Atoms & Radioactivity | 原子与放射性

📚 Atoms & Radioactivity | 原子与放射性

This chapter forms the foundation of modern physics. It explains the internal structure of atoms, the nature of ionising radiation, and how unstable nuclei decay. For the Edexcel IGCSE syllabus, you must understand the historical models of the atom, the properties of α, β and γ radiation, the concept of half-life, and the practical applications and hazards of radioactivity.

本章是现代物理学的基础，阐述了原子的内部结构、电离辐射的性质以及不稳定原子核的衰变方式。在 Edexcel IGCSE 考纲中，你必须理解原子模型的发展历史、α、β 和 γ 三种辐射的特性、半衰期的概念，以及放射性的实际应用与危害。

1. The Structure of the Atom | 原子的结构

Atoms are the smallest neutral units of matter. Each atom consists of a tiny, dense, positively charged nucleus surrounded by negatively charged electrons moving in energy levels (shells). The nucleus itself contains protons and neutrons, which are collectively called nucleons.

原子是物质中保持电中性的最小单位。每个原子都由一个极小且致密的带正电原子核，以及绕核在能级（壳层）上运动的带负电电子组成。原子核本身包含质子和中子，统称为核子。

The three subatomic particles have specific relative charges and masses as follows:

三种亚原子粒子具有特定的相对电荷与相对质量，如下表所示：

Particle	Relative Mass	Relative Charge
Proton	1	+1
Neutron	1	0
Electron	1⁄1836 (negligible)	−1

The atomic number Z is the number of protons in the nucleus. It defines which element the atom is. The mass number A is the total number of protons and neutrons in the nucleus.

原子序数 Z 是原子核中的质子数，它决定了该原子属于哪种元素。质量数 A 是原子核中质子与中子的总数。

ⁿₐX

Here, X is the chemical symbol, A (superscript) is the mass number, and Z (subscript) is the atomic number. For example, ²³₉₁Na has 11 protons and 23 − 11 = 12 neutrons.

其中 X 是元素符号，A（上标）为质量数，Z（下标）为原子序数。例如，²³₉₁Na 有 11 个质子，中子数为 23 − 11 = 12。

2. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They have identical chemical properties because the electronic arrangement is the same, but their physical properties such as mass differ.

同位素是指同一种元素中质子数相同而中子数不同的原子。由于电子排布相同，它们的化学性质完全相同，但物理性质（如质量）存在差异。

¹²₆C and ¹⁴₆C are both carbon; both have 6 protons, but they have 6 and 8 neutrons respectively.

¹²₆C 和 ¹⁴₆C 都是碳元素，都有 6 个质子，但分别含有 6 个和 8 个中子。

Some isotopes are unstable and spontaneously emit radiation to become more stable. These are called radioactive isotopes or radioisotopes. This process is called radioactive decay.

某些同位素不稳定，会自发地发射辐射以转变为更稳定的状态，这些称为放射性同位素。这一过程称为放射性衰变。

3. What is Radioactivity? | 什么是放射性？

Radioactivity is the spontaneous and random emission of radiation from the nucleus of an unstable atom. It is a completely random process that cannot be predicted or influenced by physical or chemical changes, such as heating or cooling.

放射性是不稳定原子核自发且随机地发射辐射的现象。这是一个完全随机的过程，无法预测，也不会受到加热、冷却等物理或化学变化的影响。

When an unstable nucleus decays, it may emit alpha (α) particles, beta (β) particles, or gamma (γ) rays. These three types of radiation have very different natures and behaviours.

不稳定原子核衰变时，可能发射 α 粒子、β 粒子或 γ 射线。这三种辐射在本质和性质上差异极大。

Background radiation: We are always surrounded by low-level radiation from natural and man-made sources. Natural sources include radon gas from rocks, cosmic rays from space, and carbon-14 in living things. Man-made sources mainly come from medical X-rays and nuclear waste.

本底辐射：我们周围始终存在来自天然和人工来源的低水平辐射。天然来源包括岩石释放的氡气、太空宇宙射线和生物体内的碳-14；人工来源主要来自医疗 X 射线和核废料。

4. Alpha Particles | α 粒子

An alpha particle is identical to a helium nucleus, consisting of 2 protons and 2 neutrons. It has a relative charge of +2 and a relative mass of 4.

α 粒子与氦原子核完全相同，由 2 个质子和 2 个中子组成。它的相对电荷为 +2，相对质量为 4。

When an atom emits an alpha particle, its mass number decreases by 4 and its atomic number decreases by 2:

当原子发射 α 粒子时，其质量数减少 4，原子序数减少 2：

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

Alpha particles are strongly ionising because they are relatively large, have a double positive charge, and move relatively slowly. However, they have a very short range in air (only a few centimetres) and can be stopped by a sheet of paper or the outer layers of skin.

α 粒子电离能力很强，因为其体积较大、带双正电荷且运动速度较慢。然而，它在空气中的射程很短（仅数厘米），一张纸或皮肤外层即可将其阻挡。

5. Beta Particles | β 粒子

A beta particle is a fast-moving electron emitted from the nucleus when a neutron transforms into a proton. Its relative charge is −1 and its relative mass is virtually negligible (1⁄1836).

β 粒子是从原子核中高速发射出的电子，当中子转化为质子时产生。其相对电荷为 −1，相对质量几乎可忽略（1⁄1836）。

After beta emission, the mass number remains unchanged, but the atomic number increases by 1:

发射 β 粒子后，质量数不变，但原子序数增加 1：

¹⁴₆C → ¹⁴₇N + ⁰₋₁e

Beta particles have moderate ionising power. They are more penetrating than alpha but less penetrating than gamma. They can travel up to a few metres in air and are stopped by a few millimetres of aluminium.

β 粒子电离能力中等，穿透力比 α 强但比 γ 弱。它在空气中可传播数米，几毫米厚的铝板即可挡住。

6. Gamma Rays | γ 射线

Gamma rays are electromagnetic waves of very high frequency and extremely short wavelength, emitted from the nucleus. They carry no charge and have no rest mass. They are often emitted alongside α or β decay to release excess energy.

γ 射线是从原子核发出的频率极高、波长极短的电磁波。它不带电荷，也没有静止质量，通常在 α 或 β 衰变时伴随发射，用于释放多余能量。

Gamma radiation has very weak ionising power because it has no charge and interacts only occasionally with matter. However, it is extremely penetrating: it can pass through the human body and requires several centimetres of lead or thick concrete to be stopped effectively.

γ 辐射的电离能力非常弱，因为其不带电荷，与物质相互作用的机会少。然而，它的穿透力极强，可以穿透人体，通常需要数厘米厚的铅板或厚厚的混凝土才能有效阻挡。

Property	α	β	γ
Nature	Helium nucleus	Fast electron	EM wave
Charge	+2	−1	0
Ionising power	Strong	Moderate	Weak
Penetration	Paper / skin	Few mm of Al	cm of Pb / concrete

7. Half-Life | 半衰期

Half-life, T½, is the time taken for half the unstable nuclei in a sample to decay. It can also be described as the time for the activity (count rate) of a radioactive source to halve. The half-life of a particular isotope is constant and cannot be changed by any external physical or chemical means.

半衰期（T½）是指样品中一半不稳定原子核发生衰变所需的时间，也可以定义为放射源的活度（计数率）降为一半所需的时间。特定同位素的半衰期是恒定的，不受任何外部物理或化学手段影响。

Activity is measured in becquerels (Bq), where 1 Bq = 1 decay per second. In practice, we use a Geiger-Müller (GM) tube and a counter to measure count rate in counts per second.

活度以贝克勒尔（Bq）为单位，1 Bq 等于每秒发生 1 次衰变。实际测量中，我们使用盖革-米勒（GM）管和计数器来测量每秒计数率。

Remaining fraction = (½)ⁿ, where n = number of half-lives.

剩余比例 = (½)ⁿ，其中 n 为经过的半衰期个数。

Worked example: A source has an initial activity of 800 Bq and a half-life of 3 hours. What is the activity after 9 hours? After 9 hours, n = 9 ÷ 3 = 3 half-lives. Activity = 800 × (½)³ = 800 ÷ 2 ÷ 2 ÷ 2 = 100 Bq.

例题：某放射源初始活度为 800 Bq，半衰期为 3 小时，求 9 小时后的活度。9 小时内经过 n = 9 ÷ 3 = 3 个半衰期。活度 = 800 × (½)³ = 800 ÷ 2 ÷ 2 ÷ 2 = 100 Bq。

Half-life graphs show a characteristic exponential decay curve. To find the half-life from a graph, locate the time taken for the count rate to drop from any value to half of that value.

半衰期图像呈现典型的指数衰减曲线。要从图像中求半衰期，只需找出计数率从某一值下降到它的一半所需的时间即可。

8. Uses of Radioactivity | 放射性的应用

Medical uses: Gamma rays are used to sterilise medical equipment because they kill bacteria. Cobalt-60 is a common source. Radioactive tracers such as iodine-131 are used to diagnose thyroid gland disorders; the emission is detected from outside the body to form an image. Radiotherapy uses gamma rays to destroy cancer cells by focusing high doses onto the tumour.

医学应用：γ 射线用于对医疗器械进行灭菌，因为其能杀死细菌，钴-60 是常用放射源。碘-131 等放射性示踪剂用于诊断甲状腺疾病，通过体外探测器接收其发射的辐射来成像。放射治疗则使用高剂量 γ 射线聚焦照射肿瘤，以杀死癌细胞。

Industrial uses: In thicknes control, beta or gamma sources are placed on one side of a production line (e.g. paper or metal sheets). A detector on the other side measures how much radiation passes through; if the material becomes thicker, less radiation reaches the detector, and rollers are adjusted automatically.

工业应用：在厚度控制中，将 β 或 γ 放射源放置在生产线（如纸张或金属薄板）一侧，另一侧放置探测器测量穿过的辐射量。材料变厚时，到达探测器的辐射减少，滚轮会自动调整。

Radioactive dating uses carbon-14 to estimate the age of organic materials up to about 50,000 years old. All living things absorb carbon-14; after death, the amount decreases with a half-life of 5,730 years, allowing scientists to calculate age.

放射性测年利用碳-14 估算有机材料的最长约 5 万年的年龄。所有生物都会吸收碳-14；死亡后，碳-14 含量以 5730 年的半衰期减少，科学家据此计算出年代。

9. Dangers of Radiation | 辐射的危害

Ionising radiation can damage living cells by knocking electrons out of atoms and molecules, breaking chemical bonds. This can alter DNA, potentially causing mutations, cancer, or cell death. The degree of harm depends on the type of radiation, the dose absorbed, and which organs are exposed.

电离辐射会通过将原子和分子中的电子击出、破坏化学键来损伤活细胞，可能改变 DNA，从而引发突变、癌症甚至细胞死亡。危害程度取决于辐射类型、吸收剂量以及受照射的器官。

Alpha particles are particularly dangerous if ingested or inhaled, because although they cannot penetrate the skin, their strong ionising power causes severe damage to internal tissues. Beta particles can cause burns to the skin, while gamma rays can penetrate the whole body and cause widespread internal damage.

α 粒子若被吞入或吸入则尤其危险，因为尽管它无法穿透皮肤，但其强大的电离能力会对内部组织造成严重损伤。β 粒子可灼伤皮肤，而 γ 射线可穿透全身，造成大范围的内部损伤。

Safety precautions: Sources should be handled with tongs, never directly. Radiation exposure time should be minimised, and distance from the source should be maximised. Lead shielding or concrete walls are used to absorb radiation, and radioactive waste must be stored safely and labelled clearly.

安全措施：应使用镊子操作放射源，切勿直接用手接触。应尽量缩短照射时间、加大与源的距离。使用铅屏蔽或混凝土墙吸收辐射，放射性废物必须安全存放并明确标识。

10. Nuclear Fission and Fusion | 核裂变与核聚变

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei after absorbing a neutron, with the release of energy and two or three additional neutrons. These neutrons go on to split more nuclei in a chain reaction. This process is used in nuclear power stations and in nuclear reactors.

核裂变是指一个大的不稳定原子核在吸收一个中子后分裂成两个较小的原子核，同时释放能量和两到三个中子的过程。这些中子继续引发更多原子核裂变，形成链式反应。这一过程用于核电站和核反应堆。

²³⁵₉₂U + ¹₀n → ⁹²₃₆Kr + ¹⁴¹₅₆Ba + 3¹₀n + energy

Nuclear fusion is the joining of two small light nuclei to form a larger nucleus, releasing enormous amounts of energy. This is the process that powers the Sun and other stars. Fusion has not yet been controlled commercially on Earth because it requires extremely high temperatures and pressures.

核聚变是两个小而轻的原子核结合成一个较大原子核的过程，会释放巨大能量。这正是太阳和其他恒星的能量来源。由于聚变需要极高的温度和压力，目前地球上尚未实现商用可控核聚变。

²₁H + ³₁H → ⁴₂He + ¹₀n + energy

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Atoms & Radioactivity | 原子与放射性