📚 Mastering the Periodic Table for WJEC A-Level Chemistry | A-Level WJEC 化学：元素周期表 考点精讲
The periodic table is the cornerstone of chemistry, organising over 100 elements into a powerful tool for predicting physical and chemical properties. For WJEC A-Level Chemistry, you must not only recall trends but also explain them in terms of electronic structure, nuclear charge and bonding. This comprehensive revision guide covers every key specification point, from the table’s historical development to the intricacies of Period 3 oxides and chlorides.
元素周期表是化学的基石，它将一百多种元素组织成一个强大的工具，用以预测物理和化学性质。对于 WJEC A-Level 化学考试，你不仅要记住周期性趋势，更要从电子结构、核电荷和化学键的角度解释它们。这份全面的备考指南涵盖了从周期表历史发展到第三周期氧化物和氯化物细节的所有核心考点。
1. History and Development of the Periodic Table | 元素周期表的历史与发展
Dmitri Mendeleev arranged the known elements in 1869 by increasing atomic mass, placing elements with similar chemical properties in vertical columns.
德米特里·门捷列夫于 1869 年按原子量递增的顺序排列已知元素，并将化学性质相似的元素放在同一纵列中。
Mendeleev left deliberate gaps for undiscovered elements and predicted properties of eka-aluminium (gallium), eka-silicon (germanium) and eka-boron (scandium) with remarkable accuracy.
门捷列夫刻意留出空位给未发现的元素，并惊人准确地预测了类铝（镓）、类硅（锗）和类硼（钪）的性质。
Henry Moseley later used X-ray spectra to determine the atomic number of each element, leading to the modern periodic law: elements are arranged in order of increasing atomic number, not mass.
亨利·莫塞莱后来利用 X 射线光谱测定了每个元素的原子序数，从而确立了现代周期律：元素按原子序数递增的顺序排列，而非按原子量。
WJEC examiners expect you to appreciate how this historical progression resolved anomalies such as tellurium and iodine, where ordering by mass would break the chemical grouping.
WJEC 考官期望你理解这一历史进程如何解决了碲和碘等元素的反常问题，因为按原子量排列会破坏化学分组。
2. Structure of the Periodic Table: Periods and Groups | 周期表的结构：周期与族
The modern table consists of horizontal rows called periods and vertical columns called groups, numbered 1 to 18 as recommended by IUPAC.
现代周期表由横排（周期）和纵列（族）组成，采用 IUPAC 建议的 1 至 18 族编号。
Each period begins with an element having its outermost electron in a new principal quantum shell; Period 1 contains only two elements, while Period 4 introduces the 3d transition metals.
每个周期都从最外层电子进入新主量子壳层的元素开始；第一周期仅含两种元素，而第四周期引入了 3d 过渡金属。
Elements in the same group possess the same number of outer-shell electrons and thus exhibit similar chemical behaviour. For instance, all Group 2 elements have an s² outer configuration.
同一族元素拥有相同的最外层电子数，因此表现出相似的化学行为。例如，所有第二族元素的最外层电子排布都是 s²。
Within a group, the atoms get larger down the column, causing predictable trends in reactivity and first ionisation energy.
在同一族中，从上到下原子半径增大，从而使反应性和第一电离能呈现可预测的趋势。
3. Electronic Configuration and Blocks (s, p, d, f) | 电子排布与分区 (s, p, d, f)
The periodic table is divided into four blocks based on the highest-energy sub-shell being filled: s-block (Groups 1–2), p-block (Groups 13–18), d-block (transition metals) and f-block (lanthanides and actinides).
根据最高能量子壳层的不同，周期表分为四个区：s 区（第 1-2 族）、p 区（第 13-18 族）、d 区（过渡金属）和 f 区（镧系与锕系）。
WJEC requires you to write electron configurations using 1s² 2s² 2p⁶ notation up to krypton (Z = 36), including the exceptions of chromium and copper. For chromium, the configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵ rather than 4s² 3d⁴, because the half-filled d⁵ sub-shell gives extra stability.
WJEC 要求你用 1s² 2s² 2p⁶ 的形式书写直到氪（Z = 36）的电子排布，包括铬和铜的特例。铬的电子排布是 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵，而不是 4s² 3d⁴，因为半充满的 d⁵ 子壳层提供额外的稳定性。
Copper adopts 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰, gaining the energetic advantage of a completely filled 3d sub-shell.
铜采用 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰ 的排布，获得了全满 3d 子壳层的能量优势。
You must be able to deduce an element’s block, period and group from its configuration. For example, an element ending in 3p⁴ lies in the p-block, Period 3, and Group 16 (4 p electrons + the preceding s electrons count).
你必须能从电子排布推断出某元素所在的分区、周期和族。例如，以 3p⁴ 结尾的元素位于 p 区、第三周期、第 16 族。
4. Atomic Radius Trends | 原子半径趋势
The atomic radius is half the distance between the nuclei of two adjacent atoms in a solid or a diatomic molecule. Across Period 3 from sodium to argon, the atomic radius decreases steadily.
原子半径是指固体或双原子分子中两个相邻原子核间距的一半。在第三周期中，从钠到氩，原子半径逐步减小。
The decrease occurs because the number of protons (nuclear charge) increases, pulling the outer electrons inwards, whereas the shielding provided by inner shells remains roughly constant.
半径减小是因为质子数（核电荷）增加，将外层电子拉向原子核，而内层电子提供的屏蔽效应基本保持不变。
Down Group 2 from beryllium to barium, the atomic radius increases. Each successive element adds an extra electron shell, so the outermost electrons are farther from the nucleus despite the greater nuclear charge.
第二族从上到下（从铍到钡），原子半径增大。每向下移动一个元素都会增加一个电子壳层，因此尽管核电荷增大，最外层电子离原子核却更远。
A common WJEC question asks you to explain why the radius of a sodium atom (Na) is much larger than that of its ion (Na⁺). The Na⁺ ion has lost its 3s¹ electron, leaving only two occupied shells and a stronger effective nuclear pull on the remaining electrons.
WJEC 常见考题要求解释为什么钠原子的半径远大于钠离子。Na⁺ 离子失去了 3s¹ 电子，只剩下两个被占据的电子壳层，且有效核引力更强。
5. First Ionisation Energy Trends | 第一电离能趋势
The first ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous unipositive ions.
第一电离能是指从一摩尔气态原子中移除一摩尔电子，形成一摩尔气态一价正离子所需的能量。
X(g) → X⁺(g) + e⁻
Across Period 3, the first ionisation energy shows a general increase because the atomic radius falls and the nuclear charge rises, making it harder to remove an electron.
在第三周期中，第一电离能总体上呈增大趋势，因为原子半径减小而核电荷增加，使电子更难被移走。
However, two clear dips appear: between magnesium and aluminium, and between phosphorus and sulfur. Magnesium (1s² 2s² 2p⁶ 3s²) has a full 3s subshell, whereas aluminium’s outermost electron enters a higher-energy 3p orbital that is easier to remove.
然而，图中出现两次明显下降：镁与铝之间，以及磷与硫之间。镁（3s²）具有全满的 3s 子壳层，而铝的最外层电子进入能量更高的 3p 轨道，更容易被去除。
The drop from phosphorus to sulfur occurs because in phosphorus the three 3p electrons each occupy separate p orbitals (3p_x, 3p_y, 3p_z), while sulfur has a pair of electrons in one p orbital. The mutual repulsion of the paired electrons makes the first electron easier to remove from sulfur.
从磷到硫的下降是因为磷的三个 3p 电子各自占据独立的 p 轨道，而硫在一个 p 轨道中有一对电子。成对电子的相互排斥使得硫的第一个电子更容易失去。
Down Group 2, the first ionisation energy decreases because the outermost electron is in a shell with a higher principal quantum number, farther from the nucleus and experiencing greater shielding.
在第二族中从上到下第一电离能减小，因为最外层电子位于主量子数更高的壳层，距离原子核更远且受到更多屏蔽。
6. Electronegativity Trends | 电负性趋势
Electronegativity is the ability of a bonded atom to attract the electron pair in a covalent bond. The Pauling scale is usually used, with fluorine assigned the highest value of 4.0.
电负性是键合原子吸引共价键中电子对的能力。通常采用鲍林标度，氟的电负性最高，为 4.0。
Across Period 3, electronegativity increases from sodium (0.9) to chlorine (3.0), while argon has no electronegativity value because it does not form bonds under standard conditions.
在第三周期中，电负性从钠（0.9）到氯（3.0）递增，而氩没有电负性值，因为在标准条件下它不形成共价键。
The increase is driven by the same factors as ionisation energy: greater nuclear charge and smaller atomic radius allow the nucleus to attract bonding electrons more strongly.
这种增大与电离能的原因相同：更大的核电荷和更小的原子半径使原子核对成键电子的吸引力更强。
Down a group, electronegativity falls. For example, in Group 17, fluorine is the most electronegative element and the value decreases to iodine. This occurs because the bonding electron pair is increasingly distant from the nucleus and is more heavily shielded.
在同族中，电负性从上到下减小。例如，在第 17 族中，氟的电负性最大，到碘逐渐减小。这是因为成键电子对离原子核越来越远，且屏蔽效应增强。
WJEC often links electronegativity to bond polarity and the acid–base character of Period 3 oxides.
WJEC 经常将电负性与键的极性和第三周期氧化物的酸碱特性联系起来考查。
7. Melting and Boiling Points across Period 3 | 第三周期元素的熔点与沸点变化
The melting points of Period 3 elements exhibit a striking pattern that reflects their structure and bonding.
第三周期元素的熔点呈现出显著的规律性变化，反映了它们的结构和键合方式。
Sodium, magnesium and aluminium are metallic, so their melting points rise from Na to Al because the metal ions become smaller and more highly charged (Na⁺, Mg²⁺, Al³⁺), while the number of delocalised electrons per atom increases, strengthening the metallic bond.
钠、镁和铝都是金属，因此它们的熔点从 Na 到 Al 逐渐升高，因为金属离子变小、电荷升高（Na⁺、Mg²⁺、Al³⁺），同时每个原子的离域电子数增加，增强了金属键。
Silicon has a giant covalent structure, leading to a very high melting point; each silicon atom forms four strong covalent bonds in a tetrahedral lattice.
硅具有巨共价结构，因此熔点极高；每个硅原子在四面体晶格中形成四个强共价键。
Phosphorus (P₄), sulfur (S₈) and chlorine (Cl₂) exist as simple molecular substances. Their melting points depend on van der Waals’ forces. Sulfur (S₈) has a higher melting point than phosphorus (P₄) because S₈ rings are larger and have more electrons, creating stronger London dispersion forces. Chlorine (Cl₂) is a diatomic molecule with weaker intermolecular forces than P₄ and S₈, so its melting point is much lower.
磷（P₄）、硫（S₈）和氯（Cl₂）以简单分子形式存在，它们的熔点取决于范德华力。硫（S₈）的熔点高于磷（P₄），因为 S₈ 环更大、电子更多，产生的伦敦色散力更强。氯（Cl₂）为双原子分子，分子间作用力弱于 P₄ 和 S₈，因此熔点低得多。
Argon, being monatomic, has only feeble London forces and the lowest melting point in Period 3.
氩是单原子分子，仅有极弱的伦敦力，因此是第三周期中熔点最低的元素。
8. Chemical Properties: Period 3 Oxides | 化学性质：第三周期氧化物
The oxides of Period 3 illustrate a clear transition from basic to acidic character across the period, linked to the electronegativity of the element and the oxide ion’s ability to act as a proton acceptor or donor.
第三周期氧化物清晰地展现出从左到右碱性至酸性的转变，这与元素的电负性以及氧化物作为质子接受体或给予体的能力有关。
Below is a summary of the key oxides, their reactions with water and their acid–base nature.
以下是主要氧化物的总结，包括它们与水的反应和酸碱性质。
| Oxide | Reaction with water | Acid–base nature |
|---|---|---|
| Na₂O | Na₂O(s) + H₂O(l) → 2NaOH(aq) | Basic |
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