IB化学 原子结构 同位素 电子排布 电离能
IB 文凭课程的化学在 2025 年首次评估的版本里改成结构与反应两条主线，原子结构是结构线的开头部分。按 IBO 官网公布的学科简章，Structure 1 在 SL 建议 17 小时、HL 建议 21 小时，整门课的内容课时是 SL 110 小时、HL 180 小时。这一块的知识点看着都是以前见过的内容，考试却要求用 IB 的写法重新表述一遍。原子、同位素、电子排布、电离能四项连起来，后面讲成键与周期性才有落脚点。下面按考点顺序拆开，每个小节配一个可以照着做的判断方法。
The 2025 chemistry course is organised around two concepts, structure and reactivity, and atomic structure opens the structure strand. The IBO subject brief recommends 17 teaching hours at SL and 21 at HL for Structure 1, inside 110 SL and 180 HL hours of syllabus content. Much of the material looks familiar from earlier years, yet the exam expects it restated in IB language, since bonding and periodicity are built on the vocabulary set out here.
原子核与电子：质子数决定元素身份
原子由一个带正电的原子核与核外电子组成，原子核里是质子与中子。质子数就是原子序数，它决定这是哪一种元素，也决定元素在周期表里的位置。中性原子里电子数等于质子数，所以整体电荷为零。电子质量只有质子的约一千八百四十分之一，原子质量几乎全部集中在核上。IB 的题目常给一个核素符号让你数质子、中子、电子，左上角是质量数，左下角是原子序数。中子数就是质量数减原子序数，这一步最容易在离子题里被忘记，因为电子数已经变了。很多学生以为左上角的数字只是排版习惯，其实它直接给出核子总数。
An atom is a tiny positive nucleus surrounded by electrons; the nucleus holds protons and neutrons, and the proton count is the atomic number that fixes the element and its place in the periodic table. Because an electron weighs about 1/1840 of a proton, nearly all the mass sits in the nucleus, which is why the mass number counts nucleons rather than electrons. In IB questions the symbol carries the mass number top left and the atomic number bottom left, so the neutron count is the difference, a step candidates skip whenever the particle is an ion.
质量数与同位素：同一元素不同中子数
同位素是质子数相同、中子数不同的同一种元素的原子。它们的化学性质几乎一样，因为化学反应只涉及最外层电子。物理性质与质量有关，所以密度、扩散速率会不一样。氢的三个同位素最常被拿来举例，氕没有中子，氘有一个，氚有两个。IB 要求能写出同位素符号，也能判断两个粒子是不是同位素。常见的错误是把同位素和同素异形体混在一起，金刚石与石墨是同一元素的两种结构，不是同位素。判断标准只有一条，质子数相同而中子数不同。
Isotopes are atoms of one element with identical proton numbers but different neutron counts, so their chemistry is almost the same because reactions involve the outer electrons. Mass-dependent physical properties such as density and rate of diffusion do differ, and that difference is what makes separation possible. Hydrogen is the standard illustration, with protium carrying no neutron, deuterium one and tritium two. IB expects correct isotope notation and a clear distinction from allotropes, since diamond and graphite are structures of one element rather than isotopes.
相对原子质量是加权平均：镁的算例
相对原子质量是各同位素的相对质量按丰度加权的平均值。计算时把每个同位素的质量乘以它的相对丰度，再把结果相加。丰度写成小数，百分比要先除以一百。以镁为例，镁 24 占 79.0%，镁 25 占 10.0%，镁 26 占 11.0%，加权后是 24 乘 0.790 加 25 乘 0.100 加 26 乘 0.110，得到 24.32。数据小册子里给的镁是 24.31，差别来自丰度取值的有效数字。常见的错误是把三个质量直接平均成 25.0，这样做忽略了丰度差异，答案会明显偏大。反过来，题目给 Ar 和两个丰度时，设一个未知数就能解出比例。
Relative atomic mass is a weighted mean: multiply each isotope mass by its fractional abundance, add the results, and quote the value to the precision of the data given. Magnesium is a clean worked example, where 79.0% Mg-24, 10.0% Mg-25 and 11.0% Mg-26 give 24.32 against the data booklet value of 24.31. A plain average of 25.0 is the usual wrong answer because it treats the three isotopes as equally common. When Ar and one abundance are given, set the other as x and solve for it.
离子电荷与电子数换算
离子电荷等于质子数减去电子数。阳离子失去电子，阴离子得到电子，原子核本身没有变化。镁失去两个电子成为镁离子，排布变成 1s2 2s2 2p6，与氖相同。氯得到一个电子成为氯离子，排布与氩相同。题目常问某个离子的电子数与未成对电子数，先写排布再数。这一步最容易出错的地方是只改电子数，却把质量数也一起改了，而质量数只由核子决定。另一个高频错误是电荷数字写成一，规范写法是单独一个加号或减号。
Ion charge is the gap between protons and electrons, so a cation has lost electrons and an anion has gained them while the nucleus stays untouched. Magnesium losing two electrons reaches the neon configuration, chlorine gaining one reaches argon, and that is why these ions are described as isoelectronic with noble gases. Questions often ask for the electron count and the number of unpaired electrons, so write the configuration first and count from it. Mass number never changes when a neutral atom becomes an ion, whatever the charge.
氢原子线状光谱与能级
氢原子受激发后发出的光经过棱镜会分成几条分离的谱线，而不是一条连续色带。这说明电子只能处在分立的能级上，跃迁时放出固定能量的光子。能量差与光子频率成正比，关系式是 E 等于 hf，IB 2025 版指南把频率符号改成了 f。谱线在高频端越来越密，最后收敛于一个极限，对应电子从最高能级回到同一低能级。巴尔末系落在可见光区，莱曼系在紫外区。很多学生以为线状光谱说明原子只有一个电子，其实它只说明能级是分立的。解释题要写出跃迁方向与能量差，只写放出光子通常拿不到全分。
A hydrogen discharge tube gives separate lines rather than a continuous band, which is evidence that electrons sit on discrete levels and emit photons of fixed energy when they move between them. Photon energy is proportional to frequency through E = hf, and the 2025 guide writes frequency as f instead of the older nu symbol. Lines crowd together at high frequency and converge on a limit where the electron starts from the highest level. Balmer lines fall in the visible region and Lyman lines in the ultraviolet, and an explanation mark needs both the transition and the energy difference.
主层与亚层：1s 到 4s 的填充顺序
电子排布按能级从低到高填充，顺序是 1s、2s、2p、3s、3p、4s、3d、4p。4s 排在 3d 前面，因为填充时它的能量更低。每个亚层容纳的电子数是 s 两个、p 六个、d 十个、f 十四个，相当于轨道数乘二。主层 n 最多容纳 2n 平方个电子，第三层最多十八个。写排布时上标写电子数，钠写作 1s2 2s2 2p6 3s1。常见的错误是按主层顺序把 3d 写在 4s 前面，这样写会直接扣分。过渡金属还要注意铬与铜的反常，它们的 3d 与 4s 分布不符合简单规则。
Electrons fill levels in order of increasing energy, and the sequence runs 1s, 2s, 2p, 3s, 3p, 4s and only then 3d, which surprises students who expect shells to fill strictly by number. Subshell capacity is the orbital count doubled: two in s, six in p, ten in d and fourteen in f, while shell n holds up to 2n squared. Sodium is written 1s2 2s2 2p6 3s1, and placing 3d before 4s is a common slip that costs marks. Chromium and copper are the standard exceptions, and both are worth memorising as written in the data booklet.
电子排布三条规则：构造原理 洪特规则 泡利原理
构造原理规定电子先填能量最低的轨道。洪特规则规定简并轨道先各占一个电子，自旋方向相同，然后才配对。泡利不相容原理规定一个轨道最多两个电子，而且自旋相反。三条规则一起用，才能写出正确的排布与轨道图。氮的 2p 三个电子各占一个轨道，写成三对电子就违反了洪特规则。氧的 2p 有四个电子，其中一对已经配对。阅卷时看轨道图会数方框里的箭头方向，两个同向箭头放进一个方框直接判错。描述题要写出规则名称，只画图不写说明通常拿不全分。
Three rules do the work: the Aufbau principle fills the lowest available orbital first, Hund's rule spreads electrons across degenerate orbitals with parallel spins before any pairing, and the Pauli exclusion principle allows at most two electrons per orbital with opposite spins. Nitrogen keeps its three 2p electrons in three separate orbitals, so pairing them early breaks Hund's rule. Examiners reading an orbital diagram check arrow directions as carefully as counts, and two arrows pointing the same way in one box is marked wrong. Name the rule in written answers instead of only drawing boxes.
电离能的定义与周期趋势
第一电离能是气态原子失去一个电子所需的最小能量，单位是千焦每摩尔。同一周期从左到右总体升高，因为核电荷增加而电子加在同一主层，半径缩小。同一主族从上到下降低，因为外层电子离核更远，屏蔽更强。第三周期的数据可以看出细节，钠 496、镁 738、铝 578、硅 786、磷 1012、硫 1000、氯 1251、氩 1521。铝低于镁，因为电子进入 3p，能量略高且屏蔽增加。硫低于磷，因为 3p 出现成对电子，电子间排斥降低了束缚。解释时要同时写出核电荷、屏蔽与半径三个因素。
First ionisation energy is the minimum energy needed to remove one electron from a gaseous atom, quoted in kilojoules per mole. Across a period it rises overall as nuclear charge grows while electrons stay in the same shell, and down a group it falls as the outer electron sits further out behind more shielding. Period 3 values show the detail, from sodium at 496 and magnesium at 738 through aluminium at 578, silicon at 786, phosphorus at 1012, sulfur at 1000, chlorine at 1251 and argon at 1521. Aluminium sits below magnesium because its electron enters 3p, and sulfur sits below phosphorus because pairing in 3p adds repulsion.
逐级电离能的跳跃说明壳层结构
逐级电离能是逐个移走电子所需的能量，每一级都比上一级高。当某一级突然跳升几倍，说明电子开始来自更内层的壳层。镁的前两级是 738 与 1451，第三级跳到 7733，这个比例说明最外层只有两个电子。钠的第十一级才出现大跳，因为第十一个电子来自 1s。题目常要求根据数据推断电子层数与最外层电子数，先找出最大的跳升出现在第几级。这一步最容易算错的地方是把跳升级数当成电子数，正确的电子数是跳升级数减一。
Successive ionisation energies climb steadily, and a jump of several times the previous value marks the point where electrons start coming from an inner shell. Magnesium gives 738 and 1451 for the first two removals, then 7733 for the third, the signature of two outer electrons. Sodium only jumps at the eleventh, because that electron leaves the 1s subshell. When a question asks you to deduce shell structure, locate the largest jump first and read the outer electron count as that position minus one.
质谱读数与 Ar 的计算
质谱仪把样品电离、加速、按质荷比分离，再记录各离子的相对丰度。谱图上最高的峰通常定为 100，其他峰按比例换算。读图时要看清横轴是质荷比，纵轴是相对丰度百分比。分子离子峰给出相对分子质量，碎片峰可以用来推断结构。HL 的补充内容会要求用质谱数据计算 Ar，方法与同位素丰度加权完全一致。含溴或含氯的分子会出现特征双峰，因为两种同位素丰度比是固定的。常见的错误是把最高峰的质量直接当成 Ar，忽略其他峰的贡献。
A mass spectrometer ionises the sample, accelerates the ions, separates them by mass-to-charge ratio and records relative abundances. The tallest peak is normally scaled to 100 and every other peak expressed against it, so read both axes before doing any arithmetic. The molecular ion peak gives relative molecular mass while fragment peaks hint at structure. HL extension work uses these abundances for an Ar calculation with the same weighted mean as before, and bromine or chlorine compounds give recognisable doublets because their isotope ratios are fixed.
命题用语决定要写多少：define explain deduce
IB 的每个小问都以命题用语开头，用词直接决定要写多少。Define 只要给出定义，一句话足够，多写不加分。State 只要给出答案，不需要理由。Describe 要说清是什么、有哪些特征。Explain 要给出原因与机制，通常每个理由一分。Deduce 要求从给定数据推出结论，推理过程要写出来。Calculate 要写公式、代入与单位，有效数字与数据一致。很多学生以为写得多就拿得多，其实超出命题用语要求的句子不进入评分点。答题前先圈出动词，能省下不少时间。
Every IB sub-question opens with a command term, and the verb sets how much to write. Define needs one sentence, State needs the answer alone, Describe needs the features, and Explain needs causes or mechanisms with a mark usually attached to each reason. Deduce means drawing a conclusion from the data given, so show the reasoning, while Calculate expects the equation, the substitution and units to sensible significant figures. Writing beyond the command term rarely earns extra credit and takes time you need elsewhere in the paper.
常见扣分点与两分钟自检
排布写错上标是最常见的失分来源，写完要数一遍电子总数是否等于质子数。离子题要确认电子数改动而质量数不动。氩与钾的排布顺序常被写反，注意 4s 先填。计算 Ar 时丰度要用小数，百分比忘除一百会让答案偏差巨大。解释电离能趋势要落到核电荷、屏蔽与半径三个因素。画轨道图要用方框与箭头，不要用点或叉。最后留两分钟检查单位与有效数字，很多学生会在这里丢分。做完一道题回头读一遍题干的第一句，命题用语往往就写在那里。
Counting electrons against the proton number catches most configuration slips before they cost marks, and ion questions need the electron count changed without touching mass number. Argon and potassium are often written in the wrong order, so remember that 4s fills first. For Ar calculations convert percentages into fractions, since forgetting to divide by 100 gives an answer roughly a hundred times too large. Explaining ionisation energy trends needs nuclear charge, shielding and radius together, and orbital diagrams need boxes with arrows. Two minutes on units and significant figures usually earns more than another sentence.
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