4.2.1 Atomic Structure | 4.2.1 原子结构

📚 4.2.1 Atomic Structure | 4.2.1 原子结构

Everything around you — from the water you drink to the screen you are reading — is made of atoms. Understanding atomic structure is the first step in mastering IGCSE Edexcel Science, because it explains how elements behave, why they bond, and what gives each substance its unique properties. This article will guide you through subatomic particles, nuclear symbols, isotopes, electron configurations, and how these ideas link directly to the Periodic Table and chemical reactions.

你周围的一切——从饮用水的分子到正在阅读的屏幕——都是由原子构成的。理解原子结构是掌握 IGCSE Edexcel 科学课程的第一步，因为它能解释元素的性质、它们为何会形成化学键，以及每种物质为何具有独特的特性。本文将带你系统地学习亚原子粒子、核符号、同位素、电子排布，以及这些概念如何与元素周期表和化学反应直接关联。

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1. What is an Atom? | 什么是原子？

An atom is the smallest particle of an element that can still be recognised as that element. Atoms themselves are made up of even smaller subatomic particles, but the chemical identity of an element is determined by the number of protons in its nucleus. In IGCSE Science, you must be able to describe the relative charges and masses of these particles and explain how atoms are arranged.

原子是能够被识别为某种元素的最小粒子。原子本身由更小的亚原子粒子构成，但元素的化学性质由原子核内的质子数决定。在 IGCSE 科学中，你必须能够描述这些粒子的相对电荷和质量，并解释原子的结构。

Atoms are electrically neutral overall because they contain equal numbers of positively charged protons and negatively charged electrons. The nucleus, which is tiny compared with the total size of the atom, contains protons and neutrons and holds nearly all the mass. Electrons move around the nucleus in regions called shells or energy levels.

原子整体上呈电中性，因为它们含有等量的带正电荷的质子和带负电荷的电子。原子核远小于原子的整体尺寸，几乎容纳了原子的全部质量，由质子和中子组成。电子在被称为电子层或能级的区域中绕核运动。

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2. Subatomic Particles | 亚原子粒子

You must know the three fundamental subatomic particles: the proton, the neutron and the electron. Their properties can be summarised using relative mass and relative charge. A proton has a relative mass of 1 and a relative charge of +1. A neutron also has a relative mass of 1, but it carries no charge (0). The electron has a relative mass of 1/1840 (often taken as negligible at IGCSE) and a relative charge of -1.

你必须熟悉三种基本的亚原子粒子：质子、中子和电子。它们的特性可以用相对质量和相对电荷来概括。质子的相对质量为 1，相对电荷为 +1。中子的相对质量同样是 1，但不带电荷 (0)。电子的相对质量约为 1/1840（在 IGCSE 阶段通常可忽略不计），相对电荷为 -1。

Protons and neutrons are located in the nucleus and are therefore called nucleons. Electrons are found outside the nucleus, moving rapidly in shells. The attraction between the positive nucleus and the negative electrons holds the atom together, and this model is sufficient for explaining chemical bonding and trends across the Periodic Table at this level.

质子和中子位于原子核内，因此又被称为核子。电子分布在原子核外，在电子层中快速运动。带正电的原子核与带负电的电子之间的引力将原子维系在一起，这个模型足以解释 IGCSE 阶段的化学键和元素周期表中的变化规律。

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3. Nuclear Symbols | 核符号表示

In Edexcel IGCSE Science, you must be able to interpret and write symbols such as ²³₁₁Na or ¹⁴₆C. In this notation, the mass number (also called nucleon number) is written as a superscript to the left of the element symbol, and the atomic number (proton number) is written as a subscript to the left of the symbol.

在 Edexcel IGCSE 科学中，你必须能够读懂并书写例如 ²³₁₁Na 或 ¹⁴₆C 这样的核符号。在这种表示法中，质量数（又称核子数）写在元素符号左上角，而原子序数（质子数）写在符号左下角。

The atomic number Z tells you the number of protons and, in a neutral atom, also the number of electrons. The mass number A gives the total number of protons and neutrons. Therefore the number of neutrons can be calculated by subtracting Z from A: Neutrons = A − Z.

原子序数 Z 表示质子数，对于中性原子也等于电子数。质量数 A 是质子和中子的总数。因此，中子数可以通过 A − Z 计算得出：中子数 = A − Z。

²³₁₁Na → 11 protons, 12 neutrons, 11 electrons

²³₁₁Na → 11个质子、12个中子、11个电子

This system works for any nuclide. For example, ¹⁹₉F has 9 protons, 10 neutrons and 9 electrons. When you are given an ion, remember to adjust the number of electrons: a 2+ charge means two fewer electrons than protons.

这一规则适用于任何核素。例如 ¹⁹₉F 有 9 个质子、10 个中子和 9 个电子。如果给出的是离子，记得要调整电子数：带 2+ 的电荷代表电子比质子少 2 个。

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4. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Since the chemical behaviour of an element is determined by its electron arrangement, isotopes of the same element have identical chemical properties. Their physical properties, such as density or rate of diffusion, may vary because the mass differs.

同位素是指质子数相同但中子数不同的同一种元素的不同原子。因为元素的化学性质由电子排布决定，所以同一元素的同位素具有相同的化学性质。由于质量不同，它们的物理性质——例如密度或扩散速率——可能会有所差异。

Two well-known examples are the isotopes of carbon: ¹²₆C (6 protons, 6 neutrons) and ¹⁴₆C (6 protons, 8 neutrons). Carbon-12 is the standard for relative atomic mass, while carbon-14 is radioactive and used in radiocarbon dating. Another common example is chlorine, which exists naturally as a mixture of ³⁵₁₇Cl and ³⁷₁₇Cl.

两个著名的例子是碳的同位素：¹²₆C（6 个质子、6 个中子）和 ¹⁴₆C（6 个质子、8 个中子）。碳-12 是相对原子质量的标准，而碳-14 具有放射性，常用于放射性碳年代测定。另一个常见例子是氯，它天然以 ³⁵₁₇Cl 和 ³⁷₁₇Cl 的混合物形式存在。

When answering exam questions, always stress that isotopes have the same atomic number but different mass numbers. Avoid saying they have different ‘numbers of atoms’ — it is the number of neutrons that changes, not the number of protons.

在回答考题时，一定要强调同位素具有相同的原子序数但不同的质量数。避免说它们含有不同数目的“原子”——变化的是中子数，而不是质子数。

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5. Relative Atomic Mass (Aᵣ) | 相对原子质量

Because the actual masses of atoms are incredibly small, chemists use relative atomic mass, Aᵣ, on a scale where one atom of carbon-12 is given a mass of exactly 12 units. The relative atomic mass of an element is the weighted average mass of all its isotopes compared with 1/12 of the mass of a carbon-12 atom.

由于原子的真实质量极其微小，化学家使用相对原子质量 Aᵣ，并规定一个碳-12 原子的质量为 12 个单位。某种元素的相对原子质量是该元素所有同位素相对于碳-12 原子质量 1/12 的加权平均质量。

Aᵣ = Σ (isotope mass × % abundance) / 100

相对原子质量 = 各同位素（质量 × 丰度%）之和 / 100

For example, chlorine consists of 75% ³⁵Cl and 25% ³⁷Cl. The calculation gives Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5. This value is the one you see on the Periodic Table. Understanding this calculation helps you explain why many atomic masses are not whole numbers.

例如，氯由 75% 的 ³⁵Cl 和 25% 的 ³⁷Cl 组成。计算结果为 Aᵣ = (35×75 + 37×25) /100 = 35.5。这个数值正是元素周期表上显示的值。理解这一计算过程有助于你解释为什么许多原子量并非整数。

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6. Electron Shells and Configuration | 电子层与电子排布

Electrons are arranged in shells (energy levels) around the nucleus. The first shell can hold up to 2 electrons, the second shell up to 8, and the third shell up to 8 electrons for the first 20 elements (using the 2.8.8 pattern). The arrangement is often written as a string of numbers, e.g., sodium (Na): 2,8,1.

电子在原子核外的电子层（能级）中排布。第一层最多容纳 2 个电子，第二层最多容纳 8 个电子，对于前 20 号元素，第三层也可容纳最多 8 个电子（遵循 2.8.8 规律）。排布方式常写成一串数字，例如钠 (Na)：2,8,1。

The electronic configuration determines how an element reacts. Elements with a full outer shell, such as the noble gases, are very unreactive. Elements in the same group of the Periodic Table have the same number of electrons in their outermost shell, which is why they share similar chemical properties.

电子排布决定了元素的反应活性。外层电子充满的元素，如稀有气体，非常不活泼。周期表中同一族的元素最外层电子数相同，因此它们具有相似的化学性质。

To work out the electronic configuration for the first 20 elements, simply place electrons into shells starting from the lowest energy level closest to the nucleus until you run out. For potassium (19 electrons), the configuration is 2,8,8,1; for calcium (20 electrons), it is 2,8,8,2. Note that the third shell can temporarily hold up to 18 electrons in heavier elements, but at IGCSE you only need the simple 2.8.8.2 form for calcium.

要写出前 20 号元素的电子排布，只需从最靠近原子核的最低能级开始填充电子，直到用完所有电子。钾（19 个电子）的排布是 2,8,8,1；钙（20 个电子）为 2,8,8,2。需要注意的是，第三层在更重的元素中最多可容纳 18 个电子，但在 IGCSE 阶段你只需记住钙的排布为 2.8.8.2。

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7. Linking Electron Configuration to the Periodic Table | 电子排布与元素周期表的联系

The structure of the Periodic Table is a direct map of electron configurations. The period number tells you how many electron shells are occupied. For example, all elements in Period 2 have electrons in two shells, while those in Period 3 have three occupied shells. The group number (for Groups 1 to 7 and 0) often matches the number of electrons in the outer shell.

元素周期表的结构是电子排布的直接映射。周期数告诉你原子占有多少个电子层。例如，第二周期的所有元素都有两个电子层，而第三周期的元素拥有三个电子层。族的编号（第 1 至 7 族以及第 0 族）通常与外层电子数目相匹配。

Sodium, in Group 1, has one outer electron: 2,8,1. Oxygen, in Group 6, has six outer electrons: 2,6. Neon, a noble gas in Group 0, has a full outer shell of eight electrons: 2,8. This relationship allows you to predict the electron arrangement of an element from its position in the table, and vice versa — a vital skill for IGCSE Edexcel Science.

钠位于第 1 族，有一个外层电子：2,8,1。氧位于第 6 族，有六个外层电子：2,6。氖是第 0 族的稀有气体，具有全满的八个外层电子：2,8。这种关联使你能够从元素在周期表中的位置推断其电子排布，反之亦然——这是 IGCSE Edexcel 科学的关键技能。

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8. Formation of Ions | 离子的形成

Atoms become more stable when they achieve a full outer shell of electrons. Metals tend to lose electrons to form positive ions (cations), while non-metals tend to gain electrons to form negative ions (anions). The number of electrons lost or gained is equal to the charge on the ion.

当原子获得全满的外层电子排布时，会变得更加稳定。金属倾向于失去电子形成带正电的离子（阳离子），而非金属倾向于得到电子形成带负电的离子（阴离子）。失去或得到的电子数目等于离子所带的电荷数。

Sodium (2,8,1) loses one electron to become Na⁺ with the stable configuration 2,8. Chlorine (2,8,7) gains one electron to become Cl⁻, also achieving the stable 2,8,8 arrangement. The resulting ions attract each other strongly, forming an ionic bond — the basis of compounds such as sodium chloride.

钠（2,8,1）失去一个电子变成 Na⁺，获得 2,8 的稳定排布。氯（2,8,7）得到一个电子变成 Cl⁻，同样达到 2,8,8 的稳定排布。形成的离子相互强烈吸引，产生离子键——这是氯化钠等化合物的基础。

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

In exams, you may be asked to draw the electronic configurations of ions or to explain why a particular ion carries a certain charge. Always show the original atom’s structure and then clearly indicate the loss or gain of electrons with arrows or written equations using e⁻.

在考试中，可能会要求你画出离子的电子排布图，或解释为什么某个离子带有特定的电荷。记得先画出原子原有的结构，然后用箭头或含有 e⁻ 的方程式清晰地显示电子的得失。

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9. Isotopes and Radioactivity (Extension) | 同位素与放射性（扩展）

Some isotopes are unstable because their nuclei contain an unbalanced ratio of protons to neutrons. These unstable isotopes are called radioisotopes, and they release radiation from the nucleus in the form of alpha (α) particles, beta (β) particles or gamma (γ) rays. This process is known as radioactive decay and results in the formation of a different element.

某些同位素不稳定，因为原子核内的质子与中子比例失衡。这些不稳定的同位素被称为放射性同位素，原子核会以 α 粒子、β 粒子或 γ 射线的形式释放辐射。这一过程称为放射性衰变，并会生成另一种元素。

Carbon-14 is a radioactive isotope that decays to nitrogen-14 by beta emission. It is used in archaeology to date organic materials. Uranium-235, with the symbol ²³⁵₉₂U, undergoes fission and is important in nuclear power. Although radioactivity is explored in more depth in the Physics component, a basic awareness links to your understanding of isotopes in Chemistry.

碳-14 是一种放射性同位素，通过 β 衰变转变为氮-14。它被用于考古学中有机材料的年代测定。铀-235，符号为 ²³⁵₉₂U，会发生核裂变，在核能领域至关重要。尽管放射性在物理部分会深入探讨，但基本认知与你对化学同位素的理解紧密相连。

For IGCSE Edexcel Science, you should recognise that nuclear equations show the mass number and atomic number on both sides must balance. For example, the decay of carbon-14 can be written as:

在 IGCSE Edexcel 科学中，你需要知道核反应的方程式必须满足两边质量数和原子序数守恒。例如碳-14 的衰变可以写作：

¹⁴₆C → ¹⁴₇N + ⁰₋₁β

¹⁴₆C → ¹⁴₇N + ⁰₋₁β

Note that the beta particle is represented as an electron emitted from the nucleus, with a mass number of 0 and an atomic number of −1 to balance the equation.

注意，β 粒子被表示为从原子核中发射出的电子，质量数为 0，原子序数为 −1，以使方程式守恒。

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10. Key Definitions and Summary | 关键定义与总结

To succeed in the Atomic Structure section of your IGCSE, keep these definitions at your fingertips. The atomic number is the number of protons in the nucleus of an atom. The mass number is the total number of protons and neutrons. Isotopes are atoms of the same element with the same proton number but different nucleon number. The relative atomic mass is the weighted average mass of an element’s isotopes compared with 1/12 the mass of carbon-12.

要顺利通过 IGCSE 原子结构部分，必须熟练掌握以下定义。原子序数是原子核中的质子数。质量数是质子和中子的总数。同位素是质子数相同但核子数不同的同种元素的原子。相对原子质量是元素各种同位素相对于碳-12 质量的 1/12 的加权平均质量。

An ion is a charged particle formed when an atom loses or gains electrons. Electrons occupy shells: the first holds 2, the second holds 8, and for the first 20 elements the third shell also holds up to 8. The outer-shell electrons determine the chemical reactivity of the element and its position in the Periodic Table.

离子是原子因失去或得到电子而形成的带电粒子。电子占据电子层：第一层容纳 2 个电子，第二层容纳 8 个，前 20 号元素的第三层最多也可容纳 8 个电子。外层电子决定了元素的化学活性和它在周期表中的位置。

Revise by practising nuclear symbols, calculating Aᵣ from isotope data, and drawing shell diagrams for atoms and ions. Use the Periodic Table as a reference and check that your configurations match the group and period. These fundamentals reappear throughout the entire IGCSE Science course, from bonding to electrolysis and acids.

复习时，务必多练习核符号书写、根据同位素数据计算 Aᵣ，并画出原子和离子的电子层示意图。以元素周期表为参照，确保你的电子排布与族和周期匹配。这些基础知识贯穿 IGCSE 科学课程的始终，从化学键到电解和酸的反应都会涉及。

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