⚗️ A-Level Chemistry
Chemistry Complete Cheatsheet
Atoms to organic chemistry — the complete chemistry revision cheatsheet.
📖 10 sections
⏱ 26 min read
✅ Quizzes included
🌙 Dark mode
01 Atomic Structure ▼
Proton
Positive charge, in nucleus, mass ≈1 u
Neutron
No charge, in nucleus, mass ≈1 u
Electron
Negative charge, shells, mass ≈0
Atomic number
Number of protons
Mass number
Protons + neutrons
Isotopes
Same protons, different neutrons
Ion
Atom with gained/lost electrons
Electron shells
2, 8, 8, 18... (max electrons per shell)
CHEMElectronic configuration
Na (11): 2, 8, 1
Cl (17): 2, 8, 7
Ca (20): 2, 8, 8, 2

Valence electrons = group number (main groups)
Period = number of electron shells
Group 0/18 (noble gases): full outer shells → stable
02 Periodic Table ▼
Period
Horizontal row — same number of electron shells
Group
Vertical column — same number of valence electrons
Group 1
Alkali metals: very reactive, soft, +1 ion
Group 7
Halogens: reactive non-metals, -1 ion
Group 0
Noble gases: full shells, inert, exist as single atoms
Transition metals
d-block: variable oxidation states, coloured compounds, catalysts
Electronegativity
Increases across period, decreases down group
Atomic radius
Decreases across period, increases down group
💡
Reactivity of Group 1 increases DOWN (Cs > Rb > K > Na > Li). Reactivity of Group 7 increases UP (F > Cl > Br > I).
03 Chemical Bonding ▼
Ionic
Metal + non-metal. Transfer electrons. High mp. Conducts when molten/dissolved.
Covalent
Non-metal + non-metal. Share electrons. Usually low mp. Poor conductor.
Metallic
Metal atoms. Delocalized electrons. Good conductor, malleable.
Giant ionic
NaCl: lattice of ions. Very high mp. Strong attractions.
Simple molecular
H₂O, CO₂: weak intermolecular forces. Low mp. Don't conduct.
Giant covalent
Diamond, SiO₂: very high mp. Very hard. Diamond doesn't conduct.
CHEMDot-cross diagrams
H₂O: O shares 2 pairs with H (2 lone pairs remain)
NH₃: N shares 3 pairs with H (1 lone pair)
CO₂: C forms 2 double bonds with O (linear)
CH₄: C shares 4 single bonds with H (tetrahedral, 109.5°)
04 Reactions & Equations ▼
CHEMBalancing equations
Unbalanced: H₂ + O₂ → H₂O
Balanced:   2H₂ + O₂ → 2H₂O

Mole calculations:
moles = mass / Mr
moles = concentration × volume (L)
moles = volume / 24 (gas at RTP, in dm³)

Molar mass examples:
H₂O  = 2(1) + 16 = 18 g/mol
NaCl = 23 + 35.5 = 58.5 g/mol
H₂SO₄ = 2+32+64 = 98 g/mol

Yield: actual/theoretical × 100%
💡
ALWAYS balance equations. Atoms must be equal on both sides. Only change coefficients, NEVER subscripts.
05 Acids & Bases ▼
pH Scale
CHEMAcids and bases
pH = -log[H⁺]
[H⁺] = 10⁻ᵖᴴ

pH 0-6: Acid (lower = stronger)
pH 7:   Neutral
pH 8-14: Alkali (higher = stronger)

Strong acids (fully dissociate): HCl, H₂SO₄, HNO₃
Weak acids (partially): CH₃COOH, HF

Neutralisation:
acid + base → salt + water
HCl + NaOH → NaCl + H₂O
H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O

Brønsted-Lowry: acid = proton donor, base = proton acceptor
Lewis: acid = electron pair acceptor, base = electron pair donor
Indicators
Litmus: red acid, blue alkali. Universal: pH colour chart.
Titration
Find unknown concentration using known concentration
06 Organic Chemistry ▼
Functional Groups
Alkane
C-C single bonds only. CₙH₂ₙ₊₂. Methane, ethane, propane.
Alkene
C=C double bond. CₙH₂ₙ. Ethene, propene. More reactive.
Alcohol
-OH group. Ethanol CH₃CH₂OH. Fermentation & oxidation.
Carboxylic acid
-COOH group. Ethanoic acid. Weak acid.
Ester
-COO-. Formed from acid+alcohol. Used in flavours/fragrances.
Halogenoalkane
-X (X=F,Cl,Br,I). Nucleophilic substitution reactions.
Amine
-NH₂. Basic. Amino acids contain both amine and carboxylic acid.
CHEMKey reactions
Combustion: CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O
Addition: alkene + Br₂ → dibromoalkane (decolourises bromine water)
Substitution: alkane + Cl₂ → chloroalkane + HCl (UV light)
Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Saponification: ester + NaOH → alcohol + carboxylate salt
07 Electrochemistry ▼
Oxidation
Loss of electrons / gain of oxygen (OIL)
Reduction
Gain of electrons / loss of oxygen (RIG)
Electrolysis
Splitting compound using electricity. Cathode=-. Anode=+.
Galvanic cell
Spontaneous redox → generates electricity
Electrolytic cell
Forced redox → requires electricity
Oxidation state
Hypothetical charge on atom. Sum in compound = overall charge
CHEMElectrolysis examples
Copper(II) sulfate solution:
  Cathode: Cu²⁺ + 2e⁻ → Cu (deposit copper)
  Anode:   2H₂O → O₂ + 4H⁺ + 4e⁻ (oxygen gas)

NaCl solution (brine):
  Cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻ (hydrogen gas)
  Anode:   2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas)
08 Rates of Reaction ▼
Rate Factors
Temperature
↑T → more kinetic energy → more successful collisions → ↑rate
Concentration
↑[reactant] → more particles in same space → more collisions → ↑rate
Surface area
↑SA → more exposed particles → more collisions → ↑rate
Catalyst
Provides alternative lower activation energy pathway → ↑rate, same products
Pressure
↑P (gases only) → particles closer → more collisions → ↑rate
Activation energy
Minimum energy needed for reaction to occur
CHEMRate expressions
Rate = k[A]ᵐ[B]ⁿ

Order = power in rate equation (m or n)
0th order: rate independent of [A]
1st order: rate ∝ [A]   → t½ is constant
2nd order: rate ∝ [A]²  → t½ increases

Arrhenius: k = Ae^(-Ea/RT)
09 Energetics ▼
ΔH = ΔHₚ - ΔHᵣ
Hess's Law
Enthalpy of products - reactants
Exothermic
ΔH < 0
Releases heat. Products more stable.
Endothermic
ΔH > 0
Absorbs heat. Reactants more stable.
Bond energy
ΔH = broken - formed
Using mean bond enthalpies
ΔG = ΔH - TΔS
Gibbs energy
ΔG<0 → spontaneous
Lattice enthalpy
Energy to separate 1 mol ionic lattice
Born-Haber cycle
10 Mini Quizzes ▼
❓ Quiz 1
What does pH measure?
pH = -log[H⁺]. pH 7 = neutral. Below 7 = acidic. Above 7 = alkaline. A pH of 1 means [H⁺] = 10⁻¹ = 0.1 mol/dm³.
❓ Quiz 2
Which represents oxidation?
OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). Also: oxidation = gain of O or loss of H.