IGCSE Physics: Atomic Structure and the Basics of Radioactivity | IGCSE物理：原子结构与放射性基础

📚 IGCSE Physics: Atomic Structure and the Basics of Radioactivity | IGCSE物理：原子结构与放射性基础

Understanding atomic structure is the gateway to modern physics. From the discovery of the nucleus to the mysterious emissions of radioactive substances, this topic explains how matter is built and why some materials are unstable.

理解原子结构是通往现代物理学的门槛。从原子核的发现到放射性物质的神秘辐射，本主题解释了物质是如何构成的，以及为什么有些材料是不稳定的。

1. The Basic Building Blocks of an Atom | 原子的基本组成

Every atom consists of a central nucleus surrounded by orbiting electrons. The nucleus contains protons and neutrons, collectively called nucleons. Protons carry a positive charge, neutrons are neutral, and electrons carry a negative charge.

每个原子都由位于中心的原子核和绕核运动的电子组成。原子核含有质子和中子，统称为核子。质子带正电，中子不带电，电子带负电。

The charges and masses of these particles are crucial for IGCSE. Protons and neutrons have approximately the same mass (1 atomic mass unit, u), while electrons are about 1/1836 of that mass and therefore negligible in mass calculations.

这些粒子的电荷和质量对IGCSE考试至关重要。质子和中子的质量大致相同（1个原子质量单位 u），而电子的质量约为其1/1836，因此在质量计算中可以忽略不计。

• Proton: charge +1, mass 1 u | 质子：电荷 +1，质量 1 u

• Neutron: charge 0, mass 1 u | 中子：电荷 0，质量 1 u

• Electron: charge -1, mass 1/1836 u | 电子：电荷 -1，质量 1/1836 u

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2. Atomic Number and Mass Number | 原子序数与质量数

The atomic number (Z) is the number of protons in the nucleus. This defines which element an atom belongs to. 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，它决定了原子属于哪种元素。质量数（A）是原子核中质子数和中子数的总和。

For any neutral atom, the number of electrons equals the number of protons. This balance ensures the atom has zero overall charge.

对于任何中性原子，电子数等于质子数。这种平衡保证了原子的总电荷为零。

A = Z + N

质量数 = 原子序数 + 中子数

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3. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.

同位素是同一元素中具有相同质子数但不同中子数的原子。这意味着它们具有相同的原子序数，但质量数不同。

For example, carbon-12 and carbon-14 are both isotopes of carbon. Carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons. Chemically, isotopes behave identically because chemical properties depend on electron configuration, which is the same for isotopes of the same element.

例如，碳-12和碳-14都是碳的同位素。碳-12有6个质子和6个中子，而碳-14有6个质子和8个中子。同位素的化学性质完全相同，因为化学性质取决于电子排布，而同一元素的同位素电子排布相同。

Some isotopes are stable, while others are unstable and undergo radioactive decay. Unstable isotopes are called radioisotopes.

有些同位素是稳定的，而另一些不稳定并会发生放射性衰变。不稳定的同位素被称为放射性同位素。

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4. Development of the Atomic Model | 原子模型的发展

The atomic model has evolved over centuries. John Dalton proposed that atoms were solid, indivisible spheres. J.J. Thomson discovered the electron and proposed the ‘plum pudding’ model, where negative electrons were scattered throughout a sphere of positive charge.

原子模型经历了数百年的演变。约翰·道尔顿提出原子是实心的、不可分割的球体。J.J.汤姆逊发现了电子，并提出了”葡萄干布丁”模型，即带负电的电子散布在带正电的球体中。

Ernest Rutherford’s gold foil experiment revolutionised our understanding. He fired alpha particles at thin gold foil and observed that most passed straight through, some were deflected, and a few bounced back. This led to the nuclear model: a tiny, dense, positively charged nucleus containing almost all the mass, with electrons orbiting at a relatively large distance.

欧内斯特·卢瑟福的金箔实验彻底改变了我们的认识。他用α粒子轰击薄金箔，观察到大多数粒子直穿而过，一些发生偏转，极少数反弹回来。这导致了核模型的提出：一个微小、致密、带正电荷的原子核集中了几乎全部质量，电子在较远的距离外绕核运动。

Niels Bohr then refined the model by stating that electrons orbit the nucleus in fixed energy levels or shells.

尼尔斯·玻尔随后完善了这一模型，提出电子在固定的能级或壳层中绕核运动。

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5. What is Radioactivity? | 什么是放射性？

Radioactivity is the spontaneous disintegration of unstable nuclei. During this process, the nucleus emits radiation in the form of particles or electromagnetic waves. This process is random and cannot be influenced by physical or chemical conditions such as temperature, pressure, or chemical bonding.

放射性是不稳定原子核的自发蜕变。在此过程中，原子核以粒子或电磁波的形式释放辐射。这个过程是随机的，不受温度、压力或化学键合等物理或化学条件的影响。

Radioactive decay is a random process. This means it is impossible to predict exactly which nucleus will decay next or when a particular nucleus will decay. However, for a large number of nuclei, the decay follows a predictable statistical pattern.

放射性衰变是一个随机过程。这意味着无法准确预测下一个衰变的核是哪一个，或某个核何时衰变。然而，对于大量的核，衰变遵循可预测的统计规律。

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6. Alpha, Beta and Gamma Radiation | α、β与γ辐射

There are three main types of radiation emitted during radioactive decay: alpha (α), beta (β) and gamma (γ). Each has distinct properties in terms of nature, charge, mass, ionising power and penetrating ability.

放射性衰变主要有三种辐射类型：α（阿尔法）、β（贝塔）和γ（伽马）。每种辐射在本质、电荷、质量、电离能力和穿透能力上都有不同的特性。

Alpha particles are helium nuclei, consisting of 2 protons and 2 neutrons. They carry a charge of +2 and have a mass of 4 u. Because of their relatively large mass and charge, they are strongly ionising but have very low penetrating power.

α粒子是氦原子核，由2个质子和2个中子组成。它们带+2电荷，质量为4 u。由于质量和电荷较大，它们的电离能力很强，但穿透力很弱。

Beta particles are fast-moving electrons emitted from the nucleus when a neutron converts into a proton. They carry a charge of -1 and have negligible mass. They are moderately ionising and more penetrating than alpha particles.

β粒子是快速运动的电子，在中子转化为质子时从原子核中发射出来。它们带-1电荷，质量可忽略不计。它们具有中等电离能力，穿透力比α粒子强。

Gamma rays are electromagnetic waves with very high frequency and energy. They have no charge and no mass. They are weakly ionising but highly penetrating, requiring several centimetres of lead or thick concrete to stop them.

γ射线是频率极高、能量极大的电磁波。它们不带电荷，也没有质量。它们的电离能力弱，但穿透力极强，需要数厘米厚的铅或厚混凝土才能阻挡。

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7. Penetrating Power and Ionising Ability | 穿透能力与电离能力

The table below summarises the key differences between the three types of radiation. You must be able to compare and explain these differences in exams.

下表总结了三种辐射之间的关键区别。在考试中，你需要能够比较并解释这些差异。

Property | 特性	Alpha (α) | α	Beta (β) | β	Gamma (γ) | γ
Nature | 本质	Helium nucleus | 氦核	Fast electron | 高速电子	EM wave | 电磁波
Charge | 电荷	+2	-1	0
Relative mass | 相对质量	4 u	1/1836 u	0
Ionising power | 电离能力	Strong | 强	Moderate | 中等	Weak | 弱
Penetrating power | 穿透能力	Stopped by paper | 纸可阻挡	Stopped by 3 mm aluminium | 3毫米铝可阻挡	Stopped by thick lead/concrete | 厚铅/混凝土可阻挡

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8. Nuclear Decay Equations | 核衰变方程

When a nucleus undergoes radioactive decay, the process can be represented by a nuclear equation. The total atomic number and mass number must be conserved on both sides of the equation.

当原子核发生放射性衰变时，这个过程可以用核方程来表示。方程两边的总原子序数和总质量数必须守恒。

For alpha decay, the parent nucleus loses 2 protons and 2 neutrons. For example, radium-226 decays to radon-222:

对于α衰变，母核失去2个质子和2个中子。例如，镭-226衰变为氡-222：

²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He

For beta decay, a neutron turns into a proton, so the mass number stays the same but the atomic number increases by 1. For example, carbon-14 decays to nitrogen-14:

对于β衰变，一个中子转化为质子，因此质量数不变，但原子序数增加1。例如，碳-14衰变为氮-14：

¹⁴₆C → ¹⁴₇N + ⁰₋₁e

Gamma emission usually accompanies alpha or beta decay. The nucleus releases excess energy as gamma radiation without changing its mass number or atomic number.

γ发射通常伴随α或β衰变发生。原子核以γ辐射的形式释放多余能量，其质量数和原子序数均不改变。

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9. Half-Life | 半衰期

The half-life of a radioactive isotope is the time taken for half the unstable nuclei in a sample to decay. It is also the time taken for the activity or count rate of a sample to fall to half its initial value.

放射性同位素的半衰期是指样品中一半不稳定核发生衰变所需的时间。它也是样品的活度或计数率降至初始值一半所需的时间。

Half-life is a fixed property of a particular isotope and cannot be changed. Each isotope has its own unique half-life, ranging from fractions of a second to millions of years.

半衰期是特定同位素的固定属性，无法改变。每种同位素都有自己独特的半衰期，从几分之一秒到数百万年不等。

If a sample has an initial activity of 800 counts per second and a half-life of 2 hours, after 2 hours the activity will be 400 counts per second. After another 2 hours, it will be 200 counts per second, and so on.

如果一个样品的初始活度为每秒800次计数，半衰期为2小时，那么2小时后活度将为每秒400次计数。再过2小时后为每秒200次计数，以此类推。

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

剩余比例 = (½)ⁿ，其中 n = 经过的半衰期个数

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10. Background Radiation | 背景辐射

Background radiation is the low-level radiation that is always present in the environment. It comes from a variety of natural and artificial sources.

背景辐射是环境中始终存在的低水平辐射。它来自各种天然和人工来源。

Natural sources include radon gas from rocks and soil, cosmic rays from space, and radioactive carbon-14 in the atmosphere. Artificial sources include medical X-rays and nuclear power plants. When measuring the radiation from a radioactive source, the background count must be subtracted to obtain the corrected count rate.

天然来源包括岩石和土壤中的氡气、来自太空的宇宙射线，以及大气中的放射性碳-14。人工来源包括医用X射线和核电站。在测量放射源的辐射时，必须减去背景计数以获得修正后的计数率。

A Geiger-Müller tube connected to a counter is commonly used to detect and measure radiation. The count rate is measured in counts per second.

盖革-米勒管连接到计数器上常用于探测和测量辐射。计数率以每秒计数为单位。

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11. Uses of Radioactivity | 放射性的应用

Radioactivity has many important applications in medicine, industry and archaeology.

放射性在医学、工业和考古学中都有许多重要应用。

In medicine, gamma rays are used to sterilise medical equipment and treat cancer through radiotherapy. Radioactive tracers are injected into the body to diagnose conditions. The isotopes chosen for medical use must have a short half-life so that the radioactivity does not remain in the body for long.

在医学上，γ射线用于对医疗设备进行消毒，并通过放射疗法治疗癌症。放射性示踪剂被注入体内以诊断疾病。用于医疗的同位素必须具有较短的半衰期，以便放射性不会在体内残留太久。

In industry, alpha particles are used in smoke detectors because they can ionise the air inside the detector. Beta radiation is used to measure the thickness of paper or aluminium foil during manufacturing. In archaeology, carbon-14 dating uses the half-life of carbon-14 to determine the age of ancient organic materials.

在工业上，α粒子用于烟雾探测器，因为它们可以电离探测器内部的空气。β辐射用于制造过程中测量纸张或铝箔的厚度。在考古学中，碳-14测年利用碳-14的半衰期来确定古代有机材料的年代。

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12. Safety and Protection | 安全与防护

Radiation can damage living cells and cause mutations in DNA, which may lead to cancer. Therefore, strict safety measures must be followed when handling radioactive materials.

辐射会损伤活细胞并导致DNA突变，从而可能引发癌症。因此，在处理放射性材料时必须严格遵守安全措施。

Key safety precautions involve minimising exposure time, maximising distance from the source, and using appropriate shielding. Alpha particles require only paper or skin for shielding, beta particles require aluminium, and gamma rays require lead or concrete.

关键安全措施包括尽量减少暴露时间、与放射源保持最大距离，以及使用适当的屏蔽材料。α粒子仅需纸或皮肤即可屏蔽，β粒子需要铝来屏蔽，而γ射线需要铅或混凝土。

• Use tongs to handle sources, never bare hands | 使用钳子夹取放射源，切勿徒手接触

• Point sources away from the body | 将放射源朝向远离身体的方向

• Store sources in lead-lined containers | 将放射源存放在铅衬容器中

• Wear protective clothing and dosimeters | 穿戴防护服并佩戴剂量计

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