📚 Edexcel A-Level Combined Science: Topic 105 - Atomic Structure and the Periodic Table | Edexcel A-Level 综合科学：专题 105 - 原子结构与元素周期表
Atomic structure is the foundation of all chemical and physical behaviour. In Edexcel A-Level Combined Science, Topic 105 requires you to link subatomic particles, electron configurations, ionisation energies and periodic trends. This guide walks through the key ideas, worked comparisons and common exam traps.
原子结构是所有化学与物理行为的基础。在 Edexcel A-Level 综合科学中，专题 105 要求你把亚原子粒子、电子排布、电离能和周期趋势联系起来。本指南将梳理核心概念、典型对比和常见考试陷阱。
1. Subatomic Particles and Nuclear Symbols | 亚原子粒子与核素符号
Atoms contain three main subatomic particles. Protons have a relative charge of +1 and a relative mass of 1. Neutrons have a relative charge of 0 and a relative mass of 1. Electrons have a relative charge of -1 and a relative mass of 1/1836. The nucleus contains virtually all the mass but occupies a tiny fraction of the atomic volume.
原子包含三种主要亚原子粒子。质子相对电荷为 +1，相对质量约为 1；中子相对电荷为 0，相对质量约为 1；电子相对电荷为 -1，相对质量约为 1/1836。原子核几乎集中了全部质量，但只占原子体积的极小部分。
| Particle | Relative charge | Relative mass |
| Proton 质子 | +1 | 1 |
| Neutron 中子 | 0 | 1 |
| Electron 电子 | -1 | 1/1836 |
The atomic number Z is the number of protons. The mass number A is the total number of protons and neutrons. In a neutral atom the number of electrons equals the number of protons, so the charges balance. When an atom loses or gains electrons it forms a positive or negative ion.
原子序数 Z 是质子数。质量数 A 是质子与中子总数。中性原子的电子数等于质子数，因此电荷平衡。当原子失去或得到电子时，会形成正离子或负离子。
ᴬZX written as A = Z + n
The symbol n stands for the number of neutrons. You must be able to read and write these nuclear symbols, for example ¹²₆C means an atom of carbon with 6 protons and 6 neutrons. Similarly, ²³⁵₉₂U has 92 protons and 143 neutrons, and ³⁷₁₇Cl has 17 protons and 20 neutrons.
符号 n 表示中子数。你必须能读、能写核素符号，例如 ¹²₆C 表示一个质子数为 6、中子数为 6 的碳原子；²³⁵₉₂U 有 92 个质子和 143 个中子；³⁷₁₇Cl 有 17 个质子和 20 个中子。
2. Isotopes, Mass Number and Relative Atomic Mass | 同位素、质量数与相对原子质量
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Because chemical properties depend mainly on electron configuration, isotopes of an element react almost identically; their physical properties such as density and rate of diffusion differ slightly.
同位素是质子数相同但中子数不同的同种元素的原子。由于化学性质主要取决于电子排布，同一元素的同位素化学反应几乎完全相同；但密度、扩散速率等物理性质略有差异。
The relative isotopic mass is the mass of one atom of an isotope compared with 1/12 of the mass of one atom of carbon-12. The relative atomic mass Aᵣ is the weighted mean mass of the atoms of an element compared with 1/12 of the mass of one atom of carbon-12, taking into account the abundance of each isotope.
相对同位素质量是某同位素一个原子的质量与碳-12 一个原子质量的 1/12 之比。相对原子质量 Aᵣ 是元素各原子按丰度加权的平均质量，与碳-12 一个原子质量的 1/12 之比。
Aᵣ = Σ (isotope mass × percentage abundance) ÷ 100
For chlorine, ³⁵₁₇Cl has an abundance of about 75% and ³⁷₁₇Cl about 25% ass spectrometer produces a mass spectrum: the x-axis shows mass-to-charge ratio m/z and the y-axis shows relative abundance.
对氯而言，³⁵₁₇Cl 丰度约 75%，³⁷₁₇Cl 约 25%，因此 Aᵣ = (35 × 75 + 37 × 25) ÷ 100 = 35.5。质谱仪给出质谱图：横轴为质荷比 m/z，纵轴为相对丰度。
3. Electron Configuration and Energy Levels | 电子排布与能级
Electrons occupy shells at increasing distance from the nucleus. Each shell is divided into sub-shells labelled s, p, d and f, and each sub-shell contains orbitals. Filling follows the Aufbau principle: the lowest available energy level fills first, giving the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.
电子占据离核由近及远的电子层。每一层又分为 s、p、d、f 亚层，每个亚层包含若干轨道。填充遵循能量最低原理：能量最低的能级先填，顺序为 1s、2s、2p、3s、3p、4s、3d、4p。
Notice that 4s fills before 3d because the 4s sub-shell has slightly lower energy. Sodium is 1s² 2s² 2p⁶ 3s¹, argon is 1s² 2s² 2p⁶ 3s² 3p⁶, and potassium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. Chromium and copper are exceptions: chromium is [Ar] 3d⁵ 4s¹ and copper is [Ar] 3d¹⁰ 4s¹, because a half-filled or completely filled d sub-shell is more stable.
注意 4s 先于 3d 填充，因为 4s 亚层能量略低。钠为 1s² 2s² 2p⁶ 3s¹，氩为 1s² 2s² 2p⁶ 3s² 3p⁶，钾为 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹。铬和铜是例外：铬为 [Ar] 3d⁵ 4s¹，铜为 [Ar] 3d¹⁰ 4s¹，因为半充满或全充满的 d 亚层更稳定。
Hund's rule states that electrons occupy orbitals singly with parallel spins before pairing up. This minimises electron-electron repulsion. You should be able to draw box-and-arrow diagrams for atoms and ions, for example Fe is [Ar] 3d⁶ 4s², Fe²⁺ is [Ar] 3d⁶ and Fe³⁺ is [Ar] 3d⁵.
洪特规则指出，电子先以平行自旋单独占据轨道，然后再配对，以尽量减少电子间斥力。你应能画出原子和离子的方框箭头图，例如 Fe 为 [Ar] 3d⁶ 4s²，Fe²⁺ 为 [Ar] 3d⁶，Fe³⁺ 为 [Ar] 3d⁵。
4. Orbitals, Sub-Shells and Shielding | 轨道、亚层与屏蔽效应
An orbital is a region of space that can hold up to two electrons with opposite spins. An s sub-shell has one orbital (maximum 2 electrons), a p sub-shell has three orbitals (maximum 6 electrons), a d sub-shell has five orbitals (maximum 10 electrons) and an f sub-shell has seven orbitals (maximum 14 electrons).
轨道是能容纳最多两个自旋相反电子的空间区域。s 亚层有 1 个轨道（最多 2 个电子），p 亚层有 3 个轨道（最多 6 个电子），d 亚层有 5 个轨道（最多 10 个电子），f 亚层有 7 个轨道（最多 14 个电子）。
An s orbital is spherical. A p orbital is dumbbell shaped and can be oriented along the x, y or z axis. d orbitals have more complex shapes. Inner shells shield outer electrons from the full nuclear charge, so the effective nuclear charge experienced by an outer electron is less than the actual number of protons.
s 轨道呈球形；p 轨道呈哑铃形，可沿 x、y、z 轴取向；d 轨道形状更复杂。内层电子会屏蔽外层电子所感受到的核电荷，因此外层电子感受到的有效核电荷小于实际质子数。
Shielding, distance from the nucleus and the nuclear charge together control how strongly an electron is held. This trio of factors explains almost every ionisation energy and periodic trend question in Topic 105. When you explain a trend, always quote at least two of these three factors explicitly.
屏蔽效应、离核距离和核电荷共同决定电子被束缚的强弱。这三要素几乎可以解释专题 105 中所有的电离能与周期趋势题。解释趋势时，务必明确引用这三个因素中的至少两个。
5. Ionisation Energy: Definitions and Equations | 电离能的定义与方程
The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. The equation below shows the process for sodium, where the value is endothermic because energy must be supplied to overcome the attraction between the electron and the nucleus.
第一电离能是指从一摩尔气态原子中移去一摩尔电子，生成一摩尔气态 +1 离子所需的能量。下面的方程给出钠的过程；该过程为吸热，因为必须提供能量来克服电子与原子核之间的吸引力。
Na(g) → Na⁺(g) + e⁻ ΔH = +496 kJ mol⁻¹
The second ionisation energy removes the second electron from a gaseous 1+ ion. The second value is always larger than the first because the remaining ion is more positively charged and attracts the electron more strongly, and because the electron removed is usually closer to the nucleus.
第二电离能是从气态 +1 离子中再移去一个电子所需的能量。第二电离能总是大于第一电离能，因为剩余离子带更多正电荷，对电子吸引更强，而且被移去的电子通常离核更近。
Na⁺(g) → Na²⁺(g) + e⁻
Definition questions demand precision. The word "gaseous" must appear, the amount must be one mole, and the product must be a gaseous ion. Losing a mark for writing "energy needed to remove an electron" without "one mole of gaseous atoms" is very common in Edexcel papers.
定义题要求措辞精确：必须出现「气态」，数量必须是一摩尔，产物必须是气态离子。写成「移去一个电子所需的能量」而漏掉「一摩尔气态原子」，在 Edexcel 试卷中失分非常常见。
6. Successive Ionisation Energies as Evidence for Shells | 逐级电离能作为电子层证据
Successive ionisation energies of the same element increase steadily, but there are large jumps when an electron is removed from a new inner shell. For aluminium the first three values are relatively low because the 3p and 3s electrons are well shielded, then the fourth value jumps sharply because the electron is removed from the much closer 2p sub-shell.
同一元素的逐级电离能逐级升高，但当电子从更内层移去时会出现大幅跳跃。铝的前三个值相对较低，因为 3p 和 3s 电子受到较好的屏蔽；第四个值急剧上升，因为该电子来自离核近得多的 2p 亚层。
| Electron removed | 1st | 2nd | 3rd | 4th |
| Al kJ mol⁻¹ | 578 | 1817 | 2745 | 11578 |
The pattern of jumps tells you the number of electrons in the outer shell. For aluminium the jump between the third and fourth values shows that aluminium has three electrons in its outer shell, consistent with the configuration 1s² 2s² 2p⁶ 3s² 3p¹.
跳跃的位置告诉你最外层电子数。铝在第三与第四个值之间出现跳跃，说明铝最外层有 3 个电子，与 1s² 2s² 2p⁶ 3s² 3p¹ 的排布一致。
For sodium the jump comes between the first and second values, showing a single outer electron. For magnesium the jump is between the second and third values, showing two outer electrons. Always link the jump to the shell number and to the increasing nuclear attraction on the remaining electrons.
钠的跳跃出现在第一与第二个值之间，说明只有 1 个外层电子；镁的跳跃出现在第二与第三个值之间，说明有 2 个外层电子。答题时要把跳跃与电子层序号，以及剩余电子所受核吸引增强联系起来。
7. Structure of the Periodic Table | 元素周期表的结构
The periodic table arranges elements in order of increasing atomic number. Elements in the same group have the same number of outer-shell electrons and therefore similar chemical properties. Elements in the same period have the same number of occupied shells. This is why the table is called periodic: properties repeat at regular intervals.
元素周期表按原子序数递增排列元素。同族元素最外层电子数相同，因此化学性质相似；同周期元素占据的电子层数相同。这正是周期表被称为「周期」的原因：性质按一定间隔重复出现。
Blocks show which sub-shell is being filled. Groups 1 and 2 are the s-block; groups 13 to 18 are the p-block; the transition metals are the d-block; and the lanthanides and actinides are the f-block. Hydrogen is placed in group 1 but is not an alkali metal because it has one electron in a 1s orbital rather than an s sub-shell in a higher shell.
分区显示正在填充的亚层：第 1、2 族为 s 区；第 13 至 18 族为 p 区；过渡金属为 d 区；镧系和锕系为 f 区。氢虽列在第 1 族，但不是碱金属，因为它的 1 个电子在 1s 轨道中，而不是更高层的 s 亚层。
Group number and outer electrons are directly linked. Group 1 elements have one outer electron, group 2 have two, group 7 have seven and group 0 (noble gases) have a full outer shell. Ions formed by main-group elements usually achieve the noble gas configuration of the nearest noble gas.
族序数与最外层电子数直接相关：第 1 族有 1 个外层电子，第 2 族有 2 个，第 7 族有 7 个，第 0 族（稀有气体）具有满外层。主族元素形成的离子通常达到最近稀有气体的电子排布。
8. Periodic Trends Across Period 3 | 第三周期的周期性趋势
Across period 3 from sodium to argon, the nuclear charge increases by one proton per element while the outer electrons enter the same shell, so shielding changes very little. As a result the outer electrons are pulled closer: atomic radius decreases and first ionisation energy generally increases.
从钠到氩，核电荷每元素增加一个质子，而外层电子进入同一电子层，屏蔽效应变化很小。因此外层电子被拉得更近：原子半径减小，第一电离能总体升高。
There are two dips in the ionisation energy trend. Aluminium is lower than magnesium because its outer electron is in a 3p orbital, which is higher in energy and better shielded than the 3s orbital. Sulfur is lower than phosphorus because the paired 3p electron in sulfur experiences extra electron-electron repulsion, making it easier to remove.
电离能趋势中有两处下降：铝低于镁，因为它的外层电子在能量更高、屏蔽更强的 3p 轨道；硫低于磷，因为硫的 3p 电子已成对，额外电子间斥力使其更容易被移去。
Melting point rises from sodium to silicon as the structure changes from metallic to giant covalent, then falls sharply from phosphorus to argon because these elements exist as simple molecular or monatomic substances with only weak intermolecular forces. Metallic character decreases across the period as ionisation energy rises.
熔点从钠到硅升高，因为结构由金属晶体变为巨型共价结构；随后从磷到氩急剧下降，因为这些元素是简单分子或单原子物质，只有较弱的分子间作用力。随着电离能升高，金属性沿周期减弱。
9. Oxides and Chlorides of Period 3 | 第三周期元素的氧化物与氯化物
The character of the period 3 oxides changes from basic through amphoteric to acidic as you move from left to right. Sodium oxide and magnesium oxide are basic and neutralise acids; aluminium oxide is amphoteric and reacts with both acids and bases; silicon dioxide, phosphorus(V) oxide, sulfur dioxide and chlorine(VII) oxide are acidic.
第三周期氧化物的性质从左到右由碱性经两性变为酸性。氧化钠和氧化镁是碱性氧化物，能中和酸；氧化铝是两性氧化物，既能与酸
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