C1: Atoms and Elements
Atoms, elements, compounds and mixtures
📋 Key Definitions
Atom: The smallest particle of an element that can still be recognised as that element. Atoms are the building blocks of all matter.
Element: A substance made up of only one type of atom. Each element has a unique atomic number and is listed on the Periodic Table.
Compound: A substance made from two or more elements that are chemically combined in fixed proportions. Compounds can only be separated into their elements by chemical reactions.
Mixture: Two or more substances that are mixed together but are not chemically combined. Mixtures can be separated by physical methods such as filtration, distillation or chromatography.
Chemical symbol: A one- or two-letter abbreviation used to represent an element (e.g. O for oxygen, Na for sodium).
📝 Atoms, Elements, Compounds and Mixtures
Atoms
Everything is made of atoms. An atom is the smallest part of an element that has the properties of that element. Atoms are extremely small - about 1 × 10⁻¹⁰ m in diameter. In chemical reactions, atoms are rearranged but never created or destroyed.
Elements
An element contains only one type of atom. There are about 118 known elements. Examples include oxygen (O), carbon (C), gold (Au) and iron (Fe). Elements cannot be broken down into simpler substances by chemical reactions.
Examples of Elements
- Hydrogen (H) - the lightest element, a colourless gas
- Carbon (C) - found as diamond, graphite and graphene
- Iron (Fe) - a magnetic metal used in construction
- Oxygen (O) - a gas essential for respiration
- Gold (Au) - an unreactive metal used in jewellery
Compounds
Compounds are formed when elements react and their atoms bond together. The elements in a compound are in fixed proportions - for example, water (H₂O) always has two hydrogen atoms for every one oxygen atom. Compounds have very different properties from the elements that make them.
Examples of Compounds
- Water (H₂O) - hydrogen and oxygen chemically combined
- Sodium chloride (NaCl) - sodium and chlorine chemically combined
- Carbon dioxide (CO₂) - carbon and oxygen chemically combined
- Iron sulfide (FeS) - iron and sulfur chemically combined
- Calcium carbonate (CaCO₃) - calcium, carbon and oxygen chemically combined
Mixtures
Mixtures are not chemically combined, so the substances keep their individual properties. The proportions in a mixture can vary. Mixtures can be separated using physical methods.
Examples of Mixtures
- Air - a mixture of nitrogen, oxygen, argon, carbon dioxide and other gases
- Sea water - a mixture of water and dissolved salts
- Crude oil - a mixture of different hydrocarbons
- Granite - a mixture of different minerals
- Ink - a mixture of dyes dissolved in water
| Element | Compound | Mixture | |
|---|---|---|---|
| Made of | One type of atom | Two or more elements chemically combined | Two or more substances not chemically combined |
| Proportions | N/A | Fixed proportions | Variable proportions |
| Separation | Cannot be separated | Chemical reactions only | Physical methods |
| Properties | Unique to that element | Different from constituent elements | Same as individual substances |
| Example | Iron (Fe) | Iron sulfide (FeS) | Iron and sulfur powder mixed |
🔬 Chemical Symbols for the First 20 Elements
Every element has a unique chemical symbol. Some symbols come from the element's English name (O for oxygen) and some from Latin or other languages (Na for sodium, from the Latin natrium).
| Number | Element | Symbol | Number | Element | Symbol |
|---|---|---|---|---|---|
| 1 | Hydrogen | H | 11 | Sodium | Na |
| 2 | Helium | He | 12 | Magnesium | Mg |
| 3 | Lithium | Li | 13 | Aluminium | Al |
| 4 | Beryllium | Be | 14 | Silicon | Si |
| 5 | Boron | B | 15 | Phosphorus | P |
| 6 | Carbon | C | 16 | Sulfur | S |
| 7 | Nitrogen | N | 17 | Chlorine | Cl |
| 8 | Oxygen | O | 18 | Argon | Ar |
| 9 | Fluorine | F | 19 | Potassium | K |
| 10 | Neon | Ne | 20 | Calcium | Ca |
⚛️ Word Equations and Symbol Equations
Word Equations
A word equation shows the names of the reactants and products in a chemical reaction. Reactants are on the left, products are on the right, separated by an arrow.
magnesium + oxygen → magnesium oxide
Symbol Equations
A symbol equation uses chemical symbols and formulae instead of words. Symbol equations show exactly which atoms are involved and in what numbers.
Mg + O₂ → MgO (unbalanced)
Worked Example - Writing a Symbol Equation
When carbon burns in oxygen, carbon dioxide is produced.
Step 1: Write the word equation: carbon + oxygen → carbon dioxide
Step 2: Replace names with symbols: C + O₂ → CO₂
Step 3: Check balancing: Left side = 1 C, 2 O; Right side = 1 C, 2 O ✓
This equation is already balanced.
⚖️ Balancing Equations
Law of conservation of mass: No atoms are created or destroyed in a chemical reaction, so the number of atoms of each element must be the same on both sides of the equation.
To balance an equation, add numbers in front of formulae (coefficients). Never change the formulae themselves.
Worked Example 1 - Balancing Mg + O₂ → MgO
Step 1: Count atoms on each side:
Left: Mg = 1, O = 2 | Right: Mg = 1, O = 1
Step 2: Oxygen is unbalanced. Put a 2 before MgO:
Mg + O₂ → 2MgO
Step 3: Recount: Left: Mg = 1, O = 2 | Right: Mg = 2, O = 2
Step 4: Now magnesium is unbalanced. Put a 2 before Mg:
2Mg + O₂ → 2MgO
Step 5: Check: Left: Mg = 2, O = 2 | Right: Mg = 2, O = 2 ✓ Balanced!
Worked Example 2 - Balancing Fe + O₂ → Fe₂O₃
Step 1: Count atoms: Left: Fe = 1, O = 2 | Right: Fe = 2, O = 3
Step 2: Balance Fe first by putting 2 before Fe:
2Fe + O₂ → Fe₂O₃
Step 3: Now balance O. LCM of 2 and 3 is 6:
2Fe + 3O₂ → Fe₂O₃
Wait - that gives O = 6 on the left. Need 2Fe₂O₃ on the right:
4Fe + 3O₂ → 2Fe₂O₃
Step 4: Check: Left: Fe = 4, O = 6 | Right: Fe = 4, O = 6 ✓ Balanced!
Worked Example 3 - Balancing CH₄ + O₂ → CO₂ + H₂O
Step 1: Count atoms: Left: C = 1, H = 4, O = 2 | Right: C = 1, H = 2, O = 3
Step 2: Balance H: put 2 before H₂O:
CH₄ + O₂ → CO₂ + 2H₂O
Step 3: Count O: Right = 2 + 2 = 4. Put 2 before O₂:
CH₄ + 2O₂ → CO₂ + 2H₂O
Step 4: Check: Left: C = 1, H = 4, O = 4 | Right: C = 1, H = 4, O = 4 ✓ Balanced!
🔤 State Symbols
State symbols are added to symbol equations to show the physical state of each substance. They are written in brackets after each formula.
| State Symbol | Meaning | Example |
|---|---|---|
| (s) | Solid | C(s), Fe(s) |
| (l) | Liquid | H₂O(l), Br₂(l) |
| (g) | Gas | O₂(g), CO₂(g) |
| (aq) | Aqueous (dissolved in water) | NaCl(aq), HCl(aq) |
Worked Example - Symbol Equation with State Symbols
When magnesium reacts with oxygen:
2Mg(s) + O₂(g) → 2MgO(s)
When hydrochloric acid reacts with sodium hydroxide:
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
❓ Practice Questions
Q1: Foundation Define the term compound and give one example.
Q2: Foundation Explain the difference between a mixture and a compound.
Q3: Foundation Balance the following equation: H₂ + O₂ → H₂O
Q4: Higher Write a balanced symbol equation with state symbols for the reaction of calcium with oxygen to form calcium oxide, CaO.
Q5: Higher Balance the following equation: Al + O₂ → Al₂O₃
Q6: Foundation Give the chemical symbols for: sodium, potassium, iron, chlorine, calcium.
✅ Answers
- A compound is a substance made from two or more elements that are chemically combined in fixed proportions. Example: water (H₂O) is a compound of hydrogen and oxygen.
- A compound has elements chemically combined in fixed proportions and can only be separated by chemical reactions. A mixture has substances not chemically combined, with variable proportions, and can be separated by physical methods (filtration, distillation, etc.).
- 2H₂ + O₂ → 2H₂O. Count: Left = H: 4, O: 2; Right = H: 4, O: 2. Balanced.
- 2Ca(s) + O₂(g) → 2CaO(s). Calcium is a solid metal, oxygen is a gas, and calcium oxide is a solid.
- 4Al + 3O₂ → 2Al₂O₃. Check: Left = Al: 4, O: 6; Right = Al: 4, O: 6. Balanced.
- Sodium = Na, Potassium = K, Iron = Fe, Chlorine = Cl, Calcium = Ca. (Na and K come from their Latin names: natrium and kalium.)
🎯 Exam Tips
- Learn the definition of compound precisely: examiners look for "chemically combined" and "fixed proportions" - both must be mentioned for full marks
- When asked about the difference between mixtures and compounds, always state at least two differences for full marks
- Never change subscripts in a formula when balancing - only add numbers in front of formulae
- Always check your balanced equation by counting atoms of each element on both sides
- State symbols: remember (aq) means dissolved in water, not just "watery" - it is distinct from (l)
- Common mistake: writing Na for sodium is correct, but writing NA or nA is wrong - symbols have a capital letter first, lowercase second
🔬 Required Practical
Required Practical Activity
There is no specific required practical for C1 in Combined Science. However, you should be able to distinguish between elements, compounds and mixtures from diagrams and descriptions, and practise balancing symbol equations.
🔢 Maths Skills
Mathematical Skills
Balancing equations: Count atoms of each element on both sides. Add coefficients (never change subscripts) until the number of each type of atom is equal on both sides.
Calculating Mr: Add up the relative atomic masses of every atom in the formula. E.g. Mr(H₂SO₄) = (2 × 1) + 32 + (4 × 16) = 98.
⚠️ Common Misconceptions
Watch Out!
Atoms and molecules are the same thing. Wrong: atoms and molecules are the same Correct: molecules contain two or more atoms chemically bonded together; atoms are the smallest unit of an element
Compounds can be separated by physical methods. Wrong: compounds can be separated physically Correct: compounds can only be separated by chemical reactions that break the chemical bonds
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the differences between elements, compounds and mixtures.
An element is a substance made of only one type of atom and cannot be broken down chemically (e.g. Fe, O₂). A compound is a substance made from two or more elements chemically combined in fixed proportions; it can only be separated into elements by chemical reactions (e.g. H₂O always has 2 H : 1 O). A mixture contains two or more substances not chemically combined, so the substances retain their individual properties and proportions can vary; mixtures can be separated by physical methods such as filtration, distillation or chromatography.
Mark scheme: 1 mark for each correct definition (element, compound, mixture — up to 3); 1 mark for stating compounds have fixed proportions; 1 mark for stating mixtures have variable proportions; 1 mark for correct separation methods for each; 1 mark for using examples.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student is shown three particle diagrams. Diagram A shows one type of atom only. Diagram B shows two types of atoms joined together in fixed groups. Diagram C shows two types of atoms mixed together but not joined.
Question: Classify each substance as an element, compound or mixture. Explain your reasoning for each.
Answer: A = element (one type of atom only); B = compound (two types of atom chemically bonded in fixed groups); C = mixture (two types of atom not chemically combined, randomly arranged).
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