IB化学 离子键 共价键 金属键 VSEPR 晶格
IB 化学的键与结构部分在新版指南里换了位置。2023 年 2 月发布、2023 年 8 月首次教学的课程把它收进 Structure 2，标题是 models of bonding and structure，SL 建议 20 小时、HL 建议 30 小时。旧版指南的 Topic 4 把离子键、共价键、分子间力、金属键分成四节，现在改成三个模型加一节从模型到材料。这一改动把重心从背结论挪到用模型预测性质。IBO 官网的说法是先用模型解释物质，再用模型预测没见过的材料。很多学生以为这部分只要背熔点与导电性表格，实际上题目更常问为什么。把结构到性质的因果链写清楚，比记住结论省时间。
In the 2023 DP chemistry guide, bonding sits under Structure 2 rather than the old Topic 4. The IBO allocates 20 teaching hours at SL and 30 at HL to these four sub-topics, which is a fair signal of the weight the ideas carry in the papers. Questions rarely ask you to recite a table of melting points; they ask you to pick one structural detail and use it to explain a property, so the chain from structure to behaviour is what gets marked.
三个模型各自解释什么
三个模型不是三套事实，而是三把精度不同的解释工具。离子模型处理金属与非金属之间电子完全转移的情形，共价模型处理非金属原子之间共用电子对的情形，金属模型处理外层电子离开单个原子、在整个晶格里自由跑动的情形。IBO 在 Structure 2.1 至 2.3 里分别给出每个模型的证据与适用边界。每个模型都有解释不了的材料，这正是 2.4 存在的理由。很多学生以为挑对模型就算答完，其实评分看的是模型给出的结构细节。同样说离子键，写清配位数与离子电荷才算落到结构上。只写阴阳离子互相吸引，多半拿不到解释分。
Each model is a tool with a stated scope rather than a fact to memorise. The ionic model covers electron transfer between a metal and a non-metal, the covalent model covers pairs of electrons shared between non-metals, and the metallic model covers valence electrons that leave their parent atoms and move through the whole lattice. Where a model stops working, Structure 2.4 takes over and treats bonding as a continuum, with real materials sitting between the three extremes.
离子模型：电子转移之后的巨晶格
离子模型只做一件事：金属原子把外层电子交给非金属原子，两边变成带相反电荷的离子。IBO 的表述是离子键来自阴阳离子之间的静电吸引，作用在所有方向上，因此离子化合物里没有单个分子。晶体中每个阳离子被若干阴离子围住，每个阴离子也被若干阳离子围住。NaCl 的熔点 801 摄氏度、沸点 1413 摄氏度，靠的就是这种遍布整个晶格的吸引。电荷越高、离子半径越小，吸引越强。写答案时把这两条写出来，比写离子键很强有用。常见的错误是把离子键说成两个离子之间的专属连线。
The ionic model reduces bonding to a single event: a metal atom hands outer electrons to a non-metal and the two resulting ions attract each other from every direction. Because that attraction is non-directional, an ionic solid is a giant lattice rather than a collection of molecules, and every cation is surrounded by anions in a repeating pattern. Sodium chloride melts at 801 °C and boils at 1413 °C, which is what a lattice-wide electrostatic attraction looks like once you put numbers on it.
配位数：NaCl 的 6:6 与 CsCl 的 8:8
离子晶体的结构细节里最常考的一项是配位数。半径比决定一个阳离子周围能塞进几个阴离子，因为阴离子彼此不能重叠。NaCl 型是 6:6，每个钠离子周围六个氯离子，每个氯离子周围也是六个钠离子。CsCl 型是 8:8，铯离子比较大，能容下八个氯离子而阴离子之间还不至于相撞。同是 1:1 的化学式，却能长出两种不同的晶格，原因只在离子大小。IBO 的数据手册给出离子半径表，就是让这类比较有据可依。答题时把半径比与配位数连起来写，不要只报数字。
Coordination number is the structural detail the papers ask about most often, and it is set by the radius ratio rather than by charge. In the rock salt structure each ion has six nearest neighbours of opposite charge, while caesium chloride reaches eight because the larger caesium ion can accommodate eight chloride ions without forcing them into each other. A single 1:1 formula can therefore produce two different lattices purely on the basis of ion size.
晶格能与熔点：MgO 为什么比 NaCl 高得多
离子电荷与半径的作用可以直接从熔点看出来。MgO 的熔点 2852 摄氏度，NaCl 只有 801 摄氏度，两者结构类型相同，差别在电荷。Mg2+ 与 O2- 的电荷乘积是四，Na+ 与 Cl- 只有一。晶格能与电荷乘积成正比，与离子间距成反比。半径变小会把离子拉近，吸引进一步增强。IBO 的数据手册里给出离子半径，就是为了让这类比较不必靠记。学生最常把摩尔质量当成熔点高低的理由，离子晶体熔化时并不需要拆开分子。比较两个离子化合物，先写电荷，再写半径，最后才写结论。
Lattice enthalpy scales with the product of the ionic charges and falls as the distance between the ions grows. Magnesium oxide melts at 2852 °C while sodium chloride needs only 801 °C, and both are 1:1 ionic solids, so the difference comes from a charge product of four against one rather than from the mass of the formula unit. Reaching for molar mass here is the standard wrong turn, because melting an ionic solid breaks attractions between ions instead of bonds inside molecules.
离子晶体为什么脆，熔融后才导电
离子晶体受力时，一层离子会相对另一层平移。平移之后同号离子被迫相邻，静电排斥把晶体沿滑移面劈开。所以离子晶体硬而脆，敲一下就在解理面裂开，而不是被压扁。导电性看的是有没有能移动的带电粒子。固态时离子被固定在晶格点上，接上电源也没有载流子。熔融或溶于水之后离子可以移动，导电性立刻出现。这一步最容易混到金属那里：金属固态就导电，因为它的载流子是电子而不是离子。写导电题先问一句载流子是谁，答案就清楚了。
An ionic solid shatters because a shear moves like charges next to each other, and the resulting repulsion splits the crystal along the slip plane. Conductivity depends on mobile charge carriers; in the solid state the ions are locked into the lattice, so melting or dissolving the solid is what frees them and lets current flow. Metals differ precisely because their carriers are electrons, which are already free in the solid.
共价模型：Lewis 式与八隅体例外
共价模型处理的是共用电子对。Lewis 式把每个价电子都画出来，成键对与孤对分开，因此可以检查电子有没有漏算。八隅体规则很好用，但不适用于所有分子。BF3 的中心硼只有六个价电子，SF6 的中心硫周围有十二个。IBO 的考纲把这类分子作为模型的边界写进来，不是让你背一张例外清单。画 Lewis 式时先数总价电子，再搭骨架，最后补孤对，顺序反了就会漏电子。常见的错误是给 BF3 硬加一对孤对把硼凑成八个，那会改掉分子的形状与偶极。
Lewis formulas work because every valence electron is accounted for, with bonding pairs and lone pairs kept apart. The octet rule is a useful default rather than a law; boron in BF3 is surrounded by six electrons and sulfur in SF6 by twelve, and the guide lists such cases as the limits of the model. Count the total valence electrons first, then build the skeleton and add the lone pairs last, because the reverse order loses electrons.
VSEPR：电子域决定形状，孤对压小键角
VSEPR 的做法是数中心原子周围的电子域。一个双键或三键算一个域，孤对也算一个域。域的数目决定电子域几何，再看孤对占了几个位置，就得到分子形状。CH4 的四个成键域给出 109.5 度的四面体。NH3 有一个孤对，键角压到约 107 度。H2O 有两个孤对，键角再压到约 104.5 度。孤对只受一个原子核约束，电子云更胖，排斥更强，顺序是孤对与孤对大于孤对与成键对，大于成键对与成键对。CO2 只有两个域，直线 180 度，BF3 三个域是 120 度，SF6 六个域是 90 度。
VSEPR counts electron domains around the central atom, where a double or triple bond counts as one domain and a lone pair counts as one as well. Four bonding domains give the tetrahedral 109.5 degrees of methane, while one lone pair in ammonia compresses the angle to about 107 degrees and two lone pairs in water bring it to roughly 104.5 degrees. Lone pairs repel more strongly because they are held by a single nucleus, so the order runs lone pair to lone pair first, then lone pair to bonding pair, then bonding pair to bonding pair.
键有极性不等于分子有极性
键的极性来自电负性差，分子的极性还要看几何。CO2 有两根极性键，但两根键的偶极大小相等、方向相反，向量和为零，整个分子非极性。H2O 的两根键夹角约 104.5 度，向量和不为零，分子有净偶极。判断的次序是先看中心原子有没有孤对，再看配位原子是否相同。CCl4 与 CH4 都是四面体、四个相同配体，非极性；CHCl3 换掉一个配体就变成极性。很多学生以为电负性差超过某个数字就一定是离子键，IBO 的模型并不给这条硬阈值，键的性质是连续变化的，2.4 的键三角正是为此准备。
A polar bond comes from an electronegativity difference, but a polar molecule also needs those bond dipoles to add up. Carbon dioxide has two polar bonds of equal size pointing in opposite directions, so the vectors cancel and the molecule is non-polar, whereas the 104.5 degree angle in water leaves a net dipole pointing towards the oxygen. Check for lone pairs on the central atom first, then compare the surrounding atoms, because symmetry can cancel a polar bond.
金属模型：离域电子海带来导电与延展
金属模型把外层电子看作整个晶格共有。正离子按规则排列，电子在它们之间自由移动，这种吸引没有方向性。没有方向性带来两个可观测的性质：层与层可以相对滑动而不断裂，所以金属有延展性；电子在固态就能移动，所以金属固态导电导热。金属键的强度随离域电子数目增加、离子半径减小而上升。第一族金属只有一个外层电子，熔点低、质地软；过渡金属离域电子多，熔点高、硬度大。合金是把尺寸不同的原子掺进去，层被撑得难以滑动，硬度上升。青铜是铜加锡，黄铜是铜加锌。
In the metallic model the valence electrons belong to the whole lattice, and the attraction between them and the cations has no direction. That non-directional bonding lets planes of ions slide past each other instead of shattering, so metals are malleable; the same mobile electrons carry charge in the solid state, which ionic solids cannot do. Bond strength rises with more delocalised electrons and smaller cations, which is why group 1 metals are soft and melt low while transition metals are hard and hold their shape.
从模型到材料：四类结构对照与石墨的例外
把模型用在材料上，常见的是四类结构。离子晶格熔点高、硬而脆、熔融导电。金属晶格熔点高低不一、延展、固态导电。巨共价网络熔点极高、极硬、通常不导电。分子晶体熔点低、软、不导电，因为熔化只克服分子间力。金刚石是巨共价网络，每个碳四根共价键，硬度最高。石墨是同一元素的另一种排列，每层内每个碳三根共价键，剩下一个电子离域，所以石墨沿层面导电。石墨层间只有色散力，层容易滑开，因此石墨软而且能当润滑剂。IBO 常用金刚石与石墨这一对考结构决定性质。
The four structural types follow from the models: ionic lattices, metallic lattices, giant covalent networks and molecular solids. Diamond and graphite contain the same element yet behave in completely different ways, because every carbon in diamond has four covalent bonds in a three-dimensional network while each carbon in graphite has three bonds within a layer and one delocalised electron. Graphite conducts along its layers and feels soft because those layers are held together only by dispersion forces.
HL 的键三角：键是连续谱不是三个盒子
HL 要处理模型之间的边界。键三角把物质放在平均电负性与电负性差构成的坐标里：平均电负性低的一端偏金属，电负性差大的一端偏离子，中间偏共价。NaCl 的平均电负性是 2.05、差是 2.3，落在三角上部，约 75% 离子性。AlCl3 的平均电负性是 2.4、差是 1.6，落在离子与共价的边界附近，约 50% 共价。这解释了 AlCl3 的熔点为什么只有 192 摄氏度，而 NaCl 是 801 摄氏度。铝氯键有显著的共价成分，层与层之间作用弱。答题时先算平均电负性与差，再定位，再推性质，中间一步不能跳过。
At HL the bonding triangle places a substance by the average electronegativity of its atoms and by the difference between them. Sodium chloride has an average of 2.05 and a difference of 2.3, which puts it near the ionic corner at roughly 75 per cent ionic character; aluminium chloride sits at an average of 2.4 and a difference of 1.6, close to the ionic and covalent border with about 50 per cent covalent character. That shift explains a melting point of 192 °C against 801 °C for sodium chloride, so calculate both values before you place the point.
答题动作：explain、deduce、suggest 各写什么
命令词决定答案写到哪一层。Explain 要求给因果，光写结论不得分。Deduce 要求从给定信息推，通常要用数据手册里的数值或结构图。Suggest 允许合理推测，但推测要挂在一个已知的结构特征上。阅卷时最常见的失分是答案停在术语：写了强静电吸引，却没有说清吸引发生在哪些粒子之间。把粒子、作用、后果三样写全，一段话就够。比较题里每一条都要写两边，只描述一边往往拿一半分。平时练习就把这三样按顺序写，考场不需要再想结构。
Command words set the depth expected: explain needs a cause, deduce needs a step from the data or diagram provided, and suggest allows a reasoned proposal that still has to hang on a structural feature. The commonest loss is an answer that stops at terminology, naming a strong electrostatic attraction without saying which particles attract and what follows from it. In comparison questions, give both sides of every point, because a one-sided answer usually earns half marks.
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