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

At the heart of all chemistry lies the atom. Understanding its structure is essential for explaining the behaviour of elements, the formation of compounds, and the patterns we see in the periodic table. This article breaks down the key concepts you need for your Edexcel IGCSE Science examination.

一切化学的核心都是原子。理解原子的结构，是解释元素行为、化合物形成以及元素周期表规律的基础。本文将为你梳理 Edexcel IGCSE 科学考试中必须掌握的关键概念。

1. Sub-Atomic Particles | 亚原子粒子

Every atom is made of three fundamental particles: protons, neutrons and electrons. Protons carry a positive charge, neutrons are neutral, and electrons carry a negative charge. Protons and neutrons are packed together in the nucleus, while electrons orbit the nucleus in shells (energy levels).

每个原子都由三种基本粒子组成：质子、中子和电子。质子带正电荷，中子不带电，电子带负电荷。质子和中子紧密地聚集在原子核中，而电子则在核外的壳层（能级）中运动。

Proton: charge +1, mass approximately 1 atomic mass unit.
Neutron: charge 0, mass approximately 1 atomic mass unit.
Electron: charge -1, mass approximately 1/1836 of a proton.

质子：电荷 +1，质量约为 1 个原子质量单位。
中子：电荷 0，质量约为 1 个原子质量单位。
电子：电荷 -1，质量约为质子的 1/1836。

In a neutral atom, the number of protons equals the number of electrons. This balance ensures the overall charge is zero.

在电中性的原子中，质子数等于电子数。这种平衡保证了原子整体呈电中性。

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 of an atom. It defines the element. The mass number (A) is the total number of protons and neutrons in the nucleus.

原子序数（Z）是指原子核中的质子数，它决定了元素的种类。质量数（A）是指原子核中质子数和中子数之和。

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

质量数（A）= 质子数（Z）+ 中子数

To find the number of neutrons, simply subtract the atomic number from the mass number: neutrons = A − Z.

要计算中子数，只需用质量数减去原子序数：中子数 = A − Z。

Particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	−1	1/1836

粒子	相对电荷	相对质量
质子	+1	1
中子	0	1
电子	−1	1/1836

A common exam question asks for the number of protons, neutrons and electrons in an atom such as aluminium, which has Z = 13 and A = 27. The answer: 13 protons, 13 electrons and 14 neutrons (27 − 13).

一个常见的考试题是要求计算铝原子的质子、中子、电子数目。铝的 Z = 13，A = 27。答案是：13 个质子，13 个电子，14 个中子（27 − 13）。

3. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They therefore have the same atomic number but different mass numbers.

同位素是指同一种元素中具有相同质子数但不同中子数的原子。因此，它们的原子序数相同，但质量数不同。

For example, carbon-12 and carbon-14 are isotopes of carbon. Both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons.

例如，碳-12 和碳-14 是碳的同位素。它们都有 6 个质子，但碳-12 有 6 个中子，碳-14 有 8 个中子。

Isotopes have the same chemical properties because chemical behaviour depends on the number of electrons.
Isotopes may have different physical properties, such as density and rate of diffusion.
Some isotopes are radioactive and decay over time.

同位素的化学性质相同，因为化学行为取决于电子数目。
同位素的物理性质可能不同，如密度和扩散速率。
有些同位素具有放射性，会随时间衰变。

In the periodic table, the mass number shown is usually the relative atomic mass, which takes into account the abundance of each isotope.

元素周期表中显示的质量数通常是相对原子质量，它考虑了各同位素的天然丰度。

4. Electronic Configurations | 电子排布

Electrons are arranged in shells around the nucleus. Each shell has a maximum capacity: the first shell holds up to 2 electrons, the second shell holds up to 8, and the third shell holds up to 8 (at IGCSE level, we usually stop at 8 for the third shell, though in reality it can hold more).

电子在原子核周围的壳层中排列。每个壳层有最大容纳数：第一层最多 2 个电子，第二层最多 8 个，第三层在 IGCSE 阶段通常也按最多 8 个处理（尽管实际上第三层可以容纳更多）。

Shell 1: 2, Shell 2: 8, Shell 3: 8

第一层：2，第二层：8，第三层：8

To write an electronic configuration, fill the shells from the lowest energy level outward. For example, sodium (Na) has 11 electrons: 2, 8, 1. Chlorine (Cl) has 17 electrons: 2, 8, 7.

书写电子排布时，从能量最低的壳层开始向外填充。例如，钠（Na）有 11 个电子：2, 8, 1。氯（Cl）有 17 个电子：2, 8, 7。

You may also see this written as 2.8.1 or 2,8,7. In diagrams, each shell is drawn as a circle with dots or crosses representing electrons.

你可能会看到 2.8.1 或 2,8,7 的写法。在示意图中，每个壳层画成圆圈，用点或叉表示电子。

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

The periodic table arranges elements in order of increasing atomic number. It is divided into periods (horizontal rows) and groups (vertical columns). Elements in the same group have the same number of outer-shell electrons, which gives them similar chemical properties.

元素周期表按照原子序数递增的顺序排列元素。它分为周期（横排）和族（纵列）。同一族的元素具有相同的最外层电子数，因此化学性质相似。

Group 1: Alkali metals – 1 outer electron, reactive metals.
Group 2: Alkaline earth metals – 2 outer electrons.
Group 7: Halogens – 7 outer electrons, reactive non-metals.
Group 0: Noble gases – full outer shell, unreactive.

第 1 族：碱金属 – 最外层 1 个电子，活泼金属。
第 2 族：碱土金属 – 最外层 2 个电子。
第 7 族：卤素 – 最外层 7 个电子，活泼非金属。
第 0 族：稀有气体 – 最外层电子全满，性质不活泼。

The group number tells you the number of electrons in the outer shell (for groups 1, 2 and 7; for group 0 it is 8 for helium, or 8 for the others, though helium has 2). The period number tells you how many shells the atom has.

族号告诉你最外层电子数（对于第 1、2、7 族；第 0 族中氦为 2，其余为 8）。周期号告诉你有几个电子壳层。

6. Group 1: Alkali Metals | 第 1 族：碱金属

Alkali metals (Li, Na, K, Rb, Cs, Fr) are soft, low-density metals that react vigorously with water and halogens. They all have one electron in their outermost shell.

碱金属（Li、Na、K、Rb、Cs、Fr）是质地软、密度较低的金属，与水、卤素反应剧烈。它们的最外层都只有一个电子。

Going down the group, the reactivity increases because:

从上到下，反应活性增强，原因在于：

Atomic radius increases
Outer electron is further from the nucleus
Shielding increases, so the outer electron is lost more easily

原子半径增大
最外层电子离原子核更远
屏蔽效应增强，因此最外层电子更容易失去

For example, potassium reacts more violently with water than sodium. Both produce hydrogen gas and an alkaline hydroxide solution.

例如，钾与水的反应比钠更剧烈。两者都产生氢气和碱性的氢氧化物溶液。

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)（钠与水反应生成氢氧化钠和氢气）

7. Group 7: Halogens | 第 7 族：卤素

Halogens (F, Cl, Br, I, At) are non-metals with seven outer electrons. They exist as diatomic molecules (F₂, Cl₂, Br₂, I₂). Their physical state changes from gas (fluorine, chlorine) to liquid (bromine) to solid (iodine) at room temperature as you go down the group.

卤素（F、Cl、Br、I、At）是最外层有 7 个电子的非金属，以双原子分子形式存在（F₂、Cl₂、Br₂、I₂）。在室温下，随着族往下，物理状态从气态（氟、氯）变为液态（溴）再变为固态（碘）。

Reactivity decreases down the group. Fluorine is the most reactive halogen. This is because going down, the atoms become larger and the attraction for an extra electron becomes weaker, so gaining an electron becomes harder.

卤素的反应活性从上到下递减。氟是最活泼的卤素。因为越往下原子越大，对额外电子的吸引力越弱，因此获得电子变得越难。

Halogens can displace less reactive halogens from their compounds. For example, chlorine will displace bromine from potassium bromide solution.

卤素可以把较不活泼的卤素从其化合物中置换出来。例如，氯可以将溴从溴化钾溶液中置换出来。

Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)

Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)（氯置换溴）

8. Group 0: Noble Gases | 第 0 族：稀有气体

Noble gases (He, Ne, Ar, Kr, Xe, Rn) have a full outer shell of electrons. This makes them very stable and unreactive. They are monatomic gases with very low boiling points.

稀有气体（He、Ne、Ar、Kr、Xe、Rn）最外层电子全满，因此非常稳定，几乎不参与反应。它们是单原子气体，沸点极低。

Because of their inertness, they are used in applications where reactions are unwanted, such as filling light bulbs (argon) or providing an inert atmosphere in welding (helium).

由于它们的惰性，常用于需要避免反应的场合，例如填充灯泡（氩）或焊接时提供惰性保护气氛（氦）。

The boiling point of noble gases increases down the group.
For helium, the outer shell is full with only 2 electrons.
For neon and the others, the outer shell has 8 electrons.

稀有气体的沸点随原子序数增大而升高。
氦的最外层只有 2 个电子即达到全满。
氖及其他稀有气体最外层有 8 个电子。

Noble gases are often called ‘inert gases’ because they do not easily form compounds.

稀有气体常被称为“惰性气体”，因为它们不易形成化合物。

9. Ions and the Octet Rule | 离子与八隅体规则

Atoms gain or lose electrons to achieve a full outer shell, usually the configuration of a noble gas. This is called the octet rule. When atoms lose or gain electrons, they become charged particles called ions.

原子通过得失电子达到稳定的最外层电子结构，通常是稀有气体的电子排布，这称为“八隅体规则”。当原子失去或获得电子时，就变成了带电粒子——离子。

Metals tend to lose electrons to form positive ions (cations). Non-metals tend to gain electrons to form negative ions (anions). For example:

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：

Sodium loses 1 electron → Na⁺
Magnesium loses 2 electrons → Mg²⁺
Chlorine gains 1 electron → Cl⁻
Oxygen gains 2 electrons → O²⁻

钠失去 1 个电子 → Na⁺
镁失去 2 个电子 → Mg²⁺
氯获得 1 个电子 → Cl⁻
氧获得 2 个电子 → O²⁻

The charge of an ion is determined by the difference between the number of protons and electrons. For example, a sodium atom has 11 protons and 11 electrons; after losing one electron, it has 11 protons and 10 electrons, giving a net charge of +1.

离子的电荷由质子数与电子数之差决定。例如，钠原子有 11 个质子和 11 个电子；失去一个电子后，它有 11 个质子和 10 个电子，净电荷为 +1。

10. Forming Ionic Compounds | 形成离子化合物

Ionic compounds are formed when a metal transfers electrons to a non-metal. The resulting positive and negative ions attract each other strongly through electrostatic forces. This is called an ionic bond.

离子化合物由金属向非金属转移电子而形成。带正电和带负电的离子通过强烈的静电力相互吸引，这种作用力称为离子键。

For example, sodium chloride (NaCl) forms when sodium gives its outer electron to chlorine. The formula NaCl represents the ratio of ions in the giant ionic lattice: one Na⁺ for every Cl⁻.

例如，氯化钠（NaCl）由钠将最外层电子转移给氯而形成。化学式 NaCl 表示巨大离子晶格中离子的比例：一个 Na⁺ 对应一个 Cl⁻。

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

Na → Na⁺ + e⁻（钠失去电子）

Cl + e⁻ → Cl⁻（氯得到电子）

Ionic compounds have high melting points, are usually soluble in water, and conduct electricity when molten or dissolved in aqueous solution because the ions are free to move.

离子化合物熔点高，通常溶于水，在熔融状态或水溶液中由于离子可自由移动而能导电。

11. Forming Covalent Compounds | 形成共价化合物

Non-metals can also bond by sharing electrons. When two atoms share one or more pairs of electrons, a covalent bond forms. The shared pair of electrons counts as part of the outer shell of both atoms, allowing each to achieve a stable configuration.

非金属原子之间也可以通过共享电子成键。当两个原子共享一对或多对电子时，就形成共价键。共享的电子对计入两个原子的最外层，使每个原子都达到稳定结构。

Examples include water (H₂O), carbon dioxide (CO₂), methane (CH₄) and hydrochloric acid (HCl).

例如水（H₂O）、二氧化碳（CO₂）、甲烷（CH₄）和氯化氢（HCl）。

Molecule	Shared electron pairs	Dot-and-cross diagram
Cl₂	1	:Cl· ·Cl:
O₂	2 (double bond)	:O::O:

分子	共享电子对数	点叉示意图
Cl₂	1	:Cl· ·Cl:
O₂	2（双键）	:O::O:

Simple covalent compounds have low melting points, are often gases or volatile liquids, do not conduct electricity, and may or may not be soluble in water.

简单共价化合物熔点低，常为气体或易挥发液体，不导电，在水中的溶解性不一。

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

Here are some quick revision tips and traps to avoid in the Edexcel IGCSE examination.

以下是一些快速复习技巧和考试中需要避免的陷阱。

Always remember: protons define the element. Changing the number of neutrons does not change which element it is.
For ion formation, use the periodic table group number to decide the charge: Group 1 forms +1, Group 2 forms +2, Group 6 forms −2, Group 7 forms −1.
The number of electrons in an ion is not the same as the number of protons. Count carefully.
Writing electronic configurations: check the atomic number first. Misreading 17 as 7 is a common error.
When drawing dot-and-cross diagrams, electrons in a shared pair must come from different atoms (except coordinate bonds, which are not in this syllabus).
For displacement reactions, a more reactive halogen can displace a less reactive one, but never vice versa.

始终记住：质子决定元素种类。改变中子数不会改变元素。
根据周期表族号判断离子电荷：第 1 族形成 +1，第 2 族形成 +2，第 6 族形成 −2，第 7 族形成 −1。
离子中的电子数不等于质子数，计算时要小心。
书写电子排布时，先确认原子序数。把 17 看成 7 是常见错误。
画点叉图时，共享电子对中的两个电子必须分别来自两个不同的原子。
置换反应中，较活泼的卤素可以置换较不活泼的，但反过来不行。

Make sure you practise with past papers, especially questions that ask you to explain trends in groups 1 and 7, and to draw electronic configurations with 18 electrons or fewer.

请务必多练习往年真题，特别是解释第 1 族和第 7 族趋势的题目，以及画出 18 个电子以内原子的电子排布。

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