The Periodic Table | 元素周期表

📚 The Periodic Table | 元素周期表

The periodic table is the central organising principle of chemistry. It arranges all known elements in order of increasing atomic number and groups them so that elements with similar properties sit together. For IGCSE students, mastering its layout, trends, and patterns is essential for predicting chemical behaviour.

元素周期表是化学的核心组织原则。它将所有已知元素按原子序数递增排列，并将性质相似的元素归入同一族。对于 IGCSE 学生而言，掌握其布局、规律和趋势，是预测化学行为的必备技能。

1. The Structure of the Periodic Table | 周期表的结构

The modern periodic table contains 118 elements arranged in rows called periods and columns called groups. There are 7 periods and 18 groups. Each element is shown with its symbol, atomic number (number of protons), and often its relative atomic mass.

现代周期表包含 118 种元素，排列成横向的周期和纵向的族。共有 7 个周期和 18 个族。每种元素都标有符号、原子序数（质子数），通常还有相对原子质量。

Atomic number = number of protons (small number above symbol)
原子序数 = 质子数（符号上方的小数字）
Group number = number of outer-shell electrons (for main groups)
族数 = 最外层电子数（对于主族元素）
Period number = number of occupied electron shells
周期数 = 已占用的电子层数

2. Periods: Horizontal Rows | 周期：横行的规律

A period is a horizontal row. As you move left to right across a period, the atomic number increases by one for each element. The number of electron shells stays the same, but the number of outer-shell electrons increases from 1 to 8 (except period 1).

周期是横排。从左到右穿过一个周期时，原子序数逐次加 1。电子层数保持不变，最外层电子数从 1 增加到 8（第 1 周期除外）。

Period 2: Li (2,1) → Be (2,2) → B (2,3) → C (2,4) → N (2,5) → O (2,6) → F (2,7) → Ne (2,8)

For example, period 3 elements all have three electron shells: sodium is 2,8,1 and chlorine is 2,8,7. The period number tells you how many shells the element’s atoms possess.

例如，第 3 周期元素都有三个电子层：钠为 2,8,1，氯为 2,8,7。周期数告诉你该元素原子拥有几个电子层。

3. Groups: Vertical Columns | 族：纵列的核心意义

A group is a vertical column. Elements in the same group have the same number of outer-shell electrons, which is why they show similar chemical properties. For main group elements, the group number equals the number of outer-shell electrons.

族是纵列。同一族元素具有相同的最外层电子数，因此表现出相似的化学性质。对于主族元素，族号等于最外层电子数。

Group 1: one outer-shell electron (e.g. Li 2,1)
第 1 族：1 个最外层电子（如 Li 2,1）
Group 2: two outer-shell electrons (e.g. Mg 2,8,2)
第 2 族：2 个最外层电子（如 Mg 2,8,2）
Group 17: seven outer-shell electrons (e.g. Cl 2,8,7)
第 17 族：7 个最外层电子（如 Cl 2,8,7）
Group 18: eight outer-shell electrons (except He with 2)
第 18 族：8 个最外层电子（He 为 2 个）

4. Metals and Non-metals | 金属与非金属

On the periodic table, metals occupy the left and centre, while non-metals are on the right. A diagonal ‘staircase’ line (starting near boron and ending near polonium) separates the two. Elements touching this line are called metalloids (e.g. silicon, germanium).

在周期表中，金属位于左侧和中部，非金属位于右侧。一条从硼附近延伸到钋附近的斜向“阶梯线”将两者分开。紧邻该线的元素称为 半金属（如硅、锗）。

Metals 金属	Non-metals 非金属
Shiny lustre 有光泽	Dull appearance 外观暗淡
Good conductors of heat and electricity 良好的热和电导体	Poor conductors (except graphite) 不良导体（石墨除外）
Malleable and ductile 可锻、可延展	Brittle when solid 固态时脆
High melting points (usually) 通常熔点高	Low melting points (often) 通常熔点低

5. Group 1: Alkali Metals | 第 1 族：碱金属

Group 1 elements (lithium, sodium, potassium, rubidium, caesium, francium) are soft, silvery metals with low density. 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、铷、铯、钫）是质地柔软、呈银白色的低密度金属。它们的活泼性自上而下增强，因为最外层电子离原子核更远，更容易失去。

All alkali metals react with water to produce hydrogen gas and the corresponding metal hydroxide solution:

所有碱金属都能与水反应，生成氢气和相应的金属氢氧化物溶液：

2Na + 2H₂O → 2NaOH + H₂

Lithium reacts gently with water 锂与水反应平缓
Sodium reacts vigorously, melting into a ball 钠剧烈反应并熔成小球
Potassium reacts even more violently, with a lilac flame 钾反应更剧烈，产生淡紫色火焰

Down the group, melting points decrease and atomic radius increases.

沿族向下，熔点降低，原子半径增大。

6. Group 17: Halogens | 第 17 族：卤素

Halogens (fluorine, chlorine, bromine, iodine, astatine) are non-metals with seven outer-shell electrons. They are diatomic molecules: F₂, Cl₂, Br₂, I₂. Their melting and boiling points increase down the group because the molecules are larger, so intermolecular forces become stronger.

卤素（氟、氯、溴、碘、砹）是具有 7 个最外层电子的非金属。它们以双原子分子存在：F₂、Cl₂、Br₂、I₂。它们的熔点和沸点沿族向下升高，因为分子更大，分子间作用力更强。

Halogen 卤素	Colour at room temperature 常温颜色	State at room temperature 常温状态
Fluorine 氟	Pale yellow 淡黄色	Gas 气体
Chlorine 氯	Greenish-yellow 黄绿色	Gas 气体
Bromine 溴	Reddish-brown 红棕色	Liquid 液体
Iodine 碘	Grey-black 灰黑色	Solid 固体

Reactivity decreases down the group. A more reactive halogen can displace a less reactive one from its salt solution.

活泼性沿族向下减弱。较活泼的卤素能把较不活泼的卤素从其盐溶液中置换出来。

Cl₂ + 2NaBr → 2NaCl + Br₂

7. Group 18: Noble Gases | 第 18 族：稀有气体

Noble gases (helium, neon, argon, krypton, xenon, radon) have a full outer shell of electrons (2 for helium, 8 for the rest). This makes them chemically unreactive, or often described as having a stable ‘octet’ configuration. They exist as monatomic gases.

稀有气体（氦、氖、氩、氪、氙、氡）具有全满的最外层电子（氦为 2 个，其余为 8 个）。这使它们在化学上非常不活泼，常被称为具有稳定的“八隅体”构型。它们以单原子气体存在。

Helium is used in balloons and airships 氦用于气球和飞艇
Neon is used in advertising signs 氖用于广告灯牌
Argon is used in welding to provide an inert atmosphere 氩用于焊接时提供惰性气氛

Their boiling points increase slightly down the group as atomic size increases.

它们的沸点沿族向下略微升高，因为原子尺寸增大。

8. Transition Metals | 过渡金属

Transition metals occupy the middle block (Groups 3–12) and include iron, copper, zinc, nickel, and chromium. They are harder, stronger, and have higher melting points than Group 1 metals.

过渡金属位于中部区块（第 3–12 族），包括铁、铜、锌、镍和铬。它们比第 1 族金属更硬、更强，熔点也更高。

Typical properties of transition metals:

过渡金属的典型性质：

Form coloured compounds (e.g. copper(II) sulfate is blue)
形成有色化合物（如硫酸铜(II)呈蓝色）
Can have variable oxidation states (e.g. iron(II) and iron(III))
具有可变化合价（如铁(II)和铁(III)）
Used as efficient catalysts (e.g. iron in Haber process)
常用作高效催化剂（如哈伯法中的铁）

Copper and iron are widely used in electrical wiring and construction because of their strength and conductivity.

铜和铁因其强度和导电性而被广泛用于电线电缆和建筑领域。

9. Periodic Trends in Properties | 元素性质的周期性变化

Several key properties show a clear repeating pattern across the periodic table. These trends help you to compare unknow elements and explain reaction behaviour.

多项关键性质在周期表中呈现出清晰的循环规律。这些趋势帮助你比较未知元素并解释反应行为。

Atomic radius 原子半径: across a period it decreases (because increasing nuclear charge pulls electrons closer); down a group it increases (because more electron shells are added).

原子半径：同一周期从左到右减小（因为核电荷增加，把电子拉得更紧）；同一族从上到下增大（因为电子层数增多）。

Ionisation energy 电离能: the energy needed to remove an outer-shell electron. It generally increases across a period and decreases down a group.

电离能：即移走一个最外层电子所需的能量。通常在同一周期内增大，在同一族内减小。

Electronegativity 电负性: the ability of an atom to attract bonding electrons. It increases across a period and decreases down a group.

电负性：是指原子吸引成键电子的能力。同一周期内增大，同一族内减小。

Metallic character decreases across a period 金属性沿周期向右减弱
Non-metallic character decreases down a group 非金属性沿族向下减弱

10. Using the Periodic Table to Predict Properties | 用周期表预测性质

The periodic table is not just a list; it is a predictive tool. If you know an element’s position, you can predict its valence electrons, the charge of its common ion, its physical state, and even its reactivity.

周期表不只是元素清单，更是预测工具。只要知道元素的位置，你就能预测其最外层电子数、常见离子的电荷、物理状态，甚至反应活性。

For example, calcium is in Group 2 and Period 4. It therefore has two outer-shell electrons, forms a Ca²⁺ ion, and is a reactive alkaline earth metal. Iodine is in Group 17 and Period 5; it has seven outer-shell electrons, forms an I⁻ ion, and is a dark solid halogen.

例如，钙位于第 2 族、第 4 周期。因此它有 2 个最外层电子，形成 Ca²⁺ 离子，是活泼的碱土金属。碘位于第 17 族、第 5 周期：它有 7 个最外层电子，形成 I⁻ 离子，是深色固态卤素。

Group number → outer electrons → ion charge
族号 → 最外层电子数 → 离子电荷

For main group metals, the common ion charge equals the group number. For non-metals, it equals group number minus 8. Sodium (Group 1) forms Na⁺, magnesium (Group 2) forms Mg²⁺, chlorine (Group 17) forms Cl⁻.

对于主族金属，常见离子电荷等于族号。对于非金属，离子电荷等于族号减 8。钠（第 1 族）形成 Na⁺，镁（第 2 族）形成 Mg²⁺，氯（第 17 族）形成 Cl⁻。

11. Common Exam Focus Points | 考试常见考点提醒

Edexcel IGCSE exams frequently test the following periodic table skills:

Edexcel IGCSE 考试经常考查以下周期表技能：

Identifying elements from their electron configurations
根据电子排布识别元素
Comparing reactivity within Group 1 and Group 17
比较第 1 族和第 17 族内部的反应活性
Explaining why noble gases are unreactive
解释为什么稀有气体不活泼
Predicting products of reactions involving alkali metals
预测涉及碱金属反应的产物
Using symbols and atomic numbers to locate elements
使用符号和原子序数定位元素

Memorise the electron configurations of the first 20 elements (e.g. potassium 2,8,8,1 and calcium 2,8,8,2) because these frequently appear in questions.

务必熟记前 20 号元素的电子排布（如钾 2,8,8,1 和钙 2,8,8,2），因为它们经常出现在考题中。

12. Summary: The Periodic Table as a Framework | 总结：周期表是化学的框架

In summary, the periodic table organises elements by atomic number, revealing repeating patterns in electron configuration, physical properties, and chemical reactivity. Periodic numbers give shell counts; group numbers give outer-shell electron counts; metals sit left, non-metals sit right; and trends in radius, energy, and electronegativity follow clear directions.

总而言之，周期表按原子序数组织元素，揭示了电子排布、物理性质和化学活性中的重复规律。周期数给出电子层数；族号给出最外层电子数；金属在左，非金属在右；而半径、能量和电负性的变化方向清晰明确。

Mastering this framework will make every subsequent chemistry topic—bonding, reactions, acids, electrochemistry—much easier to understand and recall.

掌握这个框架，后续的每一个化学主题——成键、反应、酸、电化学——都会变得更加容易理解和记忆。

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The Periodic Table | 元素周期表