📚 IGCSE Chemistry: Structure and Trends of the Periodic Table | IGCSE化学：元素周期表的结构与规律
The periodic table is a masterpiece of chemical organisation. It arranges all known elements in order of increasing atomic number and groups them by their properties and electron configurations. For IGCSE students, understanding its structure and trends is essential to predicting chemical behaviour.
元素周期表是化学组织的杰作。它按照原子序数递增的顺序排列所有已知元素，并根据它们的性质和电子排布进行分组。对于 IGCSE 学生而言，理解其结构与规律是预测化学行为的基础。
1. The Structure: Periods and Groups | 结构：周期与族
The periodic table is arranged in horizontal rows called periods and vertical columns called groups. Each period corresponds to a new electron shell being filled. Each group contains elements with the same number of outer-shell (valence) electrons.
周期表按横向的周期和纵向的族排列。每个周期对应一个新的电子壳层被填充。每个族中的元素具有相同的外层（价）电子数。
Period number = number of occupied electron shells.
周期数 = 被占据的电子壳层数。
Group number (for Groups I–VIII) = number of valence electrons (for main-group elements).
族数（对于第 I–VIII 族）= 价电子数（对于主族元素）。
2. Electron Shells and the Period Number | 电子壳层与周期数
Elements in period 1 have electrons in only the first shell; period 2 elements have electrons in the first and second shells; and so on. As you move down a group, a new shell is added, increasing atomic size.
第 1 周期元素只有第一壳层电子；第 2 周期元素有第一和第二壳层电子；依此类推。当沿族向下移动时，新增一个壳层，原子尺寸增大。
Na (period 3): 2.8.1 — three shells occupied
钠（第 3 周期）：2.8.1 — 占据三个壳层
This pattern explains why elements in the same period have different properties, while elements in the same group share similar chemical reactions.
这一模式解释了为什么同一周期的元素性质不同，而同一族的元素具有相似的化学反应。
3. Group Number and Valence Electrons | 族序数与价电子
For main-group elements (Groups I, II, III, IV, V, VI, VII, VIII), the group number tells you exactly how many valence electrons an atom has. For example, chlorine in Group VII has seven outer electrons.
对于主族元素（第 I、II、III、IV、V、VI、VII、VIII 族），族序数直接告诉你原子的价电子数。例如，第 VII 族的氯有 7 个外层电子。
Group I → 1 valence electron → forms ions with charge +1.
第 I 族 → 1 个价电子 → 形成 +1 电荷的离子。
Group II → 2 valence electrons → forms ions with charge +2.
第 II 族 → 2 个价电子 → 形成 +2 电荷的离子。
Group VI → 6 valence electrons → forms ions with charge –2.
第 VI 族 → 6 个价电子 → 形成 –2 电荷的离子。
Group VII → 7 valence electrons → forms ions with charge –1.
第 VII 族 → 7 个价电子 → 形成 –1 电荷的离子。
4. Metals, Non-metals, and Metalloids | 金属、非金属和类金属
The periodic table can be divided into metals, non-metals, and metalloids. Metals are on the left and centre, non-metals are on the upper right, and metalloids lie along the zigzag line separating them.
周期表可划分为金属、非金属和类金属。金属位于左侧和中部，非金属位于右上角，类金属位于分隔两者的折线两侧。
Metals: conduct electricity, shiny, malleable, lose electrons to form positive ions.
金属：导电、有光泽、可延展，失电子形成正离子。
Non-metals: poor conductors, dull, brittle when solid, gain electrons to form negative ions.
非金属：不良导体、暗淡、固态时易碎，得电子形成负离子。
Metalloids: show intermediate properties, e.g. silicon.
类金属：性质介于两者之间，例如硅。
5. Group I: The Alkali Metals | 第 I 族：碱金属
Group I elements (lithium, sodium, potassium) are soft, highly reactive metals. Reactivity increases down the group because the outer electron is farther from the nucleus and more easily lost.
第 I 族元素（锂、钠、钾）是柔软且高反应性的金属。反应性沿族向下递增，因为外层电子离核更远，更容易失去。
They react with water to form an alkaline solution and hydrogen gas.
它们与水反应生成碱性溶液和氢气。
They form ionic compounds with non-metals, e.g. NaCl → Na⁺ and Cl⁻.
它们与非金属形成离子化合物，例如 NaCl → Na⁺ 和 Cl⁻。
Melting point and density generally decrease down the group.
熔点和密度通常沿族向下递减。
6. Group VII: The Halogens | 第 VII 族：卤素
Group VII elements (fluorine, chlorine, bromine, iodine) are non-metals with seven valence electrons. They gain one electron to form –1 ions. Reactivity decreases down the group because the incoming electron is attracted less strongly as atomic radius increases.
第 VII 族元素（氟、氯、溴、碘）是具有七个价电子的非金属。它们获得一个电子形成 –1 离子。反应性沿族向下递减，因为随着原子半径增大，对外来电子的吸引减弱。
At room temperature: F₂ and Cl₂ are gases, Br₂ is a liquid, I₂ is a solid.
室温下：F₂ 和 Cl₂ 是气体，Br₂ 是液体，I₂ 是固体。
More reactive halogen can displace a less reactive one from its salt solution.
更活泼的卤素可以从盐溶液中置换出较不活泼的卤素。
7. Group VIII (0): The Noble Gases | 第 VIII（0）族：稀有气体
Noble gases have a full outer shell (helium has 2; others have 8). They are very stable and extremely unreactive.
稀有气体具有全充满的外层（氦为 2 个；其他为 8 个）。它们非常稳定，极不活泼。
They exist as single atoms (monatomic).
它们以单原子形式存在。
Their physical properties change gradually down the group: boiling point increases.
它们的物理性质沿族向下渐次变化：沸点升高。
Uses: neon in lights, argon in welding, helium in balloons.
用途：霓虹灯中的氖、焊接中的氩、气球中的氦。
8. Transition Elements | 过渡元素
The transition metals are found in the central block between Groups II and III. They are hard, dense, and have high melting points.
过渡金属位于第 II 族和第 III 族之间的中心区域。它们坚硬、致密且熔点高。
They form coloured compounds, e.g. Cu²⁺ is blue, Fe³⁺ is brown.
它们形成有色化合物，例如 Cu²⁺ 为蓝色，Fe³⁺ 为棕色。
They can have more than one oxidation state, e.g. Fe²⁺ and Fe³⁺.
它们可以有多种氧化态，例如 Fe²⁺ 和 Fe³⁺。
They and their compounds are often used as catalysts.
它们及其化合物常用作催化剂。
9. Periodic Trend: Atomic Radius | 周期性规律：原子半径
Atomic radius is the distance from the nucleus to the outermost electron. Across a period, atomic radius decreases; down a group, it increases.
原子半径是指从原子核到最外层电子的距离。在同一周期内，原子半径减小；在同一族内，原子半径增大。
Across period 3: Na (186 pm) → Cl (99 pm)
第 3 周期：Na（186 pm）→ Cl（99 pm）
Across a period, the nuclear charge increases while the same shell is being filled, so the electrons are pulled closer. Down a group, new shells are added, making the atom larger.
在同一周期中，核电荷增加而壳层数不变，因此电子被拉得更近。在族中向下移动时，新增壳层，原子变得更大。
10. Periodic Trend: Ionisation Energy | 周期性规律：电离能
Ionisation energy is the energy needed to remove one electron from a gaseous atom. It generally increases across a period and decreases down a group.
电离能是从气态原子中移走一个电子所需的能量。它通常在同一周期内增大，在同一族内减小。
Na(g) → Na⁺(g) + e⁻ — first ionisation energy = 496 kJ/mol
Na(g) → Na⁺(g) + e⁻ — 第一电离能 = 496 kJ/mol
r nuclear charge, smaller radius → electrons held more tightly → higher ionisation energy.
同一周期内：核电荷更大、半径更小 → 电子被更牢固地束缚 → 电离能更高。
ger radius, more shielding → outer electron is easier to remove → lower ionisation energy.
同一族中：半径更大、屏蔽效应更强 → 外层电子更容易移除 → 电离能更低。
11. Periodic Trend: Electronegativity | 周期性规律：电负性
Electronegativity measures how strongly an atom attracts bonding electrons in a covalent bond. It increases across a period and decreases down a group.
电负性衡量原子在共价键中吸引成键电子的强度。它在同一周期内增大，在同一族内减小。
Fluorine is the most electronegative element.
氟是电负性最大的元素。
Down Group VII, electronegativity decreases from fluorine to iodine.
在第 VII 族中，电负性从氟到碘逐渐减小。
Differences in electronegativity determine whether a bond is ionic or covalent.
电负性差异决定键是离子键还是共价键。
12. Summary: Using the Periodic Table | 总结：运用周期律
Mastering the periodic table allows you to predict atomic size, ionisation energy, electronegativity, and chemical reactivity without memorising every element. Always link structure to electron arrangement and nuclear charge.
掌握周期表能够让你不必死记每个元素即可预测原子大小、电离能、电负性和化学反应性。始终将结构与电子排布及核电荷联系起来。
| Trend | Across a period | Down a group | Cause |
| Atomic radius | Decreases | Increases | Nuclear charge vs. new shells |
| Ionisation energy | Increases | Decreases | Electron attraction |
| Electronegativity | Increases | Decreases | Attraction for shared electrons |
| Metallic character | Decreases | Increases | Ease of losing electrons |
规律：周期内变、族内递变，结构决定性质。
Remember: across a period, properties change gradually; down a group, they change systematically.
记住：同一周期内性质逐渐变化；同一族内性质规律性变化。
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