📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
In IB Chemistry, the story of matter begins with the atom. Understanding atomic structure, electron arrangement and the periodic table is essential for explaining bonding, reactivity and the properties of elements. This article draws together the core ideas found in the IB syllabus and supports the approach used in Activate Chemistry, building from simple particle ideas to the more demanding concepts of periodicity and ionisation energy.
在 IB 化学中，物质的故事从原子开始。理解原子结构、电子排布和元素周期表是解释化学键、反应活性以及元素性质的基础。本文将 IB 大纲中的核心概念整合在一起，并延续 Activate Chemistry 教材的思路，从简单的粒子观念逐步深入到周期性和电离能等更难的概念。
1. The Nuclear Atom | 原子核模型
All matter is made of atoms. An atom has a central nucleus containing positively charged protons and neutral neutrons. The electrons are negatively charged and move around the nucleus in regions of space called shells or energy levels.
所有物质都由原子构成。原子中心有一个原子核，核内有带正电的质子和不带电的中子。电子带负电，在原子核周围被称为电子层或能级的空间区域中运动。
The nucleus is extremely small compared with the total size of the atom, but it contains almost all of the mass. The relative masses are approximately 1 for a proton, 1 for a neutron, and 1/1840 for an electron.
原子核与整个原子相比体积极小，但几乎集中了原子的全部质量。相对质量大约为：质子 1、中子 1、电子 1/1840。
The atomic number Z is the number of protons in the nucleus and defines the element. The mass number A is the total number of protons and neutrons. An atom is therefore electrically neutral when the number of protons equals the number of electrons.
原子序数 Z 是原子核中的质子数，它决定了元素的种类。质量数 A 是质子数与中子数之和。因此，当质子数与电子数相等时，原子呈电中性。
2. 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. For example, carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons.
同位素是同一种元素中质子数相同但中子数不同的原子。例如，碳-12 有 6 个质子和 6 个中子，而碳-14 有 6 个质子和 8 个中子。
Isotopes have the same chemical properties because they have the same number of outer-shell electrons. However, their physical properties such as mass and density can differ slightly.
同位素具有相同的化学性质，因为它们具有相同的最外层电子数。然而，它们的质量、密度等物理性质可能略有不同。
The relative atomic mass Aᵣ is the weighted average mass of an element's isotopes relative to 1/12 of the mass of a carbon-12 atom. It is calculated using the formula:
相对原子质量 Aᵣ 是元素各同位素质量的加权平均值，以碳-12 原子质量的 1/12 为基准。计算公式为：
Aᵣ = Σ(fᵢ × mᵢ)
where fᵢ is the fractional abundance of each isotope and mᵢ is its relative isotopic mass. Worked examples often require students to calculate Aᵣ from percentage abundances.
其中 fᵢ 是每种同位素的丰度分数，mᵢ 是其相对同位素质量。常见例题要求学生根据丰度百分比计算 Aᵣ。
| Isotope | Protons | Neutrons | Relative isotopic mass |
|---|---|---|---|
| ¹²₆C | 6 | 6 | 12 |
| ¹⁴₆C | 6 | 8 | 14 |
| ³⁵₁₇Cl | 17 | 18 | 35 |
3. Electron Arrangement | 电子排布
Electrons are arranged in shells around the nucleus. The first shell can hold up to 2 electrons, the second shell up to 8, and the third shell up to 18 in principle, but the IB course usually uses the simpler 2,8,8 pattern for the first 20 elements.
电子在原子核周围分层排布。第一层最多可容纳 2 个电子，第二层最多 8 个，第三层原则上最多 18 个，但 IB 课程通常对前 20 号元素使用较为简单的 2,8,8 排布规律。
electronic configuration 1s² 2s² 2p⁶ 3s¹, while chlorine (Z = 17) is 1s² 2s² 2p⁶ 3s² 3p⁵.
IB 水平要求掌握亚层表示法：电子按 1s、2s、2p、3s、3p 和 4s 的顺序填充。例如，钠（Z = 11）的电子排布为 1s² 2s² 2p⁶ 3s¹；氯（Z = 17）为 1s² 2s² 2p⁶ 3s² 3p⁵。
The electrons in the outermost shell are called valence electrons. These electrons determine how an element reacts and which group it belongs to in the periodic table.
最外层的电子称为价电子。这些电子决定了元素的反应方式以及它在周期表中所属的族。
4. The Periodic Table Structure | 元素周期表结构
The periodic table arranges elements in order of increasing atomic number. Elements in the same group have similar chemical properties because they have the same number of outer-shell electrons.
元素周期表按原子序数递增的顺序排列元素。同一族的元素化学性质相似，因为它们具有相同的最外层电子数。
The table is divided into s-block, p-block, d-block and f-block elements. The s-block contains Groups 1 and 2, the p-block contains Groups 13 to 18, and the transition metals occupy the d-block.
周期表分为 s 区、p 区、d 区和 f 区元素。s 区包含第 1 族和第 2 族，p 区包含第 13 至第 18 族，过渡金属位于 d 区。
| Block | Groups | Examples |
|---|---|---|
| s-block | 1 and 2 | Na, Mg |
| p-block | 13 to 18 | C, N, O, F, Ne |
| d-block | Transition metals | Fe, Cu, Zn |
The position of an element in the periodic table can be predicted from its electron configuration. The period number equals the highest occupied shell, while the group number is linked to the number of outer-shell electrons.
元素在周期表中的位置可根据其电子排布预测。周期数等于最高占据的电子层数，族数则与最外层电子数相关。
5. Periodic Trends: Atomic Radius | 周期性趋势：原子半径
Atomic radius decreases across a period because the nuclear charge increases while electrons are added to the same outer shell. The increased attraction pulls the outer electrons closer to the nucleus.
原子半径在同一周期从左到右逐渐减小，因为核电荷增大，而电子被加到同一外层。增强的吸引力把外层电子拉得更靠近原子核。
Atomic radius increases down a group because extra electron shells are added. The outer electrons are farther from the nucleus and are also shielded by inner electrons.
原子半径在同一族从上到下逐渐增大，因为增加了新的电子层。外层电子离原子核更远，同时也受到内层电子的屏蔽。
When answering IB questions, always link the trend to three factors: nuclear charge, electron shielding and distance between the outer electrons and the nucleus.
回答 IB 问题时，始终将趋势与三个因素联系起来：核电荷、电子屏蔽以及外层电子与原子核之间的距离。
6. Ionisation Energy | 电离能
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 ions with a 1+ charge.
第一电离能是指从 1 mol 气态原子中移走 1 mol 电子，生成 1 mol 带 1+ 电荷的气态离子所需的能量。
X(g) → X⁺(g) + e⁻
Across a period, first ionisation energy generally increases because of the greater nuclear charge and smaller atomic radius. Down a group it decreases because outer electrons are further from the nucleus and more shielded.
同一周期从左到右，第一电离能总体增大，因为核电荷更大、原子半径更小。同一族从上到下，第一电离能降低，因为外层电子离核更远且受到更多屏蔽。
Successive ionisation energies provide evidence for electron shells. A large jump occurs when an electron is removed from an inner shell, so the number of electrons removed before the jump corresponds to the number of outer-shell electrons.
连续电离能为电子层的存在提供了证据。当从内层移走电子时会出现一个很大的跳跃，因此在跳跃前移走的电子数等于最外层电子数。
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