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原子结构

AQA · GCSE · 物理 · 知识点 4

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4.1

Atomic structure: the unstable nucleus

Radioactivity is over a century old, yet it still treats cancer, powers grids and demands strict safety rules. This reference covers AQA GCSE Physics 8463, topic 4.4 Atomic structure.

How the exam treats this topic:

  • Paper 1 (4.1–4.4) carries this topic. No equation sheet entries beyond energy; the work is notation, balancing and reasoning.
  • Background radiation, half-life hazards, uses and fission/fusion are physics only.
  • Net-decline ratios after several half-lives are Higher Tier.
  • You must write balanced nuclear equations for single alpha and beta decay (balance atomic numbers and mass numbers; daughter naming not required).
4.1

原子结构、质量数与同位素
教学大纲

原子结构；质量数与同位素（AQA 8463 陈述 4.4.1.1-4.4.1.2）。

  1. 描述原子结构：由带正电的原子核（含质子和中子）构成，外层电子分布在不同能级上。
  2. 记住原子半径的数量级，以及原子核直径小于其 1/10 000，却集中了绝大部分质量。
  3. 利用原子序数和质量数确定质子、中子和电子的数量。
  4. 定义同位素为具有不同中子数的同一元素原子，并解释正离子是失去外层电子的原子。

来源：Cambridge International 教学大纲

An atom is very small: radius about $1\times10^{-10}$ m. Its structure:

An atom: a small positive nucleus of protons and neutrons, with electrons in energy levels.
  • a positively charged nucleus containing protons and neutrons, surrounded by negatively charged electrons;
  • the nucleus's radius is less than 1/10 000 of the atom's radius, yet it holds most of the mass;
  • electrons sit at different distances (energy levels); absorbing electromagnetic radiation moves an electron further from the nucleus (higher level), emitting radiation moves it closer.

Notation: $\ ^{A}_{Z}X$ where $Z$ = atomic number (protons) and $A$ = mass number (protons + neutrons). In a neutral atom, electrons = protons; atoms have no overall charge.

  • Isotopes 同位素: atoms of the same element (same $Z$) with different numbers of neutrons (different $A$).
  • Neutrons in the nucleus = $A - Z$.
  • Atoms that lose one or more outer electrons become positive ions.

Worked example. Carbon-14: $\ ^{14}_{6}\text{C}$.

  • Protons = 6; electrons = 6 (neutral); neutrons = $14 - 6 = 8$.
  • Carbon-12 has 6 neutrons — same element, different neutrons: isotopes.
词汇 训练
English 中文 拼音
isotopes/ˈaɪsətəʊps/ 同位素 tóng wèi sù
4.2

原子模型的发展
教学大纲

原子模型的演变（AQA 8463 陈述 4.4.1.3）。

  1. 描述发展序列：不可分割球体、葡萄干布丁模型、核式模型、玻尔轨道、质子、中子。
  2. 解释α粒子散射实验证据如何导致核式模型的提出。
  3. 描述葡萄干布丁模型与核式模型之间的区别。

来源：Cambridge International 教学大纲

New experimental evidence can change or replace a scientific model:

Alpha scattering: most particles pass through; a few rebound from a tiny dense nucleus.
  1. Before the electron's discovery: atoms were tiny spheres that could not be divided.
  2. Electron discovered → the plum pudding model: a ball of positive charge with negative electrons embedded in it.
  3. Alpha scattering (Rutherford): most alpha particles passed straight through, a few bounced back → the mass and positive charge must be concentrated in a tiny centre → the nuclear model replaced the plum pudding model.
  4. Bohr adapted it: electrons orbit at specific distances; his calculations agreed with observations.
  5. Further work showed the positive charge comes in whole-number units — the proton; Chadwick's experiments (about 20 years later) proved the neutron.

Why the scattering changed the model (each clause a mark): if the pudding were right, alpha particles should all pass through with small deflections (B1); some bounced almost straight back (B1), which is only possible if the mass and positive charge sit in a tiny, dense, positive nucleus (B1).

4.3

放射性衰变与核辐射
教学大纲

放射性衰变与核辐射（AQA 8463 陈述 4.4.2.1）。

  1. 描述放射性衰变为一种随机过程，即不稳定的原子核释放辐射。
  2. 定义活度（贝克勒尔）和计数率。
  3. 说明α、β、γ和中子辐射的性质，包括穿透能力、在空气中的射程及电离能力。
  4. 运用这些性质为特定用途选择最佳放射源。

来源：Cambridge International 教学大纲

Some nuclei are unstable. They give out radiation as they change to become more stable — a random process called radioactive decay 放射性衰变.

  • Activity 放射性活度: the rate at which a source decays; unit becquerel 贝克勒尔 (Bq).
  • Count-rate 计数率: decays per second recorded by a detector (e.g. a Geiger–Muller tube).
Radiation What it is Ionising power Range in air Stopped by
alpha (α) 2 protons + 2 neutrons (helium nucleus) strongly ionising a few cm paper, skin
beta (β) high-speed electron from the nucleus (a neutron → proton) moderately tens of cm a few mm of aluminium
gamma (γ) electromagnetic radiation from the nucleus weakly many metres several cm of lead / thick concrete
neutron (n) a neutron from the nucleus (varies) far thick concrete / water

Choose a source for a use by matching these properties: alpha for smoke alarms (stopped by smoke, safe when sealed); beta for thickness control of thin sheets (passes through and is partially absorbed); gamma for tracing and sterilising (penetrates the body).

Penetration: alpha stopped by paper, beta by aluminium, gamma reduced by thick lead.
词汇 训练
English 中文 拼音
radioactive decay/ˌreɪdɪəʊˈæktɪv dɪˈkeɪ/ 放射性衰变 fàng shè xìng shuāi biàn
Activity/ækˈtɪvɪti/ 放射性活度 fàng shè xìng huó dù
becquerel/ˈbekwərəl/ 贝克勒尔 bèi kè lēi ěr
Count-rate/kaʊnt reɪt/ 计数率 jì shù lǜ
4.4

核方程、半衰期与随机衰变
教学大纲

Nuclear equations, half-lives and the random nature of decay (AQA 8463 statements 4.4.2.2-4.4.2.3).

  1. Write balanced nuclear equations for single alpha and beta decay, balancing atomic and mass numbers.
  2. Define half-life as the time for the number of nuclei or the count rate to halve.
  3. Determine half-life from given information or a graph.
  4. (HT only) Calculate the net decline, expressed as a ratio, after a given number of half-lives.

来源：Cambridge International 教学大纲

Balance mass numbers (top) and atomic numbers (bottom) on both sides:

  • Alpha decay: the nucleus loses 4 from the top and 2 from the bottom.
    $$^{238}_{\ 92}\text{U} \rightarrow\ ^{234}_{\ 90}\text{Th} +\ ^{4}_{2}\text{He}$$
  • Beta decay: a neutron turns into a proton; mass number unchanged, atomic number +1; the beta particle is $\ ^{0}_{-1}\text{e}$.
    $$^{14}_{\ 6}\text{C} \rightarrow\ ^{14}_{\ 7}\text{N} +\ ^{0}_{-1}\text{e}$$
  • Gamma emission: changes neither number.

Worked example. Polonium-210 decays by alpha emission. Write the equation.

  • Alpha removes 4 and 2: $A: 210 - 4 = 206$; $Z: 84 - 2 = 82$.
    $$^{210}_{\ 84}\text{Po} \rightarrow\ ^{206}_{\ 82}\text{X} +\ ^{4}_{2}\text{He}$$
  • Check both rows balance ✓ (the daughter's name is not required).
4.4

核方程、半衰期与随机衰变
教学大纲

Nuclear equations, half-lives and the random nature of decay (AQA 8463 statements 4.4.2.2-4.4.2.3).

  1. Write balanced nuclear equations for single alpha and beta decay, balancing atomic and mass numbers.
  2. Define half-life as the time for the number of nuclei or the count rate to halve.
  3. Determine half-life from given information or a graph.
  4. (HT only) Calculate the net decline, expressed as a ratio, after a given number of half-lives.

来源：Cambridge International 教学大纲

Decay is random: it cannot be predicted for any one nucleus; only the average behaviour of many is predictable.

A decay curve: count rate halves every half-life.

Half-life 半衰期: the time for (a) the number of nuclei of the isotope in a sample to halve, or (b) the count rate / activity to fall to half its initial level.

  • From a graph: read the time for the count rate to halve — repeat over several halvings and average.
  • After $n$ half-lives, the fraction remaining is $1/2^n$ (HT: express as a ratio).

Worked example. A sample's count rate falls from 800 Bq to 200 Bq in 12 years.

  • Halvings: $800 \to 400 \to 200$ is two halvings.
    $$t_{1/2} = \frac{12\ \text{years}}{2} = 6\ \text{years}$$
词汇 训练
English 中文 拼音
Half-life/hɑːf laɪf/ 半衰期 bàn shuāi qī
4.5

放射性污染与本底辐射
教学大纲

放射性污染（AQA 8463 陈述 4.4.2.4）。

  1. 定义放射性污染和辐照，并指出受辐照物体不会变为放射性物质。
  2. 比较污染与辐照的危害。
  3. 描述针对放射源危害的适当预防措施。
  4. 解释发表和同行评审辐射效应研究的重要性。

来源：Cambridge International 教学大纲

  • Contamination 污染: unwanted radioactive atoms on an object. The hazard lasts as long as the atoms are there, decaying on or in the body.
  • Irradiation 辐照: exposing an object to radiation. The irradiated object does not become radioactive.

Comparing the hazards (a credited pair): contamination gives a longer-lasting dose because the atoms stay and decay inside or on you, so the type of radiation matters (alpha is most dangerous inside the body); irradiation stops the moment the source is removed or shielded.

Precautions: hold sources with tongs, keep them at a distance, limit time near them, point them away from people, store in lead-lined boxes. Findings on radiation effects are published and peer-reviewed so they can be checked.

词汇 训练
English 中文 拼音
Contamination/kənˌtæmɪˈneɪʃn/ 污染 wū rǎn
Irradiation/ˌɪreɪdɪˈeɪʃn/ 辐照 fú zhào
4.5

放射性污染与本底辐射
教学大纲

放射性污染（AQA 8463 陈述 4.4.2.4）。

  1. 定义放射性污染和辐照，并指出受辐照物体不会变为放射性物质。
  2. 比较污染与辐照的危害。
  3. 描述针对放射源危害的适当预防措施。
  4. 解释发表和同行评审辐射效应研究的重要性。

来源：Cambridge International 教学大纲

Background radiation 本底辐射 is around us all the time:

  • natural: rocks (radon gas), cosmic rays from space, food, medical (natural body potassium);
  • man-made: fallout from weapons testing, nuclear accidents, medical uses.

Dose depends on occupation and location (high altitude, certain industries). Dose unit: sieverts (1000 mSv = 1 Sv; recall not required).

Measurements of a sample must subtract the background count-rate first.

词汇 训练
English 中文 拼音
background radiation/ˈbækɡraʊnd ˌreɪdɪˈeɪʃn/ 本底辐射 běn dǐ fú shè
4.6

核辐射的应用与半衰期选择
教学大纲

背景辐射、半衰期危害及应用（AQA 8463 陈述 4.4.3.1-4.4.3.3，仅限物理学科）。

  1. 描述背景辐射的自然来源和人为来源。
  2. 说明剂量取决于职业和地理位置，并从测量值中扣除背景辐射。
  3. 解释危害如何随半衰期不同而有所差异。
  4. 描述并评估核辐射在医学中的应用，包括检查内部器官和破坏病变组织。

来源：Cambridge International 教学大纲

Half-life and hazard: a very long half-life stays active for thousands of years (waste storage problem); a very short half-life is intensely active while it lasts. Choose sources to match: medical tracers need short half-lives (the dose ends quickly); smoke alarms use a long half-life source so the alarm works for years.

Medical uses (each = exploration or destruction):

  • Exploration: a gamma-emitting tracer (e.g. technetium-99m) injected so organs show on a scan; gamma escapes the body; short half-life limits dose.
  • Destruction: focused gamma beams or implanted sources kill cancer cells (radiotherapy); beta for skin conditions.

Evaluating risk: compare the dose and consequence of the procedure against the risk of the illness — with numbers from the question.

4.7

核裂变与核聚变（仅物理）
教学大纲

Nuclear fission and fusion (AQA 8463 statements 4.4.4.1-4.4.4.2, physics only).

  1. Describe nuclear fission: a neutron absorbed by a large unstable nucleus, the products, and the released energy.
  2. Explain chain reactions and the difference between controlled (reactor) and uncontrolled (weapon) versions.
  3. Draw and interpret diagrams representing fission and chain reactions.
  4. Describe nuclear fusion as the joining of two light nuclei with mass converting to radiation energy.

来源：Cambridge International 教学大纲

Fission 核裂变: the splitting of a large, unstable nucleus (uranium-235, plutonium-239).

Fission: a neutron splits a U-235 nucleus; released neutrons can form a chain reaction.
  • Spontaneous fission is rare: the nucleus usually absorbs a neutron first.
  • It splits into two smaller nuclei of roughly equal size, releasing two or three neutrons and gamma rays; energy is released and all products carry kinetic energy.
  • The released neutrons can cause further fissions — a chain reaction. A reactor controls it (control rods absorb neutrons); a weapon's explosion is an uncontrolled chain.
  • You must draw or interpret the diagram: neutron in → two fragments + neutrons out → branching chain.

Fusion 核聚变: two light nuclei join to form a heavier nucleus; some mass converts into the energy of radiation. Fusion releases more energy per kilogram than fission with no long-lived waste, but needs extreme temperature and pressure — why reactors are hard to build.

词汇 训练
English 中文 拼音
fission/ˈfɪʃn/ 核裂变 hé liè biàn
fusion/ˈfjuːʒn/ 核聚变 hé jù biàn
4.7

Checklist before you call this topic done
  • Compute protons, neutrons, electrons from $A$ and $Z$; identify isotopes.
  • Tell the atomic-model story in order and say what evidence changed it.
  • Match α/β/γ to penetration, range and ionising power; choose sources for uses.
  • Balance nuclear equations for single alpha and beta decay.
  • Find a half-life from a graph or from two count rates; (HT) compute net decline.
  • Distinguish contamination from irradiation and compare their hazards.
  • (physics only) Background sources; half-life choice for hazard and use; medical exploration/destruction; fission chain vs fusion.

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