Atomic Structure and Isotopes | 原子结构与同位素

📚 Atomic Structure and Isotopes | 原子结构与同位素

Understanding atomic structure is the foundation of all chemistry. In your Edexcel IGCSE Science course, you need to know the key particles inside an atom, how they are arranged, and why isotopes matter.

理解原子结构是学习一切化学的基础。在 Edexcel IGCSE 科学课程中，你需要知道原子内部的关键粒子、它们的排列方式，以及同位素为什么重要。

1. The Three Subatomic Particles | 三种亚原子粒子

Every atom is made of three main particles: protons, neutrons and electrons. Protons carry a positive charge, neutrons are neutral, and electrons carry a negative charge.

每个原子都由三种主要粒子构成：质子、中子和电子。质子带正电荷，中子不带电，电子带负电荷。

Proton: relative charge +1, relative mass 1.

质子：相对电荷 +1，相对质量 1。
Neutron: relative charge 0, relative mass 1.

中子：相对电荷 0，相对质量 1。
Electron: relative charge –1, relative mass 1/1840 (almost negligible).

电子：相对电荷 –1，相对质量 1/1840（几乎可忽略）。

Proton number (Z) = number of protons = number of electrons in a neutral atom

质子数 (Z) = 质子数 = 中性原子中的电子数

2. Nucleus and Electron Shells | 原子核与电子壳层

Protons and neutrons are packed together in a tiny, dense region called the nucleus. Electrons move around the nucleus in shells (energy levels). The nucleus is positively charged because of the protons, while the electrons are attracted to it.

质子和中子紧密地聚集在一个微小而致密的区域，称为原子核。电子在原子核周围的壳层（能级）中运动。由于质子的存在，原子核带正电，而电子受其吸引。

For example, a carbon atom has 6 protons and 6 neutrons in the nucleus, with 6 electrons arranged in two shells: 2 in the first shell and 4 in the second shell.

例如，一个碳原子在原子核中有 6 个质子和 6 个中子，6 个电子排列在两个壳层中：第一层 2 个，第二层 4 个。

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. The mass number (A) is the total number of protons and neutrons.

原子序数 (Z) 是原子核中的质子数。质量数 (A) 是质子和中子的总数。

Mass number = number of protons + number of neutrons

质量数 = 质子数 + 中子数

To find the number of neutrons, simply subtract the atomic number from the mass number.

要求中子数，只需用质量数减去原子序数。

Particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	–1	1/1840

4. 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不同原子，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: chlorine-35 with 17 protons and 18 neutrons, and chlorine-37 with 17 protons and 20 neutrons.

例如，氯有两种常见同位素：氯-35（17 个质子，18 个中子）和氯-37（17 个质子，20 个中子）。

¹⁷Cl³⁵ and ¹⁷Cl³⁷ are both chlorine atoms, but they have different masses.

¹⁷Cl³⁵ 和 ¹⁷Cl³⁷ 都是氯原子，但质量不同。

5. Writing Isotope Notation | 同位素的书写方式

We can show the mass number as a superscript on the left of the symbol, and the atomic number as a subscript on the left. For example, carbon-14 is written as ¹⁴₆C.

我们可以将质量数写在元素符号的左上方，原子序数写在左下方。例如，碳-14 写作 ¹⁴₆C。

Top number (left) = mass number (A)

左上角数字 = 质量数 (A)
Bottom number (left) = atomic number (Z)

左下角数字 = 原子序数 (Z)

In ¹⁴₆C, 6 is the atomic number and 14 is the mass number. The number of neutrons is 14 – 6 = 8.

在 ¹⁴₆C 中，6 是原子序数，14 是质量数。中子数为 14 – 6 = 8。

6. Chemical Properties of Isotopes | 同位素的化学性质

Because chemical properties depend on the number and arrangement of electrons, isotopes of the same element have identical chemical behaviour. The extra neutrons only affect mass, not the electron configuration.

由于化学性质取决于电子的数目和排布，所以同一元素的不同同位素具有完全相同的化学行为。多出的中子只影响质量，不改变电子排布。

For example, both carbon-12 and carbon-14 react with oxygen to form carbon dioxide in the same way.

例如，碳-12 和碳-14 与氧气反应生成二氧化碳的方式完全相同。

7. Radioactive Isotopes | 放射性同位素

Some isotopes have unstable nuclei and emit radiation. These are called radioactive isotopes or radioisotopes. They can be used for medical diagnosis, cancer treatment, and carbon dating.

有些同位素的原子核不稳定，会发射辐射，称为放射性同位素。它们可用于医学诊断、癌症治疗和碳元素测年。

Carbon-14 is used to date ancient biological materials.

碳-14 用于测定古代生物材料的年代。
Cobalt-60 is used in radiotherapy to destroy cancer cells.

钴-60 用于放射疗法以破坏癌细胞。
Iodine-131 is used to treat thyroid disorders.

碘-131 用于治疗甲状腺疾病。

8. Relative Atomic Mass | 相对原子质量

Because isotopes have different masses, the relative atomic mass (Aᵣ) of an element is the weighted average mass of all its isotopes compared with 1/12 of the mass of a carbon-12 atom.

由于同位素质量不同，元素的相对原子质量 (Aᵣ) 是其所有同位素的加权平均质量，并与碳-12 原子质量的 1/12 相比较。

Aᵣ = (mass of isotope 1 × abundance 1) + (mass of isotope 2 × abundance 2) ÷ 100

Aᵣ = (同位素 1 的质量 × 丰度 1) + (同位素 2 的质量 × 丰度 2) ÷ 100

For chlorine, 75% is ³⁵Cl and 25% is ³⁷Cl. So Aᵣ = (35 × 75 + 37 × 25) ÷ 100 = 35.5.

对于氯，75% 是 ³⁵Cl，25% 是 ³⁷Cl。所以 Aᵣ = (35 × 75 + 37 × 25) ÷ 100 = 35.5。

9. Electronic Configuration | 电子排布

Electrons occupy shells in a fixed order. The first shell holds a maximum of 2 electrons, the second shell holds 8, and the third shell holds 8 (for the first 20 elements).

电子按固定顺序占据壳层。第一壳层最多容纳 2 个电子，第二壳层最多容纳 8 个，第三壳层最多容纳 8 个（对前 20 号元素而言）。

For sodium (Na, Z = 11), the electron configuration is 2,8,1. This tells us that sodium has one electron in its outer shell, which explains why it reacts vigorously with water.

以钠（Na，Z = 11）为例，电子排布为 2,8,1。这说明钠的最外层有 1 个电子，解释了为什么它与水反应剧烈。

10. Shells and Valence Electrons | 壳层与价电子

The electrons in the outermost shell are called valence electrons. These determine how an atom bonds with other atoms.

最外层壳层中的电子称为价电子。它们决定原子如何与其他原子成键。

Atoms want to achieve a stable full outer shell, which is usually 8 electrons (the octet rule). They can gain, lose or share electrons to achieve this.

原子倾向于达到稳定的全满外层壳层，通常是 8 个电子（八隅体规则）。它们可以通过得到、失去或共用电子来实现这一目标。

11. Atomic Structure and the Periodic Table | 原子结构与元素周期表

In the periodic table, elements are arranged in order of increasing atomic number. The number of outer electrons corresponds to the group number (for Groups I to VII), and the number of shells corresponds to the period number.

元素周期表中的元素按原子序数递增排列。最外层电子数对应族号（第 I 族到第 VII 族），壳层数对应周期号。

For example, magnesium (Z = 12) has the configuration 2,8,2. It is in Group II and Period 3.

例如，镁（Z = 12）的电子排布为 2,8,2。它位于第 II 族、第 3 周期。

12. Summary and Exam Tips | 总结与考试提示

You should be able to identify the number of protons, neutrons and electrons from the atomic number and mass number. Remember that in a neutral atom, protons = electrons. Also, be careful with isotope calculations involving relative abundance.

你应该能够根据原子序数和质量数确定质子、中子和电子的数量。记住：在中性原子中，质子数等于电子数。同时，要小心涉及同位素丰度的相对原子质量计算。

When drawing electron shells, always fill the inner shell first. Practise writing electronic configurations for elements in the first 20 elements.

在画电子壳层时，一定要先填满内壳层。练习书写前 20 号元素的电子排布。

Common mistakes: confusing mass number with atomic number, forgetting that electrons have almost no mass, and miswriting isotope symbols. Practise past paper questions to avoid these traps.

常见错误：混淆质量数和原子序数，忘记电子质量几乎为零，写错同位素符号。多练习历年真题，避免这些陷阱。

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Atomic Structure and Isotopes | 原子结构与同位素