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

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

This article covers the core IGCSE Edexcel Science content on atomic structure and the periodic table. You will learn how atoms are built, how the periodic table is arranged, and why elements in the same group share similar properties. Revision tips and common exam traps are included.

本文涵盖 IGCSE Edexcel 科学科目中关于原子结构和元素周期表的核心内容。你将学习原子如何构成、周期表如何排列，以及为什么同一族的元素具有相似的性质。文中还包含复习技巧和常见考试陷阱。

1. The Structure of an Atom | 原子的结构

An atom is the smallest particle of an element that still has the chemical properties of that element. It consists of a central nucleus surrounded by electrons. The nucleus contains protons and neutrons, which are collectively called nucleons.

原子是元素中仍具有该元素化学性质的最小粒子。它由一个位于中心的原子核以及围绕核运动的电子构成。原子核中包含质子和中子，它们统称为核子。

Proton: positive charge (+1), relative mass 1

质子：带正电（+1），相对质量为 1
Neutron: no charge (0), relative mass 1

中子：不带电（0），相对质量为 1
Electron: negative charge (−1), relative mass 1/1836

电子：带负电（−1），相对质量约为 1/1836

Electrons move in energy levels (shells) around the nucleus. The number of protons in an atom is called the atomic number (Z). This determines which element the atom is.

电子在原子核周围的能级（壳层）中运动。原子中质子的数目称为原子序数（Z），它决定了该原子属于哪种元素。

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, the number of protons equals the number of electrons. The mass number (A) is the total number of protons and neutrons in the nucleus.

原子序数（Z）是原子核中的质子数。在电中性原子中，质子数等于电子数。质量数（A）是原子核中质子数和中子数的总和。

Mass number (A) = Number of protons + Number of neutrons

质量数（A）= 质子数 + 中子数

For example, a sodium atom has 11 protons and 12 neutrons, so its mass number is 23. Its atomic number is 11, and a neutral sodium atom has 11 electrons.

例如，一个钠原子有 11 个质子和 12 个中子，因此它的质量数为 23。它的原子序数为 11，电中性钠原子有 11 个电子。

3. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because they have the same number of protons and electrons, they have the same chemical properties. However, their physical properties may differ slightly because their masses are different.

同位素是指同一种元素的原子，它们具有相同的质子数但中子数不同。由于质子数和电子数相同，它们的化学性质相同。但由于质量不同，其物理性质可能略有差异。

For example, chlorine-35 and chlorine-37 are both isotopes of chlorine. Chlorine-35 has 17 protons and 18 neutrons; chlorine-37 has 17 protons and 20 neutrons.

例如，氯-35 和氯-37 都是氯的同位素。氯-35 有 17 个质子和 18 个中子；氯-37 有 17 个质子和 20 个中子。

Same number of protons → same element

质子数相同 → 同一种元素
Different number of neutrons → different mass number

中子数不同 → 质量数不同
Same electron configuration → same chemical behaviour

电子排布相同 → 化学行为相同

Isotopes of the same element may be radioactive, such as carbon-14 used in dating.

同位素可能具有放射性，例如用于测年的碳-14。

4. Electron Configuration | 电子排布

Electrons occupy energy levels (shells) around the nucleus. The first shell can hold up to 2 electrons, the second and third shells can each hold up to 8 electrons. The arrangement of electrons in these shells is called the electron configuration.

电子占据原子核周围的能级（壳层）。第一层最多可容纳 2 个电子，第二层和第三层最多可容纳 8 个电子。电子在壳层中的排列称为电子排布。

Rules for drawing electron configurations:

绘制电子排布的规则：

Fill the inner shells first.

先填满内层。
Once a shell is full, electrons go into the next shell.

一旦某层填满，电子进入下一层。
The number of electrons equals the atomic number (for a neutral atom).

电子数等于原子序数（对于电中性原子）。

Example: Oxygen has 8 electrons. Configuration: 2, 6. This means 2 electrons in the first shell and 6 in the second shell.

例如：氧有 8 个电子。排布为 2, 6，即第一层有 2 个电子，第二层有 6 个电子。

Example: Sodium has 11 electrons. Configuration: 2, 8, 1.

例如：钠有 11 个电子。排布为 2, 8, 1。

5. The Periodic Table | 元素周期表

The periodic table arranges elements in order of increasing atomic number. Elements are arranged in rows called periods and columns called groups. The number of electrons in the outermost shell of an atom determines its group number for groups 1 and 2, and for groups 3 to 7 (using the old numbering system). For example, elements in Group 1 have 1 electron in their outer shell.

元素周期表按原子序数递增的顺序排列元素。元素排列成行，称为周期；排列成列，称为族。对于第 1 族和第 2 族以及第 3 至第 7 族（使用旧编号系统）来说，原子最外层电子数决定了其族数。例如，第 1 族元素的最外层有 1 个电子。

In the modern IGCSE Edexcel specification, groups are often numbered 1 to 8 (or 0). The periodic table also shows the symbol, atomic number, and mass number of each element.

在现代 IGCSE Edexcel 教学大纲中，族通常编号为 1 到 8（或 0）。周期表还显示每种元素的符号、原子序数和质量数。

6. Group 1: Alkali Metals | 第 1 族：碱金属

Group 1 elements (lithium, sodium, potassium, etc.) are called alkali metals. They have one electron in their outer shell, so they are highly reactive. Their reactivity increases down the group because the outer electron is further from the nucleus and is more easily lost.

第 1 族元素（锂、钠、钾等）称为碱金属。它们的最外层有 1 个电子，因此非常活泼。越往下反应性越强，因为最外层电子离原子核越远，更容易失去。

They are soft and have low densities.

它们质地软，密度低。
They have low melting points compared to most metals.

与大多数金属相比，它们的熔点较低。
They react with water to produce hydrogen gas and an alkaline solution.

它们与水反应产生氢气和碱性溶液。
They form ions with a +1 charge.

它们形成带 +1 电荷的离子。

Example reaction with water: 2Na + 2H₂O → 2NaOH + H₂

与水的反应示例：2Na + 2H₂O → 2NaOH + H₂

7. Group 7: Halogens | 第 7 族：卤素

Group 7 elements (fluorine, chlorine, bromine, iodine) are called halogens. They have seven electrons in their outer shell, so they need one more electron to achieve a full outer shell. They are non-metals and form ions with a −1 charge.

第 7 族元素（氟、氯、溴、碘）称为卤素。它们的最外层有 7 个电子，因此需要再获得 1 个电子以形成满外层。它们是非金属，形成带 −1 电荷的离子。

Reactivity decreases down the group.

越往下反应性越弱。
They exist as diatomic molecules (F₂, Cl₂, Br₂, I₂).

它们以双原子分子存在（F₂、Cl₂、Br₂、I₂）。
They react with metals to form ionic salts.

它们与金属反应生成离子盐。
A more reactive halogen can displace a less reactive halogen from a solution of its salt.

更活泼的卤素可以从其盐溶液中置换出较不活泼的卤素。

Example displacement: Cl₂ + 2KBr → 2KCl + Br₂

置换反应示例：Cl₂ + 2KBr → 2KCl + Br₂

8. Group 0: Noble Gases | 第 0 族：稀有气体

Group 0 elements (helium, neon, argon) are called noble gases. They have a full outer shell of electrons (helium has 2; others have 8), making them very stable and unreactive. They exist as single atoms (monatomic). Their boiling points increase down the group.

第 0 族元素（氦、氖、氩）称为稀有气体。它们具有满外层电子（氦有 2 个；其他有 8 个），因此非常稳定，不易反应。它们以单原子形式存在。沸点随原子序数增大而升高。

Because they are unreactive, noble gases were once called “inert gases.” They are used in lighting and welding because they do not react with other materials.

由于稀有气体不活泼，它们曾经被称为“惰性气体”。由于它们不与其他物质反应，常用于照明和焊接。

9. Trends in the Periodic Table | 元素周期表中的规律

Moving across a period from left to right, elements change from metals to non-metals. The number of outer-shell electrons increases by one for each step within the same period. The atomic number increases, and the atomic radius generally decreases across a period because the increased nuclear charge pulls the electrons closer.

从左到右横过一个周期，元素从金属过渡到非金属。在同一周期内，每向右移动一步，最外层电子数增加 1。原子序数增加，原子半径通常在同一周期内减小，因为核电荷增大，将电子拉得更近。

Moving down a group, atoms get larger because extra electron shells are added. For metals, reactivity increases down a group (Group 1); for non-metals, reactivity decreases down a group (Group 7).

在一个族中向下移动，原子变大，因为增加了额外的电子壳层。对于金属，反应性沿族向下增强（第 1 族）；对于非金属，反应性沿族向下减弱（第 7 族）。

10. Common Exam Questions and Tips | 常见考题与技巧

Exam questions often ask you to:

考试常见问题要求你：

Work out the number of protons, neutrons, and electrons from the atomic and mass numbers.

根据原子序数和质量数计算质子数、中子数和电子数。
Draw or interpret electron configurations.

绘制或解释电子排布。
Explain why isotopes have the same chemical properties but different physical properties.

解释为什么同位素具有相同的化学性质但不同的物理性质。
Compare the reactivity of elements in Group 1 or Group 7 with reference to electron loss/gain.

参考电子得失比较第 1 族或第 7 族元素的反应性。
Use the periodic table to predict the properties of an element based on its position.

利用周期表根据元素的位置预测其性质。

Tip 1: For a neutral atom: protons = electrons = atomic number. Mass number = protons + neutrons. So neutrons = mass number − atomic number.

技巧 1：对于电中性原子：质子数 = 电子数 = 原子序数。质量数 = 质子数 + 中子数。因此中子数 = 质量数 − 原子序数。

Tip 2: When writing electron configurations, always list the number of electrons in each shell separated by commas, e.g., 2,8,3.

技巧 2：书写电子排布时，用逗号分隔每个壳层的电子数，例如 2,8,3。

Tip 3: Remember the rule for displacement reactions: a more reactive halogen displaces a less reactive one from its salt solution.

技巧 3：记住置换反应的规律：更活泼的卤素可以从其盐溶液中置换出较不活泼的卤素。

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