The Periodic Table | 元素周期表

📚 The Periodic Table | 元素周期表

The Periodic Table is a cornerstone of chemistry and a central topic in the Edexcel IGCSE Science syllabus. It organises all known elements in a way that reveals patterns in their properties and reactions. Mastering this topic is essential for exam success, as questions on the periodic table appear in virtually every session.

元素周期表是化学的基石，也是 Edexcel IGCSE 科学大纲中的核心主题。它以揭示元素性质和反应规律的方式排列了所有已知元素。掌握这一主题对考试成功至关重要，因为有关周期表的题目几乎出现在每一次考试中。

1. The History of the Periodic Table | 元素周期表的历史

In the 19th century, scientists sought to classify the known elements. John Newlands proposed the Law of Octaves in 1864, arranging elements in order of atomic mass and noticing that every eighth element had similar properties. However, his pattern broke down after calcium and did not leave gaps for undiscovered elements.

在19世纪，科学家们试图对已知元素进行分类。1864年，约翰·纽兰兹提出了“八音律”，按原子质量排列元素，并注意到每第八个元素具有相似的性质。然而，他的规律在钙之后就不成立了，而且他没有为未发现的元素留出空位。

Dmitri Mendeleev improved upon this by leaving gaps in his table for undiscovered elements and using the properties of surrounding elements to predict their behaviour. When gallium and germanium were discovered later, matching his predictions, his table gained widespread acceptance.

德米特里·门捷列夫在此基础上做了改进，他在表中为未发现的元素留出空位，并根据周围元素的性质预测这些元素的行为。当镓和锗后来被发现并符合他的预测时，他的周期表获得了广泛认可。

Today’s modern periodic table is arranged in order of increasing atomic number (proton number), not atomic mass. This corrected a few anomalies in Mendeleev’s table, such as tellurium and iodine, where atomic mass order conflicted with chemical property patterns.

现代周期表是按原子序数（质子数）递增的顺序排列的，而非原子质量。这修正了门捷列夫表中的一些异常情况，例如碲和碘，在这两个元素中，原子质量的顺序与化学性质的规律相冲突。

2. Periods and Groups | 周期与族

The periodic table is organised into horizontal rows called periods and vertical columns called groups. Each period corresponds to the filling of a new electron shell, while each group contains elements with the same number of outer-shell electrons.

周期表由称为周期的横行和称为族的纵列组成。每个周期对应一个新的电子壳层被填充，而每个族包含具有相同最外层电子数的元素。

Elements in the same group have the same number of outer-shell electrons, giving them similar chemical properties.

同一族的元素具有相同的最外层电子数，因此具有相似的化学性质。
Elements in the same period have the same number of electron shells.

同一周期的元素具有相同数量的电子壳层。

Group number = number of outer-shell electrons

族号 = 最外层电子数

3. The Group Number Pattern | 族号的规律

In the modern periodic table, the group number for Groups I and II equals the number of outer-shell electrons. For Groups III to VIII, this relationship also holds when reading the digits directly. For example, a Group I element like sodium (Na) has one outer-shell electron, while a Group VII element like chlorine (Cl) has seven outer-shell electrons.

在现代周期表中，第 I 族和第 II 族的族号等于最外层电子数。对于第 III 族到第 VIII 族，直接读取数字也同样适用。例如，钠（Na）是第 I 族元素，有一个最外层电子，而氯（Cl）是第 VII 族元素，有七个最外层电子。

Group	Outer-shell electrons	Example
I	1	Li, Na, K
II	2	Mg, Ca
VII	7	F, Cl, Br
VIII	8 (or 2 for He)	He, Ne, Ar

4. Group I — The Alkali Metals | 第 I 族——碱金属

Group I elements, known as the alkali metals, include lithium (Li), sodium (Na), and potassium (K). They are soft, low-density metals that react vigorously with cold water to produce alkaline solutions and hydrogen gas.

第 I 族元素称为碱金属，包括锂（Li）、钠（Na）和钾（K）。它们是柔软的、低密度的金属，能与冷水剧烈反应，生成碱溶液和氢气。

2Na + 2H₂O → 2NaOH + H₂

As you descend Group I, the elements become more reactive. This is because the outer-shell electron is further from the nucleus and more strongly shielded by inner shells, making it easier to lose. The atomic radius also increases down the group.

随着第 I 族向下移动，元素的反应活性增强。这是因为最外层电子离原子核更远，内层电子的屏蔽效应更强，使得失去这个电子更容易。同时，原子半径沿族向下增大。

5. Group VII — The Halogens | 第 VII 族——卤素

Group VII elements are called the halogens and include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). They exist as diatomic molecules (F₂, Cl₂, Br₂, I₂) and are non-metals with colour and state that vary with atomic size.

第 VII 族元素称为卤素，包括氟（F）、氯（Cl）、溴（Br）和碘（I）。它们以双原子分子存在（F₂、Cl₂、Br₂、I₂），是非金属，其颜色和状态随原子大小而变化。

Down the group, melting and boiling points increase as the molecules become larger and the van der Waals’ forces between molecules become stronger. Reactivity, however, decreases because it becomes harder for a larger atom to gain an electron — the incoming electron is further from the nucleus and experiences more shielding.

沿族向下，熔点和沸点升高，因为分子变大，分子间的范德华力增强。但反应活性降低，因为较大原子获得电子的难度更大——进入的电子离核更远，受到的屏蔽更强。

A key exam skill is predicting displacement reactions: a more reactive halogen will displace a less reactive halogen from its salt solution.

一个关键的考试技能是预测置换反应：更活泼的卤素可以将其盐溶液中的较不活泼卤素置换出来。

Cl₂ + 2KBr → 2KCl + Br₂

6. Group VIII — The Noble Gases | 第 VIII 族——稀有气体

The noble gases (He, Ne, Ar) are chemically inert because their outer shells are full. They exist as monatomic gases and are odourless and colourless. Their boiling points increase down the group due to stronger intermolecular forces.

稀有气体（He、Ne、Ar）由于最外层电子已满，化学性质稳定。它们以单原子气体存在，无臭无色。它们的沸点沿族向下升高，因为分子间作用力增强。

Exam questions may ask about their uses: helium is used in balloons and airships because of its low density and non-flammability; argon is used in welding as an inert shield, and in light bulbs to prevent the filament from oxidising.

考试题目可能会问及它们的用途：氦气用于气球和飞艇，因为密度低且不可燃；氩气在焊接中用作惰性保护气，并用于灯泡中防止灯丝氧化。

Full outer shell → stable electron configuration → inert behaviour

最外层填满 → 稳定电子构型 → 惰性行为

7. Metals and Non-Metals | 金属与非金属

The periodic table is split by a stepped line beginning near boron. Elements to the left of this line are metals; those to the right are non-metals. Some elements like silicon are metalloids, showing properties of both.

周期表被一条从硼附近开始的阶梯线分为两部分。该线左侧的元素是金属，右侧的元素是非金属。有些元素如硅是准金属（类金属），表现出两者的部分性质。

Metals tend to lose electrons in reactions, forming positive ions, whereas non-metals tend to gain electrons, forming negative ions or sharing them in covalent bonds. This electron-transfer model explains ionic compound formation and is frequently tested.

金属在反应中趋向于失去电子，形成正离子，而非金属趋向于获得电子，形成负离子或通过共价键共享电子。这种电子转移模型解释了离子化合物的形成，是常考的内容。

8. Periodic Trends in the Periodic Table | 周期表中的周期趋势

Across a period (left to right), elements change from metallic to non-metallic character. Nuclear charge increases while electron shielding remains constant, so outer-shell electrons are pulled in more tightly. This makes atomic radius decrease, ionisation energy increase, and elements become less reactive as metals and more reactive as non-metals.

在同一周期内（从左到右），元素从金属性过渡到非金属性。核电荷增加而电子屏蔽不变，因此最外层电子被更紧密地吸引。这使得原子半径减小，电离能增大，金属的反应活性减弱而非金属的反应活性增强。

Trend	Across a Period	Down a Group
Atomic radius	Decreases	Increases
Metallic character	Decreases	Increases
Outer-shell electrons	Increases by 1	Same
Electron shells	Same	Increases by 1

9. Predicting Properties of Unknown Elements | 预测未知元素的性质

A common extended-response question asks you to predict the properties of an element based on its position in the table. To score full marks, you must link the group and period to electron configuration, then relate that configuration to physical and chemical properties.

常见的拓展题要求你根据元素在周期表中的位置预测其性质。要得满分，你必须将族和周期与电子构型联系起来，然后将该构型与物理和化学性质联系起来。

For example, if element X is in Group II and Period 3, it will have three shells and two outer electrons. It will be a metal, lose two electrons when reacting, and form an ion with a charge of 2⁺. You can also predict the formula of its oxide as XO.

例如，如果元素 X 位于第 II 族、第 3 周期，它将有三个壳层和两个最外层电子。它是一种金属，反应时会失去两个电子，形成电荷为 2⁺ 的离子。你还可以预测其氧化物分子式为 XO。

10. Exam Tips and Common Pitfalls | 考试技巧与常见误区

Students often lose marks by confusing group and period: a period number equals the number of shells, while a group number equals the number of outer-shell electrons. Another common error is stating that atoms gain or lose “electrons” without specifying the number involved.

学生常在周期与族之间混淆：周期数等于壳层数，而族数等于最外层电子数。另一个常见错误是只说到原子“获得或失去电子”，而没有说明具体数量。

Always quote precise electron configurations such as 2, 8, 1 for sodium.

务必引用精确的电子构型，如钠的 2, 8, 1。
Describe trends in terms of atomic radius, not just “size”.

描述趋势时使用“原子半径”而非笼统的“大小”。
Use full word equations or balanced symbol equations when asked.

被要求时务必写出完整的文字方程式或配平的符号方程式。
Remember that the noble gases are unreactive due to a full outer shell — not because they are “stable” without explanation.

记住稀有气体的不活泼性是因为最外层填满——不能只写“稳定”而不加解释。

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The Periodic Table | 元素周期表