AQA · GCSE · Chemistry

• Atoms are the building blocks: protons and neutrons in a tiny nucleus, electrons in shells; the outer shell runs the chemistry.
• The periodic table orders elements by atomic number; groups share outer-shell electrons and so share properties.
• Group 0 is unreactive, Group 1 gets more reactive down, Group 7 less; transition metals (chemistry only) behave differently from Group 1.

All substances are made of atoms 原子 — the smallest part of an element that can exist. Each element has a chemical symbol (O, Na, …); about 100 elements exist, shown in the periodic table.

Compounds 化合物 form when elements combine chemically in fixed proportions; they are represented by formulae and can only be separated back into elements by chemical reactions. Chemical reactions always form one or more new substances, often with a detectable energy change. Equations: word equations and symbol equations with balanced formulae; (HT) balanced half-equations and ionic equations.

Mixtures 混合物: two or more elements or compounds not chemically combined; each substance keeps its properties. Separated by physical processes only — no new substances:

• filtration 过滤 — insoluble solid from liquid;
• crystallisation 结晶 — dissolved solid from solution;
• simple distillation 蒸馏 简单蒸馏 — liquid from dissolved solid (and solvents);
• fractional distillation 分馏 — liquids with different boiling points;
• chromatography 色谱法 — soluble substances in a mixture.

Development of the atom model: atoms were first thought to be indivisible spheres → the electron's discovery gave the plum pudding model 布丁模型 (ball of positive charge with negative electrons embedded) → the alpha-scattering experiment showed mass and positive charge concentrated in a central nucleus (nuclear model) → Bohr: electrons orbit at specific distances → protons identified in the nucleus → Chadwick: the neutron 中子. New evidence changes or replaces models.

In an atom protons = electrons, so no overall charge. Atomic number = protons; mass number = protons + neutrons. Atoms of the same element with different neutron numbers are isotopes 同位素. Atom radius ≈ 0.1 nm (1 × 10⁻¹⁰ m); the nucleus is less than 1/10 000 of that. Almost all mass is in the nucleus.

Relative atomic mass (Ar) averages over the isotopes' abundance — calculate it from percentage abundances. Electronic structure: electrons fill the lowest levels first — sodium is 2,8,1; represent the first twenty elements both as numbers and diagrams.

The table orders elements by atomic (proton) number 原子序数, so that elements with similar properties fall in groups (columns). For main-group elements, group position reflects outer-shell electrons 最外层电子 and similar chemical properties; helium has two outer electrons, while the other Group 0 elements have eight. Explain an element's position from its electronic structure; predict probable reactivity from position.

Development: early tables ordered by atomic weight — incomplete, with some elements misplaced. Mendeleev left gaps for undiscovered elements and sometimes changed the order; his predicted elements were found, supporting his table. Isotopes later explained why atomic-weight order was sometimes wrong.

Metals and non-metals: metals usually lose electrons to form positive ions; many non-metals gain electrons to form negative ions or share electrons in covalent bonds. Metals sit left and lower; non-metals right and upper. Know the characteristic physical and chemical differences, and link atomic structure to position.

Group 0 — noble gases: unreactive — stable outer-shell arrangements (8 outer electrons; helium 2), so they do not easily form molecules. Boiling points increase down the group (with relative atomic mass).

Group 1 — alkali metals: one outer electron. Reactions of Li, Na, K with oxygen, chlorine and water (e.g. 2 Na + 2 H₂O → 2 NaOH + H₂). Reactivity increases down the group — the outer electron is lost more easily further from the nucleus.

Group 7 — halogens: seven outer electrons; non-metals, diatomic molecules (Cl₂, Br₂, I₂). Reactivity decreases down the group; melting and boiling points increase. A more reactive halogen displaces 置换 a less reactive one from a salt solution (chlorine displaces bromine from potassium bromide).

Compared with Group 1, the transition elements (exemplify with Cr, Mn, Fe, Co, Ni, Cu) have higher melting points, densities, strength and hardness, and are less reactive with oxygen, water and halogens.

Typical properties: ions with different charges (Fe²⁺/Fe³⁺), coloured compounds, and use as catalysts 催化剂 (iron in the Haber process).

• p/n/e table, atomic and mass numbers, isotopes, Ar from abundance, 2,8,x structures for the first 20.
• The atom-model timeline: sphere → plum pudding → nuclear → Bohr → proton → neutron.
• Five separation methods each matched to a mixture; Mendeleev's gaps and the isotope explanation.
• Group 0/1/7 trends with electron explanations; halogen displacement equations.
• (Chem) transition metals vs Group 1: four differences plus ions, colours, catalysts.

• Three bonds hold matter together: ionic (transferred electrons, charged ions), covalent (shared electron pairs) and metallic (delocalised electrons).
• Structure follows bonding: giant ionic lattices, small molecules, polymers, giant covalent structure networks and metal lattices — each predicts melting point and conductivity.
• Carbon shows every trick: diamond, graphite, graphene, fullerenes; nanoparticles (chemistry only) turn surface area into properties.

Three strong chemical bonds, all explained by electrostatic forces:

• Ionic 离子键 — a metal atom transfers outer electrons to a non-metal: metal atoms lose electrons to become positive ions; non-metal atoms gain them to become negative ions; Groups 1/2 metals and 6/7 non-metals form ions with noble-gas electronic structures. Draw the electron transfer with dot and cross diagrams; deduce ion charges from group number (Group 1 → +1, Group 2 → +2, Group 6 → −2, Group 7 → −1).
• Covalent 共价键 — non-metal atoms share pairs of electrons. Know dot-and-cross diagrams for H₂, Cl₂, O₂, N₂, HCl, H₂O, NH₃, CH₄, and line representations for small molecules, polymer repeating units and giant covalent structures.
• Metallic 金属键 — a giant lattice of metal atoms with delocalised outer electrons 游离电子 free to move through the whole structure; the sharing of these electrons gives strong metallic bonding.

Know the limitations of models: dot-and-cross, ball-and-stick, 2D and 3D diagrams all simplify — no forces shown, fixed bond lengths, giant structures drawn as small fragments. Deduce empirical formulae from lattice models; molecular formulae from molecule diagrams.

States of matter: solid, liquid, gas; melting/freezing at the melting point, boiling/condensing at the boiling point. Particle theory (small solid spheres) explains the changes; the stronger the forces between particles, the higher the melting/boiling points. (HT) The simple model's limits: no forces shown, all spheres, solid particles. State symbols: (s), (l), (g), (aq).

Larger molecules → stronger intermolecular forces → higher melting/boiling points. Alloys 合金 are harder than pure metals because different-sized atoms distort the layers, stopping them sliding.

• Diamond — a giant covalent structure 巨型共价结构 in which each carbon forms four covalent bonds: very hard, very high melting point, does not conduct.
• Graphite — each carbon forms three bonds, layers of hexagonal rings with no covalent bonds between layers (soft, slippery — lubricant); one delocalised electron per atom → conducts electricity like a metal.
• Graphene — a single layer of graphite: one atom thick, strong, conducts — electronics and composites.
• Fullerenes — hollow molecules of carbon hexagons (plus 5- or 7-membered rings); Buckminsterfullerene C₆₀ 富勒烯 is spherical. Carbon nanotubes 碳纳米管 — cylindrical fullerenes with huge length-to-diameter ratios: nanotechnology, electronics, materials.

Nanoscience = structures 1–100 nm (a few hundred atoms). Nanoparticles 纳米颗粒 are smaller than fine particles (PM2.5, 100–2500 nm), which are smaller than coarse particles/dust (PM10, 1 × 10⁻⁵–2.5 × 10⁻⁶ m).

Surface area : volume: as a cube's side decreases 10×, SA:V increases 10×. Nanoparticles may have different properties from the bulk material because of this high ratio — catalysts (smaller quantities work), medicine delivery, cosmetics, electronics. Concerns: effects inside the body and in the environment are not fully known.

• The three bonds in terms of electrons and electrostatic forces; ion charges from group numbers.
• Dot-and-cross for the eight named molecules; model limitations named.
• The properties table above rebuilt from memory — which forces are overcome on melting.
• Diamond vs graphite vs graphene vs fullerenes, bonding to property.
• (Chem) SA:V factor-of-10 rule; the three particle-size bands and nanoparticle uses and concerns.

• Metals differ in how easily they lose electrons — the reactivity series 活动性顺序 orders them, drives displacement 置换 and decides how each is extracted.
• Acids donate H⁺; alkalis donate OH⁻; their reaction makes a salt 盐 — and the salt's name reads off the acid and the base.
• Electrolysis forces ions to give up or take electrons at electrodes — extracting the most reactive metals and splitting solutions.

Reactivity series (learn the order): potassium, sodium, lithium, calcium, magnesium, [carbon], zinc, iron, [hydrogen], copper. Reactivity = the metal's tendency to form positive ions.

• With water: K, Na, Li, Ca react ( fizzing, hydroxide + hydrogen); Mg very slow; Zn/Fe/Cu no reaction.
• With dilute acids: Mg, Zn, Fe react → salt + hydrogen; Cu does not.
• A more reactive metal displaces a less reactive one from its compound: Zn + CuSO₄ → ZnSO₄ + Cu.

Extraction: unreactive metals (gold) occur native; metals below carbon are extracted by reduction 还原 with carbon (loss of oxygen); metals above carbon need electrolysis 电解. Identify oxidation 氧化 (gain of oxygen) and reduction (loss of oxygen).

(HT) Redox in electrons: oxidation is loss of electrons, reduction is gain — OIL RIG. Write ionic equations for displacement: Zn + Cu²⁺ → Zn²⁺ + Cu.

Acids neutralised by alkalis (soluble hydroxides) and bases (insoluble oxides/hydroxides) → salt + water; by metal carbonates → salt + water + carbon dioxide. The acid decides the salt's negative ion: hydrochloric → chlorides, nitric → nitrates, sulfuric → sulfates; the base supplies the positive ion. Predict products and write salt formulae from ion charges.

Making a soluble salt (RP1): add the insoluble solid (metal oxide/carbonate) to warm dilute acid until no more dissolves (excess solid proves completion), filter off the excess, evaporate to the crystallisation point and leave to crystallise; dry the crystals.

pH scale: 0–14, measured with universal indicator or a pH probe; 7 neutral, <7 acid, >7 alkaline. Acids give H⁺(aq); alkalis give OH⁻(aq). Neutralisation 中和: H⁺ + OH⁻ → H₂O.

(Chem) Titrations 滴定 (RP2): measure reacting volumes of a strong acid and strong alkali accurately — burette, pipette, indicator; (HT) calculate concentrations in mol/dm³ and g/dm³ (topic 3.4 methods).

(HT) Strong and weak acids: strong acids (HCl, HNO₃, H₂SO₄) are completely ionised; weak acids (ethanoic, citric, carbonic) are partially ionised. Same concentration → stronger acid → lower pH. Each pH unit down multiplies [H⁺] by 10. Dilute/concentrated = amount of substance per volume — different axis from strong/weak.

Electrolytes 电解质: molten or dissolved ionic compounds — ions free to move, so they conduct. Positive ions → cathode (negative); negative ions → anode (positive); ions are discharged as elements.

• Molten binary compounds (lead bromide): metal at the cathode, non-metal at the anode.
• Extraction (aluminium): electrolysis of molten Al₂O₃ + cryolite — the mixture lowers the melting point, saving energy; carbon anode burns away (with the oxygen produced) and must be replaced. Electrolysis is used when the metal is too reactive for carbon reduction.
• Aqueous solutions (RP3): at the cathode, hydrogen is produced if the metal is more reactive than hydrogen (else the metal deposits); at the anode, oxygen — unless halide ions are present, when the halogen forms.

(HT) Half-equations 半方程 — balance charge with electrons:

• cathode: Cu²⁺ + 2e⁻ → Cu; 2H⁺ + 2e⁻ → H₂
• anode: 2Cl⁻ → Cl₂ + 2e⁻; 4OH⁻ → O₂ + 2H₂O + 4e⁻

• Recite the reactivity series with carbon and hydrogen in place; predict water/acid/displacement reactions.
• Extraction: carbon reduction vs electrolysis, with reasons; (HT) OIL RIG with ionic equations.
• Salt names from acid + base; RP1 method in order; H⁺ + OH⁻ → H₂O.
• (Chem) titration method; (HT) strong vs weak with the pH ×10 rule.
• Electrolysis products for molten and aqueous cases; (HT) half-equations both electrodes.

• Crude oil — ancient plankton biomass — separates into fractions; alkane 烷烃s (CₙH₂ₙ₊₂) burn as fuels; cracking 裂解 turns big molecules into small alkanes plus reactive alkene 烯烃s (CₙH₂ₙ).
• Alkenes, alcohols and carboxylic acids (chemistry only) are functional-group families with predictable reactions.
• Polymers — addition (alkene monomers) and (HT) condensation — plus nature's own: proteins, starch, cellulose, DNA.

Crude oil is a finite resource in rocks — the remains of ancient biomass, mainly plankton buried in mud. It is a mixture of mostly hydrocarbon 碳氢化合物s (hydrogen + carbon only), chiefly alkanes, general formula CₙH₂ₙ₊₂: methane, ethane, propane, butane (know names + formulae in all representations).

Fractional distillation 分馏: fractions contain molecules with similar carbon numbers; separated by evaporation and condensation at different levels of the column — small molecules at the top (low boiling point), large at the bottom. Fractions give fuels (petrol, diesel, kerosene, heavy fuel oil, LPG) and feedstock for petrochemicals — solvents, lubricants, polymers, detergents.

Properties track molecule size: bigger molecules → higher boiling point, higher viscosity, lower flammability — matching each fraction to its use.

Cracking: breaking large hydrocarbons into smaller, more useful molecules — catalytic cracking (high temperature + catalyst) or steam cracking (high temperature + steam). Products: a smaller alkane (fuel) + an alkene (polymer feedstock). Alkenes are more reactive: they decolourise bromine water (orange → colourless) — the alkene test. Balance cracking equations from given formulae.

Alkenes CₙH₂ₙ contain the C=C double bond — unsaturated (two fewer hydrogens than the alkane with the same carbons). Members: ethene, propene, butene, pentene. Reactions of a functional group 官能团 give a family its reactions:

• combustion — but smoky flames (incomplete);
• with hydrogen → alkane (nickel catalyst); with water → alcohol (steam, phosphoric acid catalyst); with halogens → dihaloalkanes (bromine water test).

Alcohols –OH: methanol, ethanol, propanol, butanol. Reactions: with sodium (fizzing, hydrogen); burning in air (clean combustion — fuels); dissolving in water; oxidised (air/oxidising agent) → carboxylic acid. Uses: fuels, solvents, alcoholic drinks. Fermentation: sugar + yeast → ethanol solution in warm, oxygen-free conditions.

Carboxylic acids –COOH: methanoic, ethanoic, propanoic, butanoic acids. React with carbonates (fizz, CO₂), dissolve in water (weakly acidic — HT: only partially ionised, so higher pH than a strong acid), react with alcohols → esters (ethyl ethanoate) + water.

Addition polymerisation 加成聚合: many monomers 单体 (alkenes, with C=C) join into one polymer — poly(ethene) from ethene, poly(propene) from propene. The repeating unit has exactly the monomer's atoms — nothing else is formed. Draw: monomer → repeating unit with the double bond opened.

(HT) Condensation polymerisation: monomers each carry two functional groups; joining loses a small molecule — usually water. Two different monomers with two of the same groups each: e.g. ethanediol + hexanedioic acid → polyester. Amino acids (H₂N…COOH) condense → polypeptides; different amino acids in one chain → proteins (glycine is the example).

Natural polymers: DNA — two polymer chains of four nucleotides in a double helix; proteins (amino-acid monomers), starch (sugars), cellulose (sugars).

• Alkane names/formulae C1–C4; fraction order with boiling point, viscosity, flammability trends.
• Cracking conditions, products and balanced equations; bromine-water test with the colour change.
• (Chem) Alkene functional group and its three addition reactions; alcohol four reactions + fermentation 发酵 conditions; carboxylic acid three reactions.
• Addition polymer from monomer and back; (HT) condensation principles with polyester; DNA nucleotides + the three other natural polymers' monomers.

• A pure substance 纯净物 in chemistry means one element or compound — melting and boiling points prove it; formulation 配方s are deliberate mixtures.
• Chromatography separates by distribution between phases — Rf values identify the spots.
• Gas tests (H₂, O₂, CO₂, Cl₂) and (chemistry only) ion tests — flame colours, hydroxide precipitate 沉淀s, carbonates, halides, sulfates — build the analyst's toolkit.

Pure substance — a single element or compound, not mixed with anything. Pure substances melt and boil at specific, sharp temperatures; mixtures melt over a range — use melting/boiling point data to tell them apart. (In everyday speech "pure" means unadulterated — pure milk — a different sense.)

Formulations — mixtures designed as useful products, each component in carefully measured amounts for a purpose: fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods. Identify one from given information.

Chromatography 色谱法 — a stationary phase 固定相 (the paper) and a mobile phase 流动相 (the solvent); separation depends on the distribution of each substance between the phases — those more attached to the stationary phase stay lower. Measure:

Different compounds have different Rf values in different solvents — use them to identify; a pure compound gives a single spot in every solvent.

Flame tests (cations): lithium — crimson; sodium — yellow; potassium — lilac; calcium — orange-red; copper — green. In a mixture some colours mask others.

Metal hydroxides (sodium hydroxide solution): Al³⁺, Ca²⁺, Mg²⁺ give white precipitates — but only Al(OH)₃ dissolves in excess NaOH; Cu²⁺ blue, Fe²⁺ green, Fe³⁺ brown precipitates.

Carbonates: add dilute acid — effervescence; the gas turns limewater milky.

Halides: add dilute nitric acid then silver nitrate — AgCl white, AgBr cream, AgI yellow precipitates.

Sulfates: add dilute hydrochloric acid then barium chloride — a white precipitate of BaSO₄.

(HT) Flame emission spectroscopy 光谱法: instrument draws the line emission spectrum; the positions of the lines identify the metal, their intensity its concentration. Advantages over flame test 焰色试验s: very sensitive, works on mixtures, rapid.

• Pure vs mixture by melting point; name five formulations.
• Rf calculated from a chromatogram; why a pure compound gives one spot.
• The four gas tests with results.
• (Chem) flame colours; hydroxide colours with the aluminium-in-excess trick; carbonate/halide/sulfate reagents and precipitate colours; (HT) spectroscopy advantages.