C1: Atoms and Elements
Atoms, elements, compounds, mixtures, chemical equations
๐ 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 and are approximately 1 ร 10โปยนโฐ m in diameter.
Element: A substance made up of only one type of atom. Each element has a unique atomic number and cannot be broken down into simpler substances by chemical reactions. There are about 118 known elements.
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, not physical methods.
Mixture: Two or more substances that are mixed together but are not chemically combined. The substances in a mixture can be separated by physical methods and the proportions can vary.
Chemical symbol: A one- or two-letter abbreviation used to represent an element. The first letter is always capital and the second letter is always lowercase (e.g. Na for sodium, Cl for chlorine).
Law of conservation of mass: No atoms are created or destroyed in a chemical reaction. The total mass of reactants equals the total mass of products. Atoms are simply rearranged.
๐ Atoms, Elements, Compounds and Mixtures
Atoms
Everything is made of atoms. An atom is the smallest part of an element that retains the properties of that element. In chemical reactions, atoms are rearranged but never created or destroyed. Atoms of each element are represented by their chemical symbol on the Periodic Table.
Elements
An element contains only one type of atom. Elements cannot be broken down into simpler substances. Some elements exist as single atoms (e.g. helium, Ne) while others form molecules (e.g. Oโ, Nโ).
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
- Gold (Au) - an unreactive metal used in jewellery
- Oxygen (O) - a gas essential for respiration
Compounds
Compounds form when elements react and their atoms bond chemically. The elements in a compound are in fixed proportions โ water (HโO) always has two hydrogen atoms for every one oxygen atom. Compounds have very different properties from the elements that make them. For example, sodium is a reactive metal and chlorine is a toxic gas, but sodium chloride (NaCl) is harmless table salt.
Examples of Compounds
- Water (HโO) - hydrogen and oxygen chemically combined
- Sodium chloride (NaCl) - sodium and chlorine chemically combined
- Carbon dioxide (COโ) - carbon and oxygen chemically combined
- Iron sulfide (FeS) - iron and sulfur chemically combined
- Calcium carbonate (CaCOโ) - 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 such as filtration, distillation or chromatography.
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 |
๐ฌ 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 |
โ๏ธ 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 โ magnesium oxide
Symbol Equations
A symbol equation uses chemical symbols and formulae instead of words. Symbol equations must be balanced to show the correct ratio of atoms involved.
Mg + Oโ โ 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 โ carbon dioxide
Step 2: Replace names with symbols: C + Oโ โ COโ
Step 3: Check balancing: Left side = 1 C, 2 O; Right side = 1 C, 2 O โ
This equation is already balanced.
โ๏ธ Balancing Equations
Balancing rule: Add numbers (coefficients) in front of formulae to make the number of atoms of each element the same on both sides. Never change the subscripts within a formula.
Worked Example 1 - Balancing Mg + Oโ โ 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 + Oโ โ 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 + Oโ โ 2MgO
Step 5: Check: Left: Mg = 2, O = 2 | Right: Mg = 2, O = 2 โ Balanced!
Worked Example 2 - Balancing Fe + Oโ โ FeโOโ
Step 1: Count atoms: Left: Fe = 1, O = 2 | Right: Fe = 2, O = 3
Step 2: Balance oxygen by finding LCM of 2 and 3 = 6. Put 3 before Oโ and 2 before FeโOโ:
Fe + 3Oโ โ 2FeโOโ
Step 3: Now count: Left O = 6, Right O = 6. Right Fe = 4, so put 4 before Fe:
4Fe + 3Oโ โ 2FeโOโ
Step 4: Check: Left: Fe = 4, O = 6 | Right: Fe = 4, O = 6 โ Balanced!
Worked Example 3 - Balancing CHโ + Oโ โ COโ + HโO
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 HโO:
CHโ + Oโ โ COโ + 2HโO
Step 3: Count O: Right = 2 + 2 = 4. Put 2 before Oโ:
CHโ + 2Oโ โ COโ + 2HโO
Step 4: Check: Left: C = 1, H = 4, O = 4 | Right: C = 1, H = 4, O = 4 โ Balanced!
๐ค 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 (pure substance, molten) | HโO(l), Brโ(l) |
| (g) | Gas | Oโ(g), COโ(g) |
| (aq) | Aqueous (dissolved in water) | NaCl(aq), HCl(aq) |
Worked Example - Symbol Equation with State Symbols
When magnesium reacts with oxygen:
2Mg(s) + Oโ(g) โ 2MgO(s)
When hydrochloric acid reacts with sodium hydroxide:
HCl(aq) + NaOH(aq) โ NaCl(aq) + HโO(l)
โ Practice Questions
Q1: Foundation Define the term compound and give one example.
A compound is a substance made from two or more elements that are chemically combined in fixed proportions. Example: water (HโO) is a compound of hydrogen and oxygen.Q2: Foundation Explain two differences between a mixture and a compound.
1) A compound has elements chemically combined in fixed proportions, whereas a mixture has substances not chemically combined with variable proportions. 2) Compounds can only be separated by chemical reactions, but mixtures can be separated by physical methods such as filtration or distillation.Q3: Foundation Balance the following equation: Hโ + Oโ โ HโO
2Hโ + Oโ โ 2HโO. Check: Left = H: 4, O: 2; Right = H: 4, O: 2. Balanced.Q4: Higher Write a balanced symbol equation with state symbols for the reaction of calcium with oxygen to form calcium oxide, CaO.
2Ca(s) + Oโ(g) โ 2CaO(s). Calcium is a solid metal, oxygen is a gas, and calcium oxide is a solid ionic compound.Q5: Higher Balance the following equation: Al + Oโ โ AlโOโ
4Al + 3Oโ โ 2AlโOโ. Check: Left = Al: 4, O: 6; Right = Al: 4, O: 6. Balanced.Q6: Foundation Give the chemical symbols for: sodium, potassium, iron, chlorine, calcium.
Sodium = Na, Potassium = K, Iron = Fe, Chlorine = Cl, Calcium = Ca. Note: Na comes from Latin natrium and K from Latin kalium.๐ฏ Exam Tips
Learn the definition of compound precisely: examiners look for "chemically combined" and "fixed proportions" โ 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 โ only add numbers in front of formulae.
State symbols: remember (aq) means dissolved in water, not just "watery" โ it is distinct from (l) which means a pure liquid.
Common mistake: writing Na for sodium is correct, but NA or nA is wrong โ symbols have a capital letter first, lowercase second.
๐งฎ Maths Skills
Balancing equations by counting atoms: Count atoms on each side. Add coefficients (never change subscripts) until equal. E.g. 2Hโ + Oโ โ 2HโO: left H=4, O=2; right H=4, O=2.
Counting atoms in formulae: In Alโ(SOโ)โ there are 2 Al, 3 S and 12 O. Multiply the subscript outside brackets by each subscript inside.
Calculating relative formula mass (Mแตฃ): Add up the relative atomic masses of all the atoms in the formula. E.g. Mแตฃ of HโO = (2 ร 1) + 16 = 18. E.g. Mแตฃ of CaCOโ = 40 + 12 + (3 ร 16) = 100. Always show your working.
Significant figures: Give final answers to the same number of significant figures as the data used. If data is given to 2 s.f., quote your answer to 2 s.f.
Standard form: Atom diameter โ 1 ร 10โปยนโฐ m. Avogadro's constant = 6.02 ร 10ยฒยณ. Use standard form for very large or very small numbers.
โ Common Misconceptions
Wrong: Atoms are the smallest possible particles โ nothing is smaller
Correct: Atoms contain subatomic particles (protons, neutrons and electrons). Electrons are much smaller than atoms, and the nucleus is only about 1/10 000 of the atom's diameter.
Wrong: Mixtures are the same as compounds
Correct: Compounds have elements chemically bonded in fixed proportions and can only be separated by chemical reactions. Mixtures are not chemically bonded, have variable proportions and can be separated by physical methods.
Wrong: Atoms and molecules are the same thing
Correct: An atom is the smallest unit of an element; a molecule is two or more atoms chemically bonded together (e.g. Oโ, HโO)
Wrong: Compounds can be separated by physical methods like filtration
Correct: Compounds can only be separated into their elements by chemical reactions, not by physical methods. Only mixtures can be separated physically.
โ๏ธ 6-Mark Extended Question
Question
Explain the differences between elements, compounds and mixtures. Give an example of each.
An element contains only one type of atom and cannot be broken down into simpler substances by chemical reactions. Example: iron (Fe) is a metal element made of only iron atoms [2 marks]. A compound is made from two or more elements chemically combined in fixed proportions. Compounds have different properties from their constituent elements and can only be separated by chemical reactions, not physical methods. Example: water (HโO) always contains two hydrogen atoms for every one oxygen atom chemically bonded together [2 marks]. A mixture contains two or more substances not chemically combined, so the proportions can vary and each substance retains its individual properties. Mixtures can be separated by physical methods such as filtration or distillation. Example: air is a mixture of nitrogen, oxygen, argon and other gases that are not chemically bonded [2 marks].
Mark scheme: 2 marks for element (definition + example), 2 marks for compound (definition + example), 2 marks for mixture (definition + example). Must include chemical bonding/fixed proportions for compound and variable proportions/physical separation for mixture.๐ AO3: Analyse and Evaluate
Classify an Unknown Substance
Scenario: A student is given an unknown substance X. When X is heated, it breaks down into two different substances that cannot be broken down further. The substances are always produced in the same mass ratio, no matter which sample of X is used. Classify substance X and the two products. Justify your answer.
Analysis: Substance X is a compound because: (1) it can be broken down into simpler substances, ruling out an element; (2) the products are always in the same mass ratio, showing fixed proportions โ a key feature of compounds, not mixtures; (3) it required heating (a chemical reaction) to break it down, not a physical method. The two products are elements because they cannot be broken down further. This is thermal decomposition of a compound into its constituent elements.
Alternative scenario: You are shown three particle diagrams. Diagram A shows identical circles. Diagram B shows two types of circle joined in fixed groups. Diagram C shows two types of circle mixed but not joined. Classify each: A = element (one type of atom only), B = compound (two types of atom chemically combined in fixed groups), C = mixture (two types of atom not chemically combined). The key evidence is whether different particles are bonded (compound) or simply mixed (mixture).
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