Atoms and the Periodic Table | 原子与元素周期表

📚 Atoms and the Periodic Table | 原子与元素周期表

Atoms are the building blocks of all matter. Understanding their structure and how they are arranged in the periodic table is a fundamental requirement for every IGCSE science student. This article covers the key concepts you need to master for your Edexcel IGCSE exam, with clear explanations in both English and Chinese.

原子是所有物质的基本组成单位。理解原子的结构以及它们在元素周期表中的排列方式，是每一位 IGCSE 科学学生都必须掌握的基础内容。本文专为 Edexcel IGCSE 考试设计，用中英双语清晰讲解核心考点。

1. The Structure of an Atom | 原子的结构

Every atom contains a central nucleus made of protons and neutrons. Electrons move around the nucleus in energy shells (energy levels). Protons are positively charged, electrons are negatively charged, and neutrons have no charge.

每个原子都有一个由质子和中子组成的原子核，电子在核外能量壳层（能级）中运动。质子带正电荷，电子带负电荷，中子不带电荷。

The three subatomic particles have different masses and charges:

三种亚原子粒子的质量和电荷各不相同：

Proton: relative mass = 1, relative charge = +1

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

中子：相对质量 = 1，相对电荷 = 0
Electron: relative mass = 1/1840 (often taken as 0), relative charge = −1

电子：相对质量 ≈ 1/1840（近似为 0），相对电荷 = −1

Atom = Protons + Neutrons + Electrons

2. 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. In a neutral atom, this equals the number of electrons. 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。在电中性原子中，它等于电子数。质量数（A）是质子数和中子数的总和。

Number of neutrons = Mass number − Atomic number

For example, a sodium atom with atomic number 11 and mass number 23 has:

例如，原子序数为 11、质量数为 23 的钠原子：

Protons = 11

质子 = 11
Electrons = 11

电子 = 11
Neutrons = 23 − 11 = 12

中子 = 23 − 11 = 12

Remember: the symbol for an atom is written with the mass number above and the atomic number below the element’s symbol.

请记住：表示原子时，质量数写在元素符号的左上角，原子序数写在左下角。

3. Isotopes | 同位素

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.

同位素是指同一种元素中，质子数相同而中子数不同的原子。因此它们的原子序数相同，但质量数不同。

For example, carbon has three common isotopes: carbon-12, carbon-13, and carbon-14. Each has 6 protons, but the number of neutrons is 6, 7, and 8 respectively.

例如，碳有三种常见同位素：碳-12、碳-13 和碳-14。它们都有 6 个质子，但中子数分别为 6、7 和 8。

Isotopes of the same element have the same chemical properties because they have the same number of electrons. However, their physical properties may differ, such as density or mass.

由于同位素电子数相同，所以化学性质相同，但物理性质（如密度、质量）可能不同。

Some isotopes are radioactive, which means they decay over time and emit radiation. This property is used in medicine, archaeology (carbon dating), and industry.

一些同位素具有放射性，即随时间衰变并释放辐射。该性质可用于医学、考古（碳定年法）和工业。

4. Electronic Configurations | 电子排布

Electrons occupy electron 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 (for the first 20 elements).

电子围绕原子核分布在电子壳层（能级）中。第一层最多容纳 2 个电子，第二层最多 8 个，第三层对于前 20 号元素最多 8 个。

You should be able to write the electronic configuration for elements 1–20 using the shell notation, for example:

你需要会写出前 20 号元素的电子排布（壳层表示法），例如：

Oxygen (8 electrons): 2, 6

氧（8 个电子）：2, 6
Calcium (20 electrons): 2, 8, 8, 2

钙（20 个电子）：2, 8, 8, 2

The electronic configuration explains how atoms gain, lose, or share electrons to form chemical bonds.

电子排布解释了原子如何获得、失去或共用电子以形成化学键。

5. The Periodic Table Layout | 元素周期表布局

The periodic table arranges all known elements in order of increasing atomic number. It has rows called periods and columns called groups.

元素周期表按照原子序数递增的顺序排列所有已知元素。每一行称为周期，每一列称为族。

The table is divided into metals (on the left and center) and non-metals (on the right, plus hydrogen). A staircase line separates metals from non-metals.

周期表分为金属（左侧和中部）和非金属（右侧，外加氢）。一条阶梯线将金属与非金属分开。

Feature	Period (周期)	Group (族)
Arrangement	Horizontal row	Vertical column
Significance	Number of electron shells	Number of outer electrons

6. Groups and Periods | 族与周期

The group number tells you the number of electrons in the outer shell of an atom. For example, all elements in Group 1 have one outer electron, while Group 7 elements have seven outer electrons.

族号告诉你原子最外层电子数。例如，第 1 族元素最外层有 1 个电子，而第 7 族元素最外层有 7 个电子。

The period number tells you the number of electron shells used by the atom. An element in period 3 has three occupied shells.

周期号告诉你原子占据的电子壳层数。第 3 周期的元素有 3 个电子壳层。

Elements in the same group have similar chemical properties because they have the same number of outer electrons. This is why the periodic table is so powerful for predicting chemical behavior.

同族元素具有相似的化学性质，因为它们最外层电子数相同。这就是周期表能够预测化学行为的原因。

7. Group 1 Elements: Alkali Metals | 第1族元素：碱金属

Group 1 elements (lithium, sodium, potassium, etc.) are known as alkali metals. They are soft, shiny, and have low densities compared to most metals.

第 1 族元素（锂、钠、钾等）被称为碱金属。它们质地软、有光泽，密度比大多数金属小。

Alkali metals react vigorously with water to produce hydrogen gas and an alkaline solution. For example:

碱金属与水剧烈反应，生成氢气和碱性溶液。例如：

2Na + 2H₂O → 2NaOH + H₂

Reactivity increases down the group. This is because the outer electron is further from the nucleus, so it is held less tightly and is lost more easily.

反应活性自上而下增强。原因是外层电子离原子核更远，受原子核束缚更弱，更容易失去。

Alkali metals form ionic compounds with non-metals, losing their single outer electron to form a +1 ion (e.g., Na⁺, K⁺).

碱金属与非金属形成离子化合物，失去最外层 1 个电子形成 +1 离子（如 Na⁺、K⁺）。

8. Group 7 Elements: Halogens | 第7族元素：卤素

Group 7 elements (fluorine, chlorine, bromine, iodine, astatine) are called halogens. They exist as molecules with two atoms, such as Cl₂, Br₂, and I₂.

第 7 族元素（氟、氯、溴、碘、砹）被称为卤素。它们以双原子分子存在，如 Cl₂、Br₂、I₂。

Halogens are non-metals with colored vapors. Chlorine is a green gas, bromine is a red-brown liquid, and iodine is a grey-black solid.

卤素是有色蒸汽的非金属。氯是黄绿色气体，溴是红棕色液体，碘是紫黑色固体。

Halogens react with metals to form ionic compounds. For example, sodium + chlorine → sodium chloride (NaCl).

卤素与金属反应生成离子化合物。例如，钠 + 氯 → 氯化钠（NaCl）。

Halogens also react with hydrogen to form hydrogen halides, which dissolve in water to form acidic solutions.

卤素也能与氢反应生成卤化氢，溶于水形成酸性溶液。

Reactivity decreases down the group. Fluorine is the most reactive halogen because it gains an electron most easily due to its small atomic size and strong nuclear attraction.

反应活性自上而下减弱。氟是最活泼的卤素，因为它的原子半径小、核引力强，最容易获得电子。

9. Group 0 Elements: Noble Gases | 第0族元素：稀有气体

Group 0 elements (helium, neon, argon, krypton, xenon) are called noble gases. They are colorless, odorless, and unreactive.

第 0 族元素（氦、氖、氩、氪、氙）被称为稀有气体。它们无色、无味、化学性质不活泼。

Noble gases have very low boiling points and exist as single atoms (monatomic). They have a full outer shell of electrons, so they do not need to gain, lose, or share electrons.

稀有气体的沸点很低，以单原子形式存在。它们最外层电子已满，因此不需要获得、失去或共用电子。

This full outer shell makes the noble gases extremely stable, which is why they are used in inert environments, such as argon in welding and neon in advertising signs.

满壳层使稀有气体非常稳定，因此可用于惰性环境，例如焊接中使用的氩气、广告灯中使用氖气。

Helium is lighter than air and is used in balloons and airships. Neon produces a red-orange glow when electricity passes through it.

氦气比空气轻，用于气球和飞艇。氖气通电时发出红橙色光。

10. Trends in Reactivity | 反应活性的趋势

Reactivity trends are a popular exam question. Always remember:

反应活性趋势是常见考点。请牢记：

Group 1 (metals): reactivity increases down the group.

第 1 族（金属）：反应活性自上而下增强。
Group 7 (non-metals): reactivity decreases down the group.

第 7 族（非金属）：反应活性自上而下减弱。

For metals, the tendency is to lose electrons. As atoms get larger, the outer electron is further away and more shielded, making it easier to lose.

对于金属，失去电子是趋势。原子越大，外层电子距离越远，屏蔽效应越强，越容易失去电子。

For non-metals, the tendency is to gain electrons. Smaller atoms attract electrons more strongly, so reactivity increases as atomic radius decreases.

对于非金属，获得电子是趋势。原子越小，对电子的吸引力越强，因此原子半径越小时反应活性越高。

You should be able to compare these trends using the periodic table and explain the reasons in terms of atomic structure.

你应该能够利用周期表比较这些趋势，并从原子结构角度解释原因。

11. Summary and Exam Tips | 总结与考试技巧

To score well in the Edexcel IGCSE exam, make sure you know:

为了在 Edexcel IGCSE 考试中取得好成绩，请确保掌握：

Relative charges and masses of protons, neutrons, and electrons.

质子、中子、电子的相对电荷与相对质量。
How to calculate the number of neutrons from mass number and atomic number.

如何从质量数和原子序数计算中子数。
How to write electronic configurations up to element 20.

如何书写前 20 号元素的电子排布。
The meaning of group number and period number.

族号和周期号的含义。
Properties and reactivity trends of Groups 1, 7, and 0.

第 1、7、0 族的性质和反应活性趋势。

Practice drawing electron shell diagrams and writing real-world applications for noble gases and isotopes. These are frequent extended-response questions.

练习绘制电子壳层图，并写出稀有气体和同位素的实际应用。这些是常见的扩展题。

Good luck with your revision!

祝你复习顺利！

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Atoms and the Periodic Table | 原子与元素周期表