📚 The Periodic Table of Elements | 元素周期表
The periodic table is one of the most important tools in chemistry. It arranges all known elements in order of increasing atomic number, showing patterns in their properties and electronic structures. For IGCSE students, understanding the periodic table is essential for predicting how elements react and why they form particular compounds.
元素周期表是化学中最重要的工具之一。它按照原子序数递增的顺序排列所有已知元素，展示元素性质和电子结构的规律。对 IGCSE 学生来说，理解周期表是预测元素反应行为及其形成特定化合物的关键。
1. What is the Periodic Table? | 什么是元素周期表？
The periodic table is a chart that organises every known element. Elements are placed in order of increasing atomic number, which is the number of protons in the nucleus of an atom. The table is called 'periodic' because similar chemical properties appear at regular intervals.
周期表是一张整理所有已知元素的图表。元素按照原子序数递增排列，原子序数即原子核中的质子数。它被称为 '周期' 表，因为相似的化学性质会周期性重复出现。
Each element is shown by its chemical symbol, and many tables also give the relative atomic mass and atomic number. The modern periodic table is based on atomic number rather than relative atomic mass.
每种元素用化学符号表示，许多周期表还标注相对原子质量和原子序数。现代周期表的基础是原子序数，而不是相对原子质量。
2. Groups and Periods | 族与周期
The periodic table is arranged into vertical columns called groups and horizontal rows called periods. Elements in the same group have the same number of electrons in their outer shell, which gives them similar chemical properties.
周期表由称为族的纵列和称为周期的横行组成。同一族元素最外层电子数相同，因此化学性质相似。
For main-group elements, the group number equals the number of outer-shell electrons. The period number equals the number of occupied electron shells in an atom.
对于主族元素，族数等于最外层电子数。周期数等于原子中已占有的电子层数。
In IGCSE chemistry, groups are usually numbered from 1 to 8, with Group 8 often also called Group 0. This numbering helps link electronic structure to chemical behaviour.
在 IGCSE 化学中，族通常编号为第1族至第8族，其中第8族常被称为第0族。这种编号有助于将电子结构与化学行为联系起来。
3. Metals and Non-metals | 金属与非金属
Most elements are metals, found on the left and centre of the periodic table. Non-metals are found on the right. A zig-zag line separates metals from non-metals; elements near this line are called metalloids or semi-metals.
大多数元素是金属，位于周期表的左侧和中部。非金属位于右侧。一条锯齿形分界线将金属与非金属分开，靠近此线的元素称为类金属或半金属。
Metals typically conduct electricity and heat, are shiny, malleable and ductile. Non-metals are usually poor conductors, and many are gases or brittle solids at room temperature.
金属通常能导电导热，有光泽，延展性好；非金属通常是热和电的不良导体，室温下许多为气体或脆性固体。
Metal oxides are often basic, whereas non-metal oxides are often acidic. This difference is useful when identifying the character of an element.
金属氧化物通常呈碱性，非金属氧化物通常呈酸性。这种差异有助于判断元素的金属性或非金属性。
4. Electron Configuration and the Periodic Table | 电子排布与周期表
Electronic configuration tells us how electrons are arranged in shells. For the first 20 elements, the first shell holds up to 2 electrons and the second and third shells hold up to 8. An element's position in the periodic table is directly linked to this arrangement.
电子排布告诉我们电子在电子层中的排列方式。对于前20号元素，第一层最多容纳2个电子，第二层和第三层最多容纳8个。元素在周期表中的位置与这种排列直接相关。
The number of outer electrons equals the group number for groups 1 to 8/0. The number of occupied shells equals the period number. For example, sodium has the electronic configuration 2,8,1, so it is in Group 1 and Period 3.
对于第1至第8/0族，最外层电子数等于族数；已占电子层数等于周期数。例如，钠的电子排布为 2,8,1，因此它位于第1族、第3周期。
Noble gases have full outer shells, which makes them stable and unreactive. This helps explain why elements in other groups tend to gain, lose or share electrons to achieve a full outer shell.
稀有气体具有满电子外层，因此稳定且不活泼。这有助于解释为何其他族的元素倾向于得到、失去或共享电子以达到满电子外层。
5. Group 1: The Alkali Metals | 第1族：碱金属
Group 1 elements are lithium, sodium, potassium, rubidium, caesium and francium. They are called alkali metals because they react with water to form alkaline solutions.
第1族元素包括锂、钠、钾、铷、铯和钫。它们被称为碱金属，因为它们与水反应生成碱性溶液。
Alkali metals have one electron in their outer shell. They are soft, low-density metals and are stored under oil to prevent reaction with air or water.
碱金属最外层只有1个电子。它们质地软、密度低，一般保存在油中以防与空气或水反应。
Reactivity increases down the group because the outer electron is further from the nucleus and more easily lost. For example, potassium reacts more vigorously with water than sodium.
反应活性沿族自上而下增强，因为最外层电子离核更远、更容易失去。例如，钾与水的反应比钠更剧烈。
2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)
In the general equation above, M represents an alkali metal. The products are a metal hydroxide and hydrogen gas. This reaction is exothermic and becomes more violent down the group.
在上面的通式中，M 代表碱金属。生成物是金属氢氧化物和氢气。该反应放热，且沿族向下反应越剧烈。
6. Group 7: The Halogens | 第7族：卤素
Group 7 elements are fluorine, chlorine, bromine, iodine and astatine. They are non-metals known as halogens, meaning 'salt formers', because they form salts with metals.
第7族元素包括氟、氯、溴、碘和砹。它们是非金属，称为卤素，意为 '成盐者'，因为它们与金属形成盐。
Halogens have seven electrons in their outer shell, so they gain one electron to form halide ions with a 1− charge, such as Cl⁻ and Br⁻.
卤素最外层有7个电子，因此容易获得1个电子形成带1−电荷的卤离子，如 Cl⁻ 和 Br⁻。
Reactivity decreases down the group because the outer shell is further from the nucleus, making it harder to attract an extra electron. Chlorine is more reactive than bromine, and bromine is more reactive than iodine.
反应活性沿族自上而下减弱，因为最外层离核更远，较难吸引额外电子。氯比溴活泼，溴比碘活泼。
| Halogen | State and colour at room temperature |
|---|---|
| Chlorine | Pale green gas |
| Bromine | Red-brown liquid |
| Iodine | Dark grey solid, purple vapour on heating |
In a displacement reaction, a more reactive halogen can displace a less reactive halogen from its salt solution. For example, chlorine displaces bromine from sodium bromide solution.
在置换反应中，较活泼的卤素能将较不活泼的卤素从其盐溶液中置换出来。例如，氯能从溴化钠溶液中置换出溴。
Cl₂(aq) + 2NaBr(aq) → 2NaCl(aq) + Br₂(aq)
7. Group 8/0: The Noble Gases | 第8/0族：稀有气体
Group 8/0 elements are helium, neon, argon, krypton, xenon and radon. They are called noble gases or inert gases because they are extremely unreactive.
第8/0族元素包括氦、氖、氩、氪、氙和氡。它们被称为稀有气体或惰性气体，因为极不活泼。
Noble gases have a full outer shell of electrons. This stable arrangement explains why they do not need to gain, lose or share electrons under normal conditions.
稀有气体具有稳定的满电子外层。这种稳定结构解释了它们为何在正常条件下不需要得到、失去或共享电子。
Helium has only 2 outer electrons, but this is a full first shell. All other noble gases have 8 outer electrons, which is a stable octet.
氦最外层只有2个电子，但这已填满第一电子层；其他稀有气体最外层都有8个电子，形成稳定八隅体。
These gases have uses based on their lack of reactivity: helium is used in balloons, argon in light bulbs, and neon in advertising signs.
这些气体因其不活泼性而有多种用途：氦用于气球，氩用于灯泡，氖用于广告招牌。
8. Transition Elements | 过渡元素
The transition elements are the block of metals in the middle of the periodic table, between Group 2 and Group 3. They include common metals such as iron, copper, zinc and nickel.
过渡元素是周期表中部、第2族与第3族之间的金属区域。它们包括铁、铜、锌、镍等常见金属。
Transition metals are hard, dense, shiny and good conductors of heat and electricity. They are often used as structural materials and for electrical wiring.
过渡金属坚硬、密度大、有光泽且导热导电性好，常用作结构材料和电线。
Many transition elements form coloured compounds, such as blue copper sulfate and green iron(II) compounds. They often have variable oxidation states and act as catalysts in industrial reactions.
许多过渡元素能形成有色化合物，如蓝色的硫酸铜和绿色的铁(II)化合物。它们常具有可变的氧化态，并在工业反应中用作催化剂。
For example, iron is used as a catalyst in the Haber process, and nickel is used in the hydrogenation of oils. This catalytic ability is a characteristic feature of transition metals.
例如，铁在哈伯法合成氨中用作催化剂，镍用于油脂加氢。这种催化能力是过渡金属的典型特征。
9. Periodic Trends Across a Period | 周期内的变化趋势
Across a period from left to right, metallic character decreases and non-metallic character increases. This is why metals are found on the left and non-metals on the right of the periodic table.
在同一周期中从左到右，金属性减弱、非金属性增强。这就是金属在左、非金属在右的原因。
The oxides of elements also change from basic to acidic across a period. Sodium oxide is basic, aluminium oxide is amphoteric, and sulfur dioxide is acidic.
同一周期元素的氧化物也从碱性变为酸性。氧化钠呈碱性，氧化铝呈两性，二氧化硫呈酸性。
Atomic radius generally decreases across a period because the nuclear charge increases while the outer electrons remain in the same shell, pulling them closer to the nucleus.
同一周期从左到右原子半径通常减小，因为核电荷增加，而外层电子仍在同一电子层，因此被拉得更靠近原子核。
These trends allow chemists to compare elements and predict properties even before experimental data is collected.
这些趋势使化学家能够比较元素，并在获得实验数据之前预测其
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