letter H. The box is labelled two water groups. The key names the circles: oxygen and hydrogen.OOHHHHtwo water groupsOoxygenHhydrogen

A wrong idea many people have

Water is just hydrogen and oxygen mixed together.

Why it feels right: Water is made of hydrogen and oxygen. So it seems the two gases must sit in there side by side.

The test that breaks it: Hydrogen burns. Oxygen helps things burn. If water held the two gases side by side, it would feed a fire. But water puts fires out.

What is true: In water the atoms are joined in groups of H₂O. That makes a new substance. It does not behave like hydrogen or oxygen.

Key words

atoms
the smallest bits that most materials are made of.
element
a substance with only one kind of atom.
symbol
one or two letters that stand for an element.
compound
a substance with atoms of two or more elements joined together.

Back to your guess

The answer is A: Some materials have one kind of atom. Others have different kinds of atom joined together.

Gold has one kind of atom. Water has two kinds joined together. There are only about a hundred kinds of atom, and they build everything.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. One box holds oxygen gas, with atoms joined in pairs. The other box holds water. Which box shows an element?

    Two boxes of joined groups side by side, with a key under them. In the box labelled oxygen, the circles are joined in pairs, the pairs are far apart, and every circle has the letter O. In the box labelled water, each group is one large circle with the letter O joined to two small circles with the letter H. The key names the circles: oxygen and hydrogen.OOOOOOoxygenOOOOHHHHHHHHwaterOoxygenHhydrogen
    Tap your answer.

    1. Yes, that is right. The pairs in the oxygen box are joined, but every atom is oxygen. So it is an element. Water has two kinds of atom, hydrogen and oxygen.

    2. Not this one. Atoms of two kinds joined together make a compound, not an element. An element has one kind of atom only. Go to S3.6.1. Try another.

    3. Not this one. Water has hydrogen atoms too. An element has only one kind of atom, so water is not an element. Go to S3.6.1. Try another.

    See the answer

    The oxygen box. It has only one kind of atom.

    The pairs in the oxygen box are joined, but every atom is oxygen. So it is an element. Water has two kinds of atom, hydrogen and oxygen.

    • If you chose B: Atoms of two kinds joined together make a compound, not an element. An element has one kind of atom only. Go to S3.6.1.
    • If you chose C: Water has hydrogen atoms too. An element has only one kind of atom, so water is not an element. Go to S3.6.1.

  2. Which list has only elements?

    Tap your answer.

    1. Not this one. Water is a compound. It has hydrogen atoms and oxygen atoms joined together. Go to S3.6.1. Try another.

    2. Yes, that is right. Oxygen, carbon, iron and gold each have one kind of atom. Water and carbon dioxide each have two kinds, so they are compounds.

    3. Not this one. Carbon dioxide is a compound. It has carbon atoms and oxygen atoms joined together. Go to S3.6.1. Try another.

    See the answer

    oxygen, carbon, iron and gold

    Oxygen, carbon, iron and gold each have one kind of atom. Water and carbon dioxide each have two kinds, so they are compounds.

    • If you chose A: Water is a compound. It has hydrogen atoms and oxygen atoms joined together. Go to S3.6.1.
    • If you chose C: Carbon dioxide is a compound. It has carbon atoms and oxygen atoms joined together. Go to S3.6.1.

  3. A formula shows one group of water as H₂O. How many atoms are joined in one group?

    Tap your answer.

    1. Not this one. The 2 counts only the hydrogen. Count the oxygen atom too. Go to S3.6.1. Try another.

    2. Yes, that is right. One oxygen atom and two hydrogen atoms are joined. 1 + 2 = 3.

    3. Not this one. One group of water is a single group, but it holds more than one atom. Count the H and the O. Go to S3.6.1. Try another.

    See the answer

    3 atoms

    One oxygen atom and two hydrogen atoms are joined. 1 + 2 = 3.

    • If you chose A: The 2 counts only the hydrogen. Count the oxygen atom too. Go to S3.6.1.
    • If you chose C: One group of water is a single group, but it holds more than one atom. Count the H and the O. Go to S3.6.1.

  4. Why is water called a compound?

    Tap your answer.

    1. Not this one. In water the atoms are joined. They are not separate gases side by side. Go to S3.6.1. Try another.

    2. Not this one. One kind of atom makes an element. Water has hydrogen atoms and oxygen atoms. Go to S3.6.1. Try another.

    3. Yes, that is right. A compound has atoms of two or more elements joined together. In water, each group is one oxygen atom joined to two hydrogen atoms.

    See the answer

    Its hydrogen and oxygen atoms are joined together in groups.

    A compound has atoms of two or more elements joined together. In water, each group is one oxygen atom joined to two hydrogen atoms.

    • If you chose A: In water the atoms are joined. They are not separate gases side by side. Go to S3.6.1.
    • If you chose B: One kind of atom makes an element. Water has hydrogen atoms and oxygen atoms. Go to S3.6.1.

  5. Hydrogen burns and oxygen helps things burn. Water is made from both, yet it puts fires out. What does this show?

    Tap your answer.

    1. Not this one. If it did, it would feed a fire. The atoms are joined, so a new substance has formed. Go to S3.6.1. Try another.

    2. Not this one. One substance can still be a compound. Water has two kinds of atom, hydrogen and oxygen, so it is a compound. Go to S3.6.1. Try another.

    3. Yes, that is right. When atoms join, they make a new substance with its own properties. Water is nothing like hydrogen or oxygen.

    See the answer

    A compound can behave very differently from the elements in it.

    When atoms join, they make a new substance with its own properties. Water is nothing like hydrogen or oxygen.

    • If you chose A: If it did, it would feed a fire. The atoms are joined, so a new substance has formed. Go to S3.6.1.
    • If you chose B: One substance can still be a compound. Water has two kinds of atom, hydrogen and oxygen, so it is a compound. Go to S3.6.1.

  6. A tray holds 5 water groups. Each group is H₂O. How many hydrogen atoms are on the tray?

    See the answer

    10 hydrogen atoms

    Each group has 2 hydrogen atoms. 5 × 2 = 10.

Keep it

Copy this into your exercise book.

An element has one kind of atom. A compound has different atoms joined, such as water, H₂O. A compound is not like the elements in it.

Draw: Draw one water group: a big circle O joined to two small circles H. Label it H₂O.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (936 KB). It opens on any phone.