Atomic Structure | 原子结构

📚 Atomic Structure | 原子结构

Understanding the structure of the atom is the foundation of GCSE Chemistry. In the Edexcel specification, you need to describe the relative charges and masses of protons, neutrons and electrons, explain what atomic number and mass number tell us, define isotopes, and work out electronic configurations for the first 20 elements. This guide covers every key point to help you score full marks in your exam.

理解原子结构是 GCSE 化学的基础。根据 Edexcel 考纲，你需要描述质子、中子和电子的相对电荷与质量，解释原子序数和质量数的含义，定义同位素，并能写出前 20 号元素的电子排布。本文涵盖所有关键考点，助你在考试中拿下高分。

1. Basic Structure of the Atom | 原子的基本结构

All matter is made of atoms. An atom consists of a tiny, dense nucleus at the centre, surrounded by electrons moving in shells. The nucleus contains protons and neutrons, which are collectively called nucleons. Most of the atom is empty space – the nucleus is about 10 000 times smaller than the atom itself.

所有物质都由原子构成。原子中心有一个极小且致密的原子核，核外电子分层运动。原子核内含质子和中子，二者统称核子。原子绝大部分是空的——原子核比原子本身大约小 10 000 倍。

2. Subatomic Particles: Protons, Neutrons, Electrons | 亚原子粒子：质子、中子、电子

You must remember the relative masses and charges of the three subatomic particles. A proton has a relative mass of 1 and a relative charge of +1. A neutron also has a relative mass of 1 but a relative charge of 0. An electron has a very small relative mass (about 1/1835, often taken as negligible) and a relative charge of –1. In a neutral atom, the number of protons equals the number of electrons.

你必须记住三种亚原子粒子的相对质量和相对电荷。质子的相对质量为 1，相对电荷为 +1。中子的相对质量也是 1，但相对电荷为 0。电子的相对质量极小（约 1/1835，常忽略不计），相对电荷为 –1。中性原子中，质子数等于电子数。

Particle	Relative mass	Relative charge
Proton	1	+1
Neutron	1	0
Electron	0 (negligible)	–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. It defines the element: all atoms of the same element have the same atomic number. In a neutral atom, Z also equals the number of electrons. The mass number (A) is the total number of protons and neutrons in the nucleus. It is always a whole number. You can use A and Z to find the number of neutrons: Number of neutrons = A – Z.

原子序数（Z）是原子核内的质子数。它决定了元素种类：同种元素的所有原子具有相同的原子序数。在中性原子中，Z 也等于电子数。质量数（A）是原子核内质子数与中子数的总和，总为整数。你可以用 A 和 Z 求中子数：中子数 = A – Z。

Number of neutrons = Mass number (A) – Atomic number (Z)

4. Isotopes: Same Element, Different Mass | 同位素：同元素，不同质量

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers. Isotopes have identical chemical properties because they have the same electron arrangement – chemical reactions involve electrons, not the nucleus. However, physical properties like density or rate of diffusion can differ slightly. Carbon‑12 and carbon‑14 are classic examples: both have 6 protons, but carbon‑12 has 6 neutrons while carbon‑14 has 8 neutrons.

同位素是同种元素中质子数相同而中子数不同的原子。它们具有相同的原子序数，但质量数不同。同位素的化学性质相同，因为电子排布一致——化学反应涉及电子，与原子核无关。但密度或扩散速率等物理性质可能略有差异。碳‑12 和碳‑14 是经典例子：两者均有 6 个质子，但碳‑12 有 6 个中子，碳‑14 则有 8 个中子。

5. Relative Atomic Mass (Aᵣ) | 相对原子质量 (Aᵣ)

Relative atomic mass (Aᵣ) is the weighted mean mass of an atom of an element compared with 1/12 of the mass of an atom of carbon‑12. Because most elements exist as a mixture of isotopes, Aᵣ is rarely a whole number. For example, chlorine consists of about 75% chlorine‑35 and 25% chlorine‑37, giving it an Aᵣ of 35.5. You should be able to calculate Aᵣ from isotopic abundances using the formula: Aᵣ = Σ (isotopic mass × percentage abundance) / 100.

相对原子质量（Aᵣ）是某元素一个原子的加权平均质量与碳‑12 原子质量的 1/12 之比。由于多数元素以同位素混合物形式存在，Aᵣ 很少是整数。例如，氯约由 75% 的氯‑35 和 25% 的氯‑37 组成，故其 Aᵣ 为 35.5。你应能利用同位素丰度计算 Aᵣ：Aᵣ = Σ (同位素质量 × 百分比丰度) / 100。

Aᵣ = (mass₁ × %₁ + mass₂ × %₂ + …) ÷ 100

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

Electrons occupy shells (energy levels) around the nucleus. The first shell can hold a maximum of 2 electrons, the second shell up to 8, and the third shell up to 8 (for the first 20 elements). Electrons fill the lowest energy level first – the inner shell. The electronic configuration of an atom is written as a sequence of numbers, e.g., sodium (11 electrons) is 2,8,1. For the exam, you must be able to deduce the electron arrangement of any element up to calcium (atomic number 20).

电子占据原子核外的壳层（能级）。第一层最多容纳 2 个电子，第二层最多 8 个，第三层最多 8 个（前 20 号元素适用）。电子优先填充最低能级，即最内层。原子的电子排布以数字序列表示，如钠（11 个电子）为 2,8,1。考试中，你必须能推断出到钙（原子序 20）为止任意元素的电子排布。

Hydrogen: 1
Carbon: 2,4
Oxygen: 2,6
Neon: 2,8
Potassium: 2,8,8,1
Calcium: 2,8,8,2

7. Electronic Configuration and the Periodic Table | 电子排布与元素周期表

The number of electrons in the outermost shell (valence electrons) determines the group of an element. For example, elements in Group 1 all have 1 electron in their outer shell, leading to similar chemical properties. The number of occupied shells tells you the period: sodium (2,8,1) has three occupied shells, so it is in Period 3. This link between electronic structure and the Periodic Table is a crucial exam topic, especially for explaining trends in reactivity.

最外层电子数（价电子数）决定了元素所在的族。例如，第 1 族元素的最外层都只有 1 个电子，因此化学性质相似。已占用的电子层数则表明周期数：钠（2,8,1）有三层电子，故位于第三周期。电子结构与周期表的联系是重要的考试主题，尤其用于解释反应性递变规律。

8. Ions: Gaining and Losing Electrons | 离子：得失电子

Atoms become ions by losing or gaining electrons to achieve a full outer shell – the stable electronic structure of a noble gas. Metals tend to lose electrons and form positive ions (cations). Non‑metals tend to gain electrons and form negative ions (anions). For example, a sodium atom (2,8,1) loses one electron to become Na⁺ with the electronic structure 2,8. A chlorine atom (2,8,7) gains one electron to become Cl⁻ with the structure 2,8,8. The charge on an ion equals the number of electrons lost or gained.

原子通过失去或得到电子达到满外壳——即稀有气体的稳定电子结构——从而形成离子。金属倾向于失去电子形成正离子（阳离子）。非金属倾向于获得电子形成负离子（阴离子）。例如，钠原子（2,8,1）失去一个电子变成 Na⁺，电子结构为 2,8。氯原子（2,8,7）得到一个电子变成 Cl⁻，结构为 2,8,8。离子所带电荷数等于失去或得到的电子数。

9. Summary Checklist for Exam Success | 成功应考要点总结

Before the exam, make sure you can recall the relative masses and charges of protons, neutrons and electrons. Know how to use atomic number and mass number to work out the number of each subatomic particle. Be confident defining isotopes and calculating relative atomic mass from isotopic data. Practise writing electronic configurations for the first 20 elements and linking them to the Periodic Table. Finally, explain ion formation in terms of electron transfer and full outer shells. These core skills underpin many later topics, from bonding to electrolysis.

考前请确认你能回忆质子、中子和电子的相对质量与电荷。知道如何利用原子序数和质量数计算各亚原子粒子的数目。能有把握地定义同位素，并能根据同位素数据计算相对原子质量。练习写出前 20 号元素的电子排布并将其与周期表联系起来。最后，能用电子转移和满外壳解释离子的形成。这些核心技能是后续众多课题的基础，从化学键到电解都离不开它们。

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Atomic Structure | 原子结构