Chemistry Topics: Complete A-Z Guide

All major chemistry topics — atomic structure, bonding, stoichiometry, electrochemistry, organic chemistry, thermochemistry, and more. Free explanations and worked examples for GCSE, IGCSE, A-Level, IB, WAEC, and JAMB.

Atomic Structure & Electron Configuration

Every substance in the universe is built from atoms. Understanding atomic structure is the gateway to all of chemistry — it explains how elements differ, why isotopes exist, and how electrons drive every chemical reaction.

  • Atoms consist of protons (+), neutrons (0), and electrons (−). Protons and neutrons occupy the nucleus; electrons occupy shells around it.
  • Atomic number = number of protons. Mass number = protons + neutrons. Isotopes share the same atomic number but differ in neutron count.
  • Electrons fill shells in order: shell 1 holds max 2, shell 2 holds max 8, shell 3 holds max 8 (for elements 1–20).
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Chemical Bonding: Ionic, Covalent & Metallic

Chemical bonding determines the physical and chemical properties of every substance — hardness, solubility, conductivity — all come back to how atoms are bonded.

  • Ionic bonds form between metals and non-metals. Metals transfer electrons to non-metals, creating oppositely charged ions.
  • Covalent bonds form between non-metals sharing electron pairs. Simple molecular covalent substances have low melting/boiling points.
  • Metallic bonding: a sea of delocalised electrons explains electrical conductivity, thermal conductivity, malleability, and ductility.
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Stoichiometry & The Mole Concept

Stoichiometry is the mathematical language of chemistry. It lets chemists calculate exactly how much of each reactant is needed and how much product will form.

  • One mole = 6.022 × 10²³ particles (Avogadro's number).
  • Molar mass = the mass of one mole of a substance in grams, numerically equal to the relative molecular mass. E.g., H₂O = 18 g/mol.
  • n = m/M: moles = mass ÷ molar mass. The key equation in stoichiometry.
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Acids, Bases, pH & Neutralisation

Acid-base chemistry underpins biochemistry, environmental science, and industrial processes. From stomach acid (pH 1–2) to bleach (pH 13), the pH scale describes a world of chemical interactions.

  • pH = −log[H⁺]. At pH 7 (pure water), [H⁺] = 10⁻⁷ mol/L. Each pH unit = a 10-fold change in hydrogen ion concentration.
  • Strong acids (HCl, H₂SO₄, HNO₃) fully dissociate. Weak acids (e.g., ethanoic acid) only partially dissociate.
  • Neutralisation: Acid + Base → Salt + Water.
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Electrochemistry & Redox Reactions

Electrochemistry powers modern technology — from lithium-ion batteries to industrial aluminium production. It connects electrical energy and chemical change through electron movement.

  • OIL RIG: Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons). Redox reactions always happen together.
  • In a galvanic cell, chemical energy converts to electrical energy spontaneously.
  • Electrolysis uses electrical energy to force a non-spontaneous chemical reaction.
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Organic Chemistry: Carbon Compounds

Organic chemistry studies carbon-containing compounds — over 10 million of them. Carbon's ability to form four bonds gives rise to the molecules of life: proteins, DNA, fats, and carbohydrates.

  • Alkanes (CₙH₂ₙ₊₂): saturated hydrocarbons. Undergo combustion and halogen substitution.
  • Alkenes (CₙH₂ₙ): contain C=C double bond. Bromine water decolorises — the test for unsaturation.
  • Esterification: carboxylic acid + alcohol ⇌ ester + water.
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Thermochemistry & Energy Changes

All chemical reactions involve energy changes. Understanding whether a reaction releases or absorbs heat is critical in food science, materials engineering, and pharmaceutical development.

  • Exothermic reactions release energy to the surroundings (ΔH negative): combustion, neutralisation, respiration.
  • Endothermic reactions absorb energy from the surroundings (ΔH positive): thermal decomposition, photosynthesis.
  • Bond energies: ΔH ≈ ΣBE(bonds broken) − ΣBE(bonds formed).
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Chemical Equilibrium & Le Chatelier's Principle

Many reactions are reversible. When forward and reverse rates balance, a dynamic equilibrium is established. Controlling equilibrium is vital in industrial chemistry.

  • Dynamic equilibrium: forward and reverse reactions occur at equal rates; concentrations remain constant.
  • Le Chatelier's Principle: if a system at equilibrium is disturbed, it shifts to oppose the change.
  • The Haber process (N₂ + 3H₂ ⇌ 2NH₃) uses 200 atm, 450°C, and an iron catalyst — a compromise between yield, rate, and economics.
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Rates of Reaction & Catalysis

Reaction rate is controlled by how often particles collide with sufficient energy. Temperature, concentration, surface area, and catalysts all affect speed.

  • Collision theory: a reaction occurs only when particles collide with sufficient energy (≥ activation energy) and correct orientation.
  • Temperature: a 10°C rise roughly doubles many reaction rates.
  • Catalysts provide an alternative reaction pathway with lower activation energy and are not consumed.
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The Periodic Table: Trends & Groups

The periodic table reveals patterns in physical properties, reactivity, and chemical behaviour that allow chemists to predict properties of elements they have never tested.

  • Across a period: atomic radius decreases, ionisation energy increases, metallic character decreases.
  • Group 1 (Alkali Metals): react vigorously with water → metal hydroxide + hydrogen. Reactivity increases: Li < Na < K.
  • Group 7 (Halogens): reactivity decreases down the group: F₂ > Cl₂ > Br₂ > I₂.
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Gas Laws & Kinetic Theory

Gas behaviour is remarkably predictable. The gas laws describe relationships between pressure, volume, temperature, and amount.

  • Boyle's Law: P₁V₁ = P₂V₂ (constant T). Pressure and volume are inversely proportional.
  • Charles's Law: V₁/T₁ = V₂/T₂ (constant P, T in Kelvin). Volume and absolute temperature are directly proportional.
  • Ideal Gas Law: PV = nRT (R = 8.314 J/mol·K).
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Environmental Chemistry & Green Chemistry

Chemistry drives climate change challenges and provides the solutions — from greenhouse gases to sustainable industrial processes.

  • Greenhouse gases (CO₂, CH₄, N₂O) absorb outgoing infrared radiation, warming the lower atmosphere.
  • Acid rain forms when SO₂ and NOₓ react with water vapour.
  • Atom economy = (molar mass of desired product ÷ total molar mass of reactants) × 100%.
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