📚 The Periodic Table: Patterns, Trends, and Predictions | 元素周期表：规律、趋势与预测
The Periodic Table is one of the most powerful tools in science. It organises all known elements in a way that reveals patterns in their properties, allowing chemists to predict how elements will behave. In this revision guide, we will explore the structure of the table, the key trends across periods and down groups, and how to use these patterns to answer exam questions with confidence.
元素周期表是科学中最强大的工具之一。它以一种能揭示元素性质规律的方式排列所有已知元素，使化学家能够预测元素的行为。在本复习指南中，我们将探讨周期表的结构、周期和族中的关键趋势，以及如何利用这些规律自信地解答考试题目。
1. Structure of the Periodic Table | 周期表的结构
The Periodic Table arranges elements in order of increasing atomic number (the number of protons). Each horizontal row is called a period, and each vertical column is called a group.
周期表按原子序数（质子数）递增的顺序排列元素。每一横排称为一个周期，每一竖列称为一个族。
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Elements in the same group have the same number of outer-shell electrons, which gives 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。
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The group number for Groups 1 and 2 equals the number of outer electrons. For Groups 3 to 0, the group number often relates to the outer-shell electron arrangement (e.g. Group 7 elements have 7 outer electrons, Group 0 has a full outer shell).
第1族和第2族的族号等于最外层电子数。对于第3族到第0族，族号通常与最外层电子排布有关（例如，第7族元素有7个最外层电子，第0族元素有满外层电子壳层）。
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The period number tells you how many electron shells are occupied. For example, sodium (Na) in Period 3 has three shells.
周期数表示已占用的电子壳层数量。例如，第三周期的钠（Na）有三个电子壳层。
2. Metals and Non-Metals | 金属与非金属
A clear diagonal ‘staircase’ can be drawn on the Periodic Table separating metals (on the left) from non-metals (on the right).
在周期表上可以画出一条清晰的“阶梯线”，将金属（左侧）与非金属（右侧）分开。
| Property | Metals | Non-metals |
| Appearance | Shiny (lustrous) | Dull |
| Conductivity | Good conductors of heat and electricity | Poor conductors (except graphite) |
| Malleability | Malleable and ductile | Brittle when solid |
| Oxides | Basic oxides | Acidic oxides |
Most elements are metals. Non-metals are found on the right side of the table, including gases like oxygen and solids like sulfur.
大多数元素是金属。非金属位于周期表右侧，包括氧等气体和硫等固体。
3. Group 1: Alkali Metals | 第1族：碱金属
Group 1 elements are soft, highly reactive metals with low densities. Their reactivity increases as you go down the group because the outer electron is further from the nucleus and is more easily lost.
第1族元素是柔软、高反应性的金属，密度低。它们的反应性随族向下而增强，因为最外层电子离原子核更远，更容易失去。
2M + 2H₂O → 2MOH + H₂
For example, lithium reacts gently with water, sodium melts and fizzes, potassium ignites with a lilac flame. All form hydroxides that are strong alkalis.
例如，锂与水反应温和，钠熔化并嘶嘶作响，钾着火并产生淡紫色火焰。它们都形成强碱性的氢氧化物。
4. Group 7: Halogens | 第7族：卤素
Halogens are non-metals with seven outer electrons. They exist as diatomic molecules (F₂, Cl₂, Br₂, I₂). Reactivity decreases down the group because the atoms get larger and it becomes harder to gain an electron.
卤素是具有七个最外层电子的非金属。它们以双原子分子（F₂、Cl₂、Br₂、I₂）存在。反应性随族向下减弱，因为原子变大，获得电子变得更困难。
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At room temperature: fluorine is a pale yellow gas, chlorine is a green gas, bromine is a red-brown liquid, iodine is a grey-black solid.
室温下：氟是淡黄色气体，氯是绿色气体，溴是红棕色液体，碘是灰黑色固体。
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A more reactive halogen can displace a less reactive one from its salt solution. For example, chlorine displaces bromine from potassium bromide solution.
较活泼的卤素可以从其盐溶液中置换出较不活泼的卤素。例如，氯可以从溴化钾溶液中置换出溴。
Cl₂ + 2KBr → 2KCl + Br₂
5. Group 0: Noble Gases | 第0族：稀有气体
Noble gases are monatomic and chemically inert because they have a full outer shell of electrons. Their boiling points increase down the group as the atoms get larger and the van der Waals’ forces become stronger.
稀有气体是单原子的，化学性质惰性，因为它们具有满的最外层电子壳层。它们的沸点随族向下升高，因为原子变大，分子间作用力变强。
Helium is used in balloons, neon in signs, and argon in inert atmospheres for welding.
氦用于气球，氖用于霓虹灯，氩用于焊接中的惰性气氛。
6. Trends Across a Period | 周期内的趋势
Moving from left to right across a period, the number of protons increases, the number of shells stays the same, and the outer electrons increase. This means the nuclear charge increases, pulling the outer shell closer and making the atomic radius smaller.
在一个周期内从左向右移动，质子数增加，壳层数不变，最外层电子增加。这意味着核电荷增加，将外层电子拉得更近，使原子半径变小。
| Trend across a period (left → right) | Explanation |
| Atomic radius decreases | Stronger attraction from larger nuclear charge |
| Electronegativity increases | Greater attraction for bonding electrons |
| Behaviour changes from metallic to non-metallic | Metals lose electrons, non-metals gain or share electrons |
These trends allow you to predict properties of elements not directly studied.
这些趋势使你可以预测未直接研究的元素的性质。
7. Predicting Properties Using the Table | 利用周期表预测性质
Given the position of an element, you can predict its group number, period number, electron arrangement, and likely reactivity.
根据元素的位置，你可以预测其族号、周期数、电子排布和可能的反应性。
For example, element with atomic number 20 (calcium) is in Period 4, Group 2. It has an electron arrangement 2,8,8,2. It will form a 2+ ion by losing two electrons. Its oxide will be basic.
例如，原子序数为20的元素（钙）位于第4周期、第2族。它的电子排布为2,8,8,2。它将通过失去两个电子形成2+离子。它的氧化物将是碱性的。
Similarly, an element with atomic number 17 (chlorine) is in Period 3, Group 7. It has seven outer electrons and will gain one electron to form a 1⁻ ion. Its hydride will be acidic.
类似地，原子序数为17的元素（氯）位于第3周期、第7族。它有七个最外层电子，将获得一个电子形成1⁻离子。它的氢化物将是酸性的。
8. Isotopes and Relative Atomic Mass | 同位素与相对原子质量
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties but different physical properties.
同位素是同一元素中质子数相同但中子数不同的原子。它们具有相同的化学性质但不同的物理性质。
Relative atomic mass (Aᵣ) = (Σ isotope mass × abundance) / 100
For example, chlorine has two isotopes: ³⁵Cl (75%) and ³⁷Cl (25%). Its Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5.
例如，氯有两种同位素：³⁵Cl（75%）和³⁷Cl（25%）。其相对原子质量 = (35 × 75 + 37 × 25) / 100 = 35.5。
9. Electron Arrangements in Ions | 离子的电子排布
When atoms form ions, they gain or lose electrons to achieve a stable noble gas configuration.
当原子形成离子时，它们获得或失去电子以达到稳定的稀有气体构型。
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Metals lose electrons from their outer shell to form positive cations. For example, Na → Na⁺ + e⁻.
金属从其最外层壳失去电子形成阳离子。例如，Na → Na⁺ + e⁻。
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Non-metals gain electrons to form negative anions. For example, Cl + e⁻ → Cl⁻.
非金属获得电子形成阴离子。例如，Cl + e⁻ → Cl⁻。
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The charge on an ion can be predicted from its group number: Group 1 forms 1+, Group 2 forms 2+, Group 6 forms 2⁻, Group 7 forms 1⁻.
离子的电荷可以根据族号预测：第1族形成1+，第2族形成2+，第6族形成2⁻，第7族形成1⁻。
10. Exam Tips and Common Mistakes | 考试技巧与常见错误
Students often confuse group number with period number. Remember: group = outer electrons, period = number of shells.
学生经常混淆族号和周期号。记住：族 = 最外层电子数，周期 = 壳层数。
Another common error is saying reactivity of Group 1 increases, but reactivity of Group 7 decreases. Always explain using the distance of the outer shell from the nucleus and the nuclear charge.
另一个常见错误是认为第1族反应性增加而第7族反应性减小。始终用最外层壳与原子核的距离和核电荷来解释。
When writing electron arrangements, use commas and no spaces, e.g. 2,8,7 for chlorine. Do not use brackets unless asked.
书写电子排布时，使用逗号且不加空格，例如氯为2,8,7。除非要求，否则不要使用括号。
11. Practice Questions | 练习问题
Q1. Element X has an atomic number of 12 and is in Group 2. (a) Write its electron arrangement. (b) Predict the formula of its oxide. (c) State whether its oxide is acidic or basic.
问题1. 元素X的原子序数为12，位于第2族。(a) 写出它的电子排布。(b) 预测其氧化物的化学式。(c) 说明其氧化物是酸性的还是碱性的。
Q2. The relative atomic mass of a sample of element Y is 20.2. The sample contains two isotopes, ²⁰Y and ²²Y. Calculate the percentage abundance of each isotope.
问题2. 元素Y样品的相对原子质量为20.2。样品含有两种同位素，²⁰Y和²²Y。计算每种同位素的丰度百分比。
Q3. Explain why the boiling point of argon is higher than that of helium.
问题3. 解释为什么氩的沸点高于氦。
12. Summary | 总结
The Periodic Table is a map of chemical behaviour. By understanding its structure and trends — across periods and down groups — you can predict electron arrangements, ion charges, reactivity, and the properties of oxides. Master these patterns and you will be well prepared for your IGCSE chemistry questions.
元素周期表是化学行为的图谱。通过理解其结构以及跨周期和沿族向下的趋势，你可以预测电子排布、离子电荷、反应性以及氧化物的性质。掌握这些规律，你就能为IGCSE化学问题做好充分准备。
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