📚 Understanding Atomic Structure | 理解原子结构
Atomic structure is the foundation of all chemistry and physics. In your IGCSE Science course, you need to understand how atoms are built, how to represent them, and how their structure determines the properties of elements. This article covers everything you need for your Edexcel IGCSE exam.
原子结构是所有化学和物理的基础。在IGCSE科学课程中，你需要理解原子是如何构成的、如何表示它们，以及原子结构如何决定元素的性质。本文涵盖了Edexcel IGCSE考试所需的全部内容。
1. The Atom | 原子
An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are extremely small — about 0.1 nanometres in diameter. They consist of three subatomic particles: protons, neutrons and electrons.
原子是能参与化学反应的元素的最小粒子。原子极小——直径约为0.1纳米。它们由三种亚原子粒子组成：质子、中子和电子。
Protons are positively charged and are found in the nucleus.
质子带正电，位于原子核中。
Neutrons have no charge and are also in the nucleus.
中子不带电，也位于原子核中。
Electrons are negatively charged and move around the nucleus in energy levels (shells).
电子带负电，在原子核周围的能级（电子层）中运动。
2. Subatomic Particles | 亚原子粒子
Each subatomic particle has a specific mass and charge that you must remember for the exam.
每种亚原子粒子都有特定的质量和电荷，你需要在考试中牢记。
| Particle | Charge | Mass (relative) |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | -1 | 1/1836 (≈0) |
Notice that protons and neutrons have almost the same mass, while electrons are much lighter. Because electrons are so light, most of the mass of an atom is concentrated in the nucleus.
注意质子和中子的质量几乎相同，而电子要轻得多。由于电子非常轻，原子的大部分质量都集中在原子核中。
3. Atomic Number and Mass Number | 原子序数与质量数
Every element is defined by its number of protons, called the atomic number (Z). The mass number (A) is the total number of protons and neutrons in the nucleus.
每种元素由其质子数定义，这个数称为原子序数（Z）。质量数（A）是原子核中质子数与中子数的总和。
A = number of protons + number of neutrons
Z = number of protons
For a neutral atom, the number of electrons equals the number of protons. For example, an oxygen atom has 8 protons, 8 neutrons and 8 electrons. Its atomic number is 8 and its mass number is 16.
对于中性原子，电子数等于质子数。例如，一个氧原子有8个质子、8个中子和8个电子。它的原子序数是8，质量数是16。
4. Standard Notation | 标准表示法
Scientists use a shorthand notation to show the mass number and atomic number of an atom. The mass number is written as a superscript before the element symbol, and the atomic number is written as a subscript before the symbol.
科学家使用一种简写符号来显示原子的质量数和原子序数。质量数写在元素符号的左上角，原子序数写在左下角。
^A Z X
carbon-12 is written as ¹²₆C. This tells us it has 6 protons and 6 neutrons (12 − 6 = 6).
其中X是元素符号。例如，碳-12写成¹²₆C。这告诉我们它有6个质子和6个中子（12 − 6 = 6）。
5. Numbers of Particles | 粒子数的计算
You can calculate the number of neutrons by subtracting the atomic number from the mass number:
你可以通过质量数减去原子序数来计算中子数：
number of neutrons = A − Z
For a neutral atom, the number of electrons is equal to Z. For an ion, add or subtract electrons according to the charge. For example, an oxide ion O²⁻ has 8 protons, 8 neutrons and 10 electrons (2 extra electrons).
对于中性原子，电子数等于Z。对于离子，根据电荷增减电子。例如，氧化物离子O²⁻有8个质子、8个中子和10个电子（多了2个电子）。
6. 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.
同位素是同一元素的原子，它们具有相同的质子数但中子数不同。这意味着它们的原子序数相同但质量数不同。
For example, chlorine has two common isotopes: ³⁵₁₇Cl and ³⁷₁₇Cl. Both have 17 protons, but one has 18 neutrons and the other has 20 neutrons.
例如，氯有两种常见同位素：³⁵₁₇Cl和³⁷₁₇Cl。两者都有17个质子，但一个含有18个中子，另一个含有20个中子。
Isotopes have the same chemical properties because they have the same number of electrons.
同位素具有相同的化学性质，因为它们的电子数相同。
They may have different physical properties such as density or rate of diffusion.
它们可能有不同的物理性质，如密度或扩散速率。
Some isotopes are radioactive, such as carbon-14, which is used for dating ancient objects.
有些同位素具有放射性，例如碳-14，用于测定古代物体的年代。
7. Electron Configurations | 电子排布
Electrons are arranged in energy levels (shells) around the nucleus. The first shell can hold up to 2 electrons, the second shell can hold up to 8, and the third shell can also hold up to 8 (for the first 20 elements).
电子在原子核周围的能量层（电子壳层）中排列。第一层最多能容纳2个电子，第二层最多8个，第三层最多8个（对前20号元素而言）。
To write the electron configuration, fill the lowest energy shells first. For example, sodium (Na) has 11 electrons: 2, 8, 1. This is often written as 2.8.1.
要写出电子排布，先填充最低能量的壳层。例如，钠（Na）有11个电子：2，8，1。这通常写作2.8.1。
2.8.1
Electron configuration explains why some elements are reactive and why others are stable. A full outer shell (2 or 8 electrons) makes an atom stable, like noble gases.
电子排布解释了为什么有些元素活泼，而另一些元素稳定。满外层电子（2或8个）使原子稳定，如稀有气体。
8. Ions and Electron Transfer | 离子与电子转移
Atoms can lose or gain electrons to achieve a stable electron configuration. When an atom loses or gains electrons, it becomes an ion — a charged particle.
原子可以通过失去或获得电子来达到稳定的电子排布。当原子失去或获得电子时，它变成离子——一种带电粒子。
f an atom loses electrons, it becomes a positive ion (cation). For example, Na → Na⁺ + e⁻.
如果原子失去电子，它变成正离子（阳离子）。例如，Na → Na⁺ + e⁻。
If an atom gains electrons, it becomes a negative ion (anion). For example, Cl + e⁻ → Cl⁻.
如果原子获得电子，它变成负离子（阴离子）。例如，Cl + e⁻ → Cl⁻。
Ionic charges are written using superscript plus or minus signs. The number of electrons lost or gained is shown by the number before the sign. For example, Mg²⁺ means a magnesium ion that has lost 2 electrons.
离子电荷用上标正负号表示。失去或获得的电子数由符号前的数字表示。例如，Mg²⁺表示镁离子失去了2个电子。
9. Relative Atomic Mass | 相对原子质量
The relative atomic mass (Aᵣ) of an element is the weighted average mass of its isotopes compared with 1/12 of the mass of a carbon-12 atom. It has no units.
相对原子质量（Aᵣ）是元素同位素的加权平均质量，与碳-12原子质量的1/12相比。它没有单位。
To calculate Aᵣ, use the masses and abundance of each isotope:
要计算Aᵣ，使用每种同位素的质量和丰度：
Aᵣ = (mass₁ × abundance₁ + mass₂ × abundance₂ + ...) / total abundance
For example, chlorine has 75% ³⁵Cl and 25% ³⁷Cl. So Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5.
例如，氯有75%的³⁵Cl和25%的³⁷Cl。所以Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5。
10. The Periodic Table | 元素周期表
The periodic table arranges elements in order of increasing atomic number. Elements in the same column (group) have the same number of outer-shell electrons, so they have similar chemical properties.
元素周期表按照原子序数递增的顺序排列元素。同一列（族）的元素具有相同的外层电子数，因此它们具有相似的化学性质。
Group 1 — alkali metals: 1 outer electron, very reactive.
第1族——碱金属：1个外层电子，非常活泼。
Group 2 — alkaline earth metals: 2 outer electrons.
第2族——碱土金属：2个外层电子。
Group 7 — halogens: 7 outer electrons, reactive non-metals.
第7族——卤素：7个外层电子，活泼的非金属。
Group 0 — noble gases: full outer shell (2 or 8), very stable.
第0族——稀有气体：满外层（2或8），非常稳定。
The number of occupied shells increases down a group, and the total number of electrons increases along a period.
在族中向下，占据的电子层数增加；在周期中向右，电子总数增加。
11. Key Exam Tips | 考试要点
Here are some common traps and tips for your IGCSE atomic structure questions:
以下是一些IGCSE原子结构题目的常见陷阱和提示：
Remember: protons = electrons in a neutral atom, not in an ion.
记住：中性原子中质子数=电子数，离子中不相同。
The mass number is always the larger number in the notation.
质量数总是表示法中较大的那个数。
Isotopes have identical chemical properties because their electronic structure is the same.
同位素具有相同的化学性质，因为它们的电子结构相同。
When asked to draw an atom, show electrons in the correct shells (2, 8, 8) and label the nucleus with protons and neutrons.
当要求画出原子时，在正确的壳层中显示电子（2，8，8），并在原子核中标注质子和中子。
Use the correct superscript and subscript notation in your answers.
在答案中使用正确的上标和下标符号。
12. Practice Question | 练习题
Try this question before your exam:
在考试前试试这道题：
Q: An atom of potassium has the symbol ³⁹₁₉K. State the number of protons, neutrons and electrons in this atom. Also give the electron configuration.
题：钾原子的符号为³⁹₁₉K。说出这个原子的质子数、中子数和电子数，并给出电子排布。
Protons = 19, neutrons = 39 − 19 = 20, electrons = 19. Electron configuration: 2, 8, 8, 1.
答案：质子数=19，中子数=39−19=20，电子数=19。电子排布：2，8，8，1。
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