Key Stage 3 science · Chemistry – Atoms, elements and compounds
KS3 atoms, elements and compounds: a guide for parents
Everything around us is made of atoms, and chemistry begins by sorting out how atoms, elements and compounds relate to each other. In Key Stage 3 children learn a simple atomic model, the differences between an atom, an element and a compound, how to read chemical symbols and formulae such as H₂O and NaCl, and why the total mass stays the same in a chemical reaction and in a change of state.
Atoms, elements and compounds, explained
The atomic model taught first is the one proposed by John Dalton in the early 1800s. In it, every substance is made of tiny atoms, pictured as solid spheres that cannot be broken down. All the atoms of one element are the same, and atoms of different elements differ from each other, for example in mass. Atoms are rearranged in chemical reactions but are not created or destroyed. It is a simple model. Many schools introduce a more detailed picture, with a nucleus, protons, neutrons and electrons, towards the end of Key Stage 3 or at GCSE, but the Dalton model is what this part of the curriculum requires.
An element is a substance made of only one type of atom. Iron is made of iron atoms and oxygen is made of oxygen atoms. There are 118 known elements, arranged in the periodic table, and everything else is built from their combinations. A compound is a substance in which atoms of two or more different elements are chemically joined together in a fixed ratio. Water has two hydrogen atoms for each oxygen atom. A compound has different properties from the elements that make it: sodium is a soft, reactive metal, chlorine is a poisonous gas, and sodium chloride (table salt) is neither. A mixture is different again: its parts are not chemically joined, so air is a mixture of gases and can be separated by physical means.
Each element has a chemical symbol of one or two letters, with the first a capital and the second, if there is one, in lower case: H for hydrogen, O for oxygen, C for carbon, Na for sodium, Cl for chlorine and Fe for iron. Some come from older Latin names, which is why iron is Fe. A formula shows which elements are in a substance and in what numbers. In CO₂ there is one carbon atom and two oxygen atoms, in H₂O two hydrogen and one oxygen and in NaCl one sodium and one chlorine. A small number written lower, a subscript, shows how many atoms of the element before it. Children also write word equations such as magnesium + oxygen → magnesium oxide. Using formulae in symbol equations belongs to the separate chemical reactions topic, and many schools leave balancing them until Year 9 or GCSE.
The last idea is conservation of mass. In a change of state, such as ice melting, no particles are lost or gained, so the mass is the same before and after. In a chemical reaction the atoms are rearranged to form new substances but the same atoms are still there, so the total mass of the products equals the total mass of the reactants. Apparent exceptions turn up often in school practicals. If a gas escapes into the air, the mass of what is left appears to go down. If a solid burns in air and combines with oxygen, the mass of the solid appears to go up. In both cases a gas has been left out of the measurement.
Words your child will hear
- Atom
- The smallest particle of an element. In Dalton’s model atoms are tiny solid spheres.
- Element
- A substance made of only one type of atom, such as iron, oxygen or carbon.
- Compound
- A substance made of two or more elements chemically joined, such as water or salt.
- Molecule
- A group of atoms joined together, such as the two atoms in oxygen gas, O₂, or the three in water, H₂O.
- Chemical symbol
- One or two letters standing for an element, such as Na for sodium.
- Formula
- A way of showing the atoms in a substance, such as CO₂ for carbon dioxide.
- Conservation of mass
- The total mass stays the same during a change of state or chemical reaction.
What the National Curriculum says
The programme of study for Key Stage 3 says:
- Pupils should be taught about a simple (Dalton) atomic model
- Pupils should be taught about differences between atoms, elements and compounds
- Pupils should be taught about chemical symbols and formulae for elements and compounds
- Pupils should be taught about conservation of mass in changes of state and chemical reactions
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
Try it: explaining a change in mass
In a school demonstration, 2.4 g of magnesium ribbon is heated strongly in a crucible with the lid slightly open. After the reaction it has turned into a white powder, magnesium oxide, with a mass of 4.0 g. Why did the mass go up, and how much oxygen was involved?
- Write the word equation: magnesium + oxygen → magnesium oxide. The magnesium combines with oxygen from the air.
- Say what was and was not weighed at the start. The oxygen is a gas in the air, so it was not in the crucible when the mass was measured. Only the magnesium was.
- Apply conservation of mass. The mass of the reactants must equal the mass of the product. Mass of magnesium + mass of oxygen = mass of magnesium oxide.
2.4 g + oxygen = 4.0 g
- Calculate the oxygen by subtracting.
oxygen = 4.0 g − 2.4 g = 1.6 g
- Explain the result. The mass went up because oxygen atoms from the air joined the magnesium atoms. No atoms were created, so the total mass is conserved.
1.6 g of oxygen combined with the magnesium. The mass rose because the oxygen from the air became part of the magnesium oxide, so the total mass of reactants equals the total mass of product.
Common mistakes and how to help
Using “atom”, “molecule”, “element” and “compound” as if they all mean the same thing
They are all words for “a tiny bit of stuff”, and in everyday speech they are used loosely.
What helps: An atom is one particle. An element contains only one kind of atom. A compound contains two or more kinds, joined. A molecule is a group of atoms bonded together. Sorting examples such as O₂, H₂O, Fe and CO₂ into these groups is good practice.
Thinking a compound is just a mixture of its elements
Children picture sodium and chlorine sitting side by side, so the compound seems like a blend of the two.
What helps: In a compound the atoms are chemically joined, and the properties change completely. Compare salt in your food with the gas and metal it is made from. Mixtures, like air or salty water, keep the properties of their parts.
Believing mass is lost when something burns or fizzes
A burning candle gets smaller and a fizzy drink left open goes flat, so it looks as if matter is destroyed.
What helps: The missing mass has escaped as gases: carbon dioxide and water vapour from the candle, and carbon dioxide from the drink. If the reaction is contained and weighed, the mass is the same. Ask where the atoms went.
Misreading formulae, such as seeing CO₂ as “carbon to the power of 2” or writing Co for carbon
The small number looks like an index in maths, and capital letters are easy to miss.
What helps: The small number counts the atoms of the element just before it. A capital letter starts each new symbol, so CO is carbon and oxygen, but Co is cobalt, one element. Say formulae aloud: “C, O, two”.
How to help at home
- Weigh a melting ice cube. Put an ice cube in a jar with a lid on kitchen scales, note the mass, and weigh again once it has melted. Ask why the mass has not changed and what has happened to the particles.
- Weigh a fizzing reaction in an open cup. With an adult, stand a large jug holding about 100 ml of vinegar and a small cup holding a heaped teaspoon of bicarbonate of soda on kitchen scales, on a tray in case it froths over. Note the total mass, tip the powder into the jug, put the empty cup back on the scales and weigh again once the fizzing stops. Ask where the missing mass has gone. Never seal the container.
- Find elements at home. Aluminium foil and copper pipes or wire are close to pure elements. Pencil “lead” is mostly graphite, a form of carbon, mixed with clay, and steel nails and most jewellery are mixtures of metals based on iron, gold or silver, which makes a good talking point. Then look for compounds, such as salt, water and sugar, and ask what each is made from.
- Read the formulae. Look at labels on salt, bicarbonate of soda and fizzy drinks for chemical names such as sodium chloride, sodium bicarbonate or carbonated water (water with carbon dioxide dissolved in it). Ask your child to write the formula for each one they know, and to say what it tells you, such as the two hydrogen atoms and one oxygen atom in water.
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