📚 The Structure of the Atom and Isotopes | 原子结构与同位素
Everything around you is made of atoms, the smallest particles of an element that still keep its chemical identity. Understanding the structure of the atom is essential for explaining chemical reactions, radioactivity and many everyday applications. In this article, we will explore the subatomic particles, how atoms are written in notation, the meaning of isotopes and how they are used in science and medicine.
你周围的一切都由原子构成，原子是保持元素化学性质的最小微粒。理解原子结构是解释化学反应、放射性以及许多日常应用的基础。本文将带你探索亚原子粒子、原子的表示符号、同位素的含义，以及它们在科学和医学中的应用。
1. Introduction to the Atom | 原子简介
An atom is the basic building block of matter. It is electrically neutral because it contains equal numbers of positively charged protons and negatively charged electrons. The majority of the atom, however, is empty space, with the tiny nucleus at the centre.
原子是物质的基本组成部分。它呈电中性，因为带正电的质子与带负电的电子数目相等。然而，原子的大部分区域是空的，中心是极小的原子核。
The nucleus contains protons and neutrons, which are collectively called nucleons. Electrons move around the nucleus in regions called shells or energy levels, travelling incredibly fast.
原子核含有质子和中子，统称为核子。电子在原子核外称为电子壳层或能级的区域中运动，速度极快。
2. Subatomic Particles | 亚原子粒子
There are three important subatomic particles you must know: protons, neutrons and electrons. Each has a different mass and electric charge.
你需要掌握三种重要的亚原子粒子：质子、中子和电子。三种粒子的质量和电荷各不相同。
| Particle | Charge | Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | 1/1836 (≈ 0) | Electron shells |
Remember that the mass of an electron is so small that it is usually ignored when calculating the mass of an atom. Protons and neutrons each have a relative mass of 1.
请记住，电子的质量非常小，在计算原子质量时通常忽略不计。质子和中子的相对质量均为 1。
3. 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 of an atom. In a neutral atom, this also equals the number of electrons. 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。在电中性原子中，它也等于电子数。质量数（A）是原子核中质子数与中子数的总和。
The number of neutrons can be found using the following relationship:
Number of neutrons = Mass number − Atomic number
因此，中子数可以用以下关系式求出：
中子数 = 质量数 − 原子序数
These two numbers are written together with the chemical symbol of the element, for example carbon-14 is written as ¹⁴₆C or C-14.
这两个数字通常与元素的化学符号一同书写，例如碳-14 写作 ¹⁴₆C 或 C-14。
4. Electron Arrangements | 电子排布
Electrons are arranged in shells around the nucleus. Each shell can hold only a limited number of electrons: the first shell holds 2, the second shell holds 8, and the third shell holds 8 (for the elements you will meet at IGCSE level).
电子在原子核外按壳层排列。每个壳层能容纳的电子数有限：第一层最多 2 个电子，第二层最多 8 个，第三层最多 8 个（在 IGCSE 阶段所学的元素范围内）。
For example, sodium has 11 electrons. They are arranged as 2, 8, 1. The electrons in the outer shell are called valence electrons, and they determine the chemical properties of the element.
例如，钠有 11 个电子，它们的排布是 2, 8, 1。外层电子称为价电子，它决定了元素的化学性质。
The element group number in the Periodic Table tells you the number of valence electrons, while the period number tells you the number of occupied electron shells.
元素在元素周期表中的族号告诉你价电子数，而周期号告诉你电子壳层的层数。
5. 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。因此，它们具有相同的原子序数，但质量数不同。
For example, chlorine has two common isotopes: chlorine-35 (¹⁷₃₅Cl) and chlorine-37 (¹⁷₃₇Cl). Both have 17 protons, but one has 18 neutrons while the other has 20 neutrons.
例如，氯有两种常见的同位素：氯-35（¹⁷₃₅Cl）和氯-37（¹⁷₃₇Cl）。两者都有 17 个质子，但中子数分别为 18 和 20。
The chemical properties of isotopes are identical because they have the same electron arrangement. However, their physical properties may differ, such as density or rate of diffusion.
由于同位素具有相同的电子排布，它们的化学性质完全相同。然而，物理性质可能不同，例如密度或扩散速率。
6. Calculating Relative Atomic Mass | 计算相对原子质量
Relative atomic mass (Aᵣ) is the weighted average mass of the isotopes of an element compared to one-twelfth of the mass of a carbon-12 atom. It takes into account the abundance of each isotope.
相对原子质量（Aᵣ）是元素的同位素相对于一个碳-12 原子质量的十二分之一的加权平均质量。它考虑到了每种同位素的相对丰度。
For chlorine, if about 75% is chlorine-35 and 25% is chlorine-37, then:
Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5
Therefore, chlorine is written in the Periodic Table with Aᵣ = 35.5.
因此，氯在元素周期表中的相对原子质量写作 35.5。
The general formula for relative atomic mass is:
Aᵣ = Σ(isotopic mass × abundance) / total abundance
相对原子质量的一般公式为：
Aᵣ = Σ(同位素质量 × 丰度) / 总丰度
7. Properties of Isotopes | 同位素的性质
Isotopes of the same element share the same number of protons and electrons. As a result, they take part in the same chemical reactions and form the same types of compounds.
同一元素的同位素具有相同的质子数和电子数。因此，它们参与相同的化学反应，并形成相同类型的化合物。
However, differences in neutron number can affect physical properties such as mass, density and the rate at which they diffuse. For example, water containing heavy hydrogen (deuterium) has a higher density than ordinary water.
不过，中子数的差异会影响物理性质，如质量、密度以及扩散速率。例如，含有重氢（氘）的水比普通水密度更高。
Some isotopes are radioactive, meaning their nuclei are unstable and can break down spontaneously. These are known as radioisotopes.
某些同位素具有放射性，意味着它们的原子核不稳定，可以自发发生衰变。这些同位素称为放射性同位素。
8. Uses of Isotopes | 同位素的应用
Radioactive isotopes have many useful applications in medicine, industry and archaeology. One well-known example is carbon-14, which is used in radiocarbon dating to determine the age of ancient objects.
放射性同位素在医学、工业和考古学中有许多重要应用。一个著名的例子是碳-14，它被用于放射性碳定年法，以确定古代物体的年代。
Cobalt-60 is used in radiotherapy to treat cancer by killing cancer cells with gamma rays.
Cobalt-60（钴-60）用于放射治疗，通过伽马射线杀死癌细胞来治疗癌症。
Iodine-131 is used in the diagnosis and treatment of thyroid disorders.
碘-131 用于甲状腺疾病的诊断和治疗。
Technetium-99m is used as a medical tracer to image organs like the heart or brain.
锝-99m 用作医用示踪剂，对心脏或大脑等器官进行成像。
Carbon-14 is used to date fossils and archaeological specimens.
碳-14 用于测定化石和考古样品的年代。
9. Radioactive Decay | 放射性衰变
Some isotopes have nuclei that are unstable. To become more stable, they emit radiation in a process called radioactive decay. The three types of radiation you need to know are alpha (α), beta (β) and gamma (γ).
某些同位素的原子核不稳定。为了变得更加稳定，它们会通过一种称为放射性衰变的过程释放辐射。你需要掌握三种辐射类型：α（阿尔法）、β（贝塔）和 γ（伽马）。
Alpha particles are composed of 2 protons and 2 neutrons, with a charge of +2 and low penetrating power.
α 粒子由 2 个质子和 2 个中子组成，带 +2 电荷，穿透能力较弱。
Beta particles are fast-moving electrons with a charge of −1 and moderate penetrating power.
β 粒子是快速运动的电子，带 −1 电荷，穿透能力中等。
Gamma rays are electromagnetic waves with no mass and very high penetrating power.
γ 射线是电磁波，没有质量，穿透能力极强。
During decay, a parent nucleus changes into a different nucleus or a lower-energy state, and in the process mass may be converted into energy.
在衰变过程中，母核会转变成另一种核或更低能态，在此过程中部分质量可能转化为能量。
10. Half-Life | 半衰期
The half-life of a radioactive isotope is the average time taken for half of the radioactive nuclei in a sample to decay. It is a fixed property for each isotope and does not depend on temperature or pressure.
放射性同位素的半衰期是指样品中一半放射性原子核发生衰变所需的平均时间。每种同位素的半衰期是固定的，不受温度和压强影响。
For example, if a sample starts with 80 g of iodine-131 and its half-life is 8 days, then after 8 days only 40 g remains. After another 8 days, 20 g remains, and so on.
例如，如果一份样品初始有 80 g 碘-131，其半衰期为 8 天，那么 8 天后只剩下 40 g。再过 8 天，剩下 20 g，以此类推。
Half-life calculations are common in exam questions. Always remember to count the number of half-lives, not the total time, to find the remaining mass.
半衰期计算是考试中常见的题型。请记住，应计算半衰期的次数，而不是总时间，才能求出剩余质量。
11. Deduce Composition from Notation | 从符号推断组成
You should be able to read nuclear notation such as ²³₃₈U? Wait, that is uranium-238, written as ²³⁸₉₂U. The superscript is the mass number and the subscript is the atomic number.
你应该能阅读核素符号，例如铀-238 写作 ²³⁸₉₂U。右上角标为质量数，左下角标为原子序数。
For the notation ᴬ the number of electrons in a neutral atom is also Z, and the number of neutrons is A − Z.
对于符号 ᴬ_Z X 而言，质子数为 Z，中性原子中电子数也为 Z，中子数为 A − Z。
Example: sodium-23, ²³₁₁Na. Protons = 11, electrons = 11, neutrons = 23 − 11 = 12.
示例：钠-23，即 ²³₁₁Na。质子数 = 11，电子数 = 11，中子数 = 23 − 11 = 12。
Practising these conversions quickly is crucial for scoring marks in IGCSE Science papers.
快速进行这些换算的练习对于在 IGCSE 科学考试中获得分数至关重要。
12. Common Pitfalls in Exams | 考试常见错误
Students often confuse protons with neutrons or forget that the mass number is not the same as the atomic number. Another common error is using the relative atomic mass as the mass number for a specific isotope.
学生常常混淆质子和中子，或者忘记质量数并不等于原子序数。另一个常见错误是把相对原子质量当作某一特定同位素的质量数。
Do not state that electrons have negligible charge. They have a charge of −1, but their mass is negligible.
不要说电子电荷为 0。电子带 −1 电荷，只是质量可忽略。
When writing electron configurations, always count the total electrons carefully for ions.
书写电子排布时，对离子一定要仔细数总电子数。
In half-life questions, check whether the answer asks for mass remaining or the number of half-lives.
在半衰期题型中，要看清楚题目问的是剩余质量还是半衰期次数。
Remember that isotopes of the same element have identical chemical properties, not different ones.
记住，同一元素的同位素具有相同的化学性质，而不是不同的性质。
By mastering these key ideas, you will be well prepared for questions on atomic structure and isotopes in your Edexcel IGCSE Science exam.
掌握了这些核心概念，你就能充分准备好应对 Edexcel IGCSE 科学考试中关于原子结构和同位素的问题。
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