AQA GCSE Combined Science (8464), Higher tier · Chemistry › Atomic structure and the periodic table › A simple model of the atom, symbols, relative atomic mass,
Practise Atoms, elements and compounds. 11 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The basic language of chemistry: atoms, elements and compounds, chemical symbols and formulae, and how to write word equations and balanced symbol equations. It is tested all through both papers, often as tick-box, 'name the compound' and 'balance the equation' questions. Higher tier students must also write half equations and ionic equations.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Tell elements from compounds using formulaeAn element's formula has one symbol (Fe, O2); a compound's formula has two or more different symbols (CO2, NaCl).
3
Recall symbols of the first 20 elementsAlso the Group 1 and Group 7 elements and others in the course, with the correct capital and lower-case letters.
4
Name compounds from their formulaeA metal with a non-metal ends in -ide (NaCl, sodium chloride); -ate means oxygen is also present (CaCO3, calcium carbonate).
4
Count the atoms in a formulaIn Ca(OH)2 the 2 multiplies everything in the brackets: 1 Ca, 2 O and 2 H, so 5 atoms.
5
Write word equations for reactionsReactants on the left, products on the right, joined by an arrow, e.g. magnesium + oxygen → magnesium oxide.
6
Balance symbol equationsChange only the large numbers in front of formulae until every element has the same number of atoms on each side.
7
Write ionic equations (Higher tier)Show only the particles that change and leave out spectator ions, e.g. H+ + OH− → H2O.
8
Write half equations (Higher tier)Show the electrons lost or gained by one substance, with atoms and charges balanced, e.g. Cl2 + 2e− → 2Cl−.
Notes
Atoms and elements
All substances are made of atoms. An atom is the smallest part of an element that can exist.
An element contains only one sort of atom. There are about 100 different elements, all shown in the periodic table.
Each element has a chemical symbol: one capital letter, sometimes followed by one lower-case letter, e.g. C (carbon), Cl (chlorine), Cu (copper).
Some elements exist as molecules of two atoms, e.g. H2, N2, O2 and Cl2. They are still elements, because all the atoms are the same.
Compounds
A compound contains two or more elements chemically combined in fixed proportions. Water is always H2O: two hydrogen atoms for every oxygen atom.
Compounds are made from elements by chemical reactions. A reaction always makes at least one new substance and often has an energy change you can detect, e.g. the mixture gets hot.
A compound can only be separated into its elements by another chemical reaction, not by physical processes such as filtering or distilling.
Naming: a metal with one non-metal ends in -ide (MgO, magnesium oxide). An ending of -ate means oxygen is also present (CuSO4, copper sulfate; KNO3, potassium nitrate).
Formulae and equations
A formula shows which elements are in a substance and the ratio of their atoms. In 2H2O the large 2 means two molecules; the small 2 means two H atoms in each molecule.
Everything inside brackets is multiplied by the number after them: Ca(OH)2 contains 1 Ca, 2 O and 2 H.
A word equation names the reactants and products: magnesium + oxygen → magnesium oxide.
A balanced symbol equation has the same number of atoms of each element on both sides, because atoms are not made or destroyed in a reaction: 2Mg + O2 → 2MgO.
To balance, change only the large numbers in front of formulae. Never change a small number in a formula: that would make it a different substance.
Half equations and ionic equations (Higher tier only) grade 7+
An ionic equation shows only the particles that change. Ions that are the same on both sides (spectator ions) are left out, e.g. for any acid and alkali: H+ + OH− → H2O.
A half equation shows electrons being lost or gained by one substance: Na → Na+ + e−, or Cl2 + 2e− → 2Cl−. grade 8+
In ionic and half equations the total charge must be the same on both sides, as well as the numbers of atoms.
Cheatsheet
Atom: the smallest part of an element that can exist
Element: contains only one sort of atom
Compound: two or more elements chemically combined in fixed proportions
A compound can only be separated into its elements by chemical reactions
-ide: usually just the two elements named, e.g. NaCl sodium chloride (hydroxide, OH−, is an exception)
-ate: oxygen is also present, e.g. CaCO3 calcium carbonate
Balanced equation: same number of each type of atom on both sides
Elements made of two-atom molecules: H2, N2, O2, F2, Cl2, Br2, I2
Half equation (HT): Mg → Mg2+ + 2e−grade 8+
How to answer each type of question
Identify elements and compounds, and explain why
1 to 2 marks3
Element: only one type of atom (single atoms, or molecules such as O2).
Compound: two or more different types of atom bonded together.
To explain 'why it is a compound', give both ideas: different elements, and chemically combined.
Example. The formulae of four substances are: N2, NaF, Fe, SO2 (a) Give the formulae of the two elements. (b) SO2 is a compound. Explain why.
Show the model answer
(a) N2 and Fe (1) (both needed) (b) It contains two different elements, sulfur and oxygen (1), which are chemically combined / bonded together (1).
Name a compound or write a word equation
1 mark4
Name the metal first, unchanged.
Change the non-metal ending to -ide, or use the -ate name for a group containing oxygen (sulfate, carbonate, nitrate).
In a word equation, use an arrow (→), never an equals sign.
Example. (a) Potassium reacts with bromine to form potassium bromide. Write a word equation for the reaction. (b) Name the compound with the formula MgSO4.
Balance one element at a time, starting with one that appears in only one substance on each side.
Leave elements that are on their own (e.g. O2) until last.
Recount every element at the end.
Example. Balance the equation for the reaction of aluminium with oxygen. ....Al + ....O2 → ....Al2O3
Show the model answer
4Al + 3O2 → 2Al2O3 (1) Check: Al 4 on each side; O 6 on each side.
Write an ionic equation or half equation (Higher tier only)
1 to 3 marks8
Split soluble ionic compounds into their ions.
Cross out the ions that appear unchanged on both sides (spectator ions).
For a half equation, add electrons (e−) to the side that makes the charges balance.
Check both atoms and charges balance.
Example. Chlorine reacts with potassium iodide solution: Cl2 + 2KI → 2KCl + I2 (a) Write the ionic equation for this reaction. (b) Complete the half equation for the iodide ions: 2I− → I2 + ......
Show the model answer
(a) Cl2 + 2I− → 2Cl− + I2: correct particles (1), balanced (1). The K+ ions are spectator ions, so they are left out. (b) 2e− (1)
Shortcuts and memory tricks
Symbols: one capital letter, then at most one lower-case letter. Co is cobalt; CO is carbon monoxide.
Two-atom elements: 'HON plus the halogens': H2, O2, N2 and all of Group 7 (F2, Cl2, Br2, I2).
When balancing, write a tally of each element under both sides and update it every time you change a number.
Charge check for half equations: add up the charges on each side; they must match.
Where marks are lost
Changing the small numbers in a formula (e.g. writing H2O2 for water) to balance an equation. This changes the substance.
Naming NaCl 'sodium chlorine'. The compound is sodium chloride.
Saying a compound contains elements that are 'mixed'. They are chemically combined (bonded).
Wrong capital letters: CO (carbon monoxide) and Co (cobalt) are different substances.
Using = instead of → in an equation.
Exam technique
To explain why a substance is a compound, use both key phrases: 'two or more different elements' and 'chemically combined'.
Copy formulae from the question exactly as they are printed; don't change them to make balancing easier.
Balancing questions are usually 1 mark for the whole equation, so check every element before you move on.
Higher tier: in an ionic or half equation, check that the total charge is the same on both sides.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Chalk is mainly calcium carbonate. The formula of calcium carbonate is CaCO3. Give the chemical symbol for calcium.
Ca
Lithium is the only Group 1 metal that reacts with nitrogen at room temperature. The product is lithium nitride, Li3N. Write a word equation for the reaction.
lithium + nitrogen → lithium nitride
A student added zinc powder to silver nitrate solution. Silver and zinc nitrate solution were produced. Zn + ....AgNO3 → Zn(NO3)2 + ....Ag Balance the equation.
Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
Chlorine gas can be made in a fume cupboard by reacting potassium manganate(VII), KMnO4, with concentrated hydrochloric acid. ....KMnO4 + ....HCl → ....KCl + ....MnCl2 + ....Cl2 + ....H2O Name the compound with the formula MnCl2.
manganese chloride / manganese(II) chloride
When a mixture of aluminium powder and iron(III) oxide is ignited, it glows white hot and molten iron is formed. 2Al + Fe2O3 → Al2O3 + 2Fe Name the compound Al2O3.
aluminium oxide
The iron filings in the mixture were attracted to the magnet. Explain why.
The iron and sulfur are not chemically combined, so the iron keeps its magnetic property.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Chalk is mainly calcium carbonate. The formula of calcium carbonate is CaCO3.
(a) Which of these substances is an element? Tick (✓) one box.[1]
Carbon dioxide
Magnesium
Sodium chloride
Water
(b) Give the chemical symbol for calcium.[1]
(c) Name the three elements in calcium carbonate.[1]
(d) How many atoms are shown in the formula CaCO3?[1]
(e) Which statement describes an atom? Tick (✓) one box.[1]
A particle that always has an overall positive charge
The smallest part of an element that can exist
Two or more elements chemically combined
A substance that contains more than one type of particle
Show the answer and mark scheme
(a)Answer: Magnesium
(b)Answer: Ca
Ca
(c)Answer: calcium, carbon and oxygen
calcium, carbon and oxygen
(d)Answer: 5
5
(e)Answer: The smallest part of an element that can exist
Question 2Medium5 marks
Lithium is the only Group 1 metal that reacts with nitrogen at room temperature. The product is lithium nitride, Li3N.
(a) Write a word equation for the reaction.[1]
(b) Balance the symbol equation for the reaction. ....Li + N2 → ....Li3N[1]
(c) Compounds contain elements in fixed proportions. Give the ratio of lithium atoms to nitrogen atoms in lithium nitride.[1]
(d) Explain why lithium nitride can only be separated into lithium and nitrogen by a chemical reaction.[2]
Show the answer and mark scheme
(a)Answer: lithium + nitrogen → lithium nitride
lithium + nitrogen → lithium nitride
(b)Answer: 6Li + N2 → 2Li3N
6Li + N2 → 2Li3N
(c)Answer: 3 : 1
3 : 1
(d)Answer: The lithium and nitrogen are chemically combined (bonded); physical processes such as filtration or distillation cannot break these bonds.
the elements / lithium and nitrogen are chemically combined / bonded
physical processes (such as filtration or distillation) cannot separate them / bonds must be broken (by a chemical reaction)
Question 3Hard6 marks
A student added zinc powder to silver nitrate solution. Silver and zinc nitrate solution were produced. Zn + ....AgNO3 → Zn(NO3)2 + ....Ag
(a) Balance the equation.[1]
(b) Zinc nitrate is an ionic compound. Give the formulae of the two ions in zinc nitrate.[2]
(c) The nitrate ions do not change during the reaction. Write the ionic equation for the reaction.[2]
(d) Complete the half equation for the change to the silver ions. Ag+ + .......... → Ag[1]