📚 IB Chemistry: Core Rules of Chemical Reactivity | IB化学：化学反应活性核心规律
Chemical reactivity is not a random collection of isolated facts. In the IB Chemistry course, you are expected to predict how elements and compounds behave by combining periodic patterns, electron transfer rules, thermodynamic driving forces, and kinetic barriers. When these four ideas are used together, the apparent complexity of inorganic and redox chemistry becomes a set of simple trends.
化学反应活性并非一堆孤立的零散事实。IB化学课程要求你将周期规律、电子转移规则、热力学驱动力和动力学势垒结合起来，预测元素和化合物的行为。当这四个观念协同使用时，无机化学和氧化还原化学的复杂表象就会变成一组简单规律。
1. The Periodic Table as a Predictive Framework | 元素周期表：反应活性预测框架
The periodic table is the most powerful reactivity tool in chemistry. A period tells you the number of occupied electron shells; a group tells you the number of valence electrons. Elements on the left have low ionization energies and low electronegativities, so they tend to form cations. Elements on the right, especially the halogens, have high electronegativities and tend to form anions.
周期表是化学中最重要的反应活性工具。周期数告诉你电子占据的壳层数，族数告诉你价电子数。左侧元素电离能低、电负性低，容易形成阳离子；右侧元素特别是卤素电负性高，容易形成阴离子。
The valence electron configuration also determines the common oxidation state. Sodium has one valence electron and forms Na⁺; magnesium has two and forms Mg²⁺; aluminium has three and forms Al³⁺. Chlorine has seven valence electrons and usually forms Cl⁻. This simple group-based logic is usually the first prediction a student should make.
价电子构型还决定常见氧化态。钠有1个价电子，形成Na⁺；镁有2个，形成Mg²⁺；铝有3个，形成Al³⁺；氯有7个价电子，通常形成Cl⁻。这种基于族序数的简单逻辑，应当是学生做出的第一层预测。
- Group 1 metals: reactivity increases down the group. 第1族金属：同族向下反应活性增大。
- Group 17 nonmetals: oxidizing strength decreases down the group. 第17族非金属：同族向下氧化能力减弱。
2. Atomic Radius | 原子半径
Atomic radius is essential for explaining reactivity because it controls how tightly the outer electrons are held. Across a period, nuclear charge increases while the electrons are added to the same shell. The increased effective nuclear charge pulls the electron cloud closer, so atomic radius generally decreases from left to right.
原子半径是解释反应活性的关键，因为半径决定外层电子被束缚的紧密程度。同一周期内，核电荷增加而电子仍进入同一壳层，有效核电荷增大使电子云收缩，因此原子半径从左向右一般减小。
Down a group, each new shell is farther from the nucleus, and inner electrons shield the outer electrons from the full nuclear charge. The atomic radius therefore increases down a group. For metals, a larger radius usually means the outer electron is more easily lost, which increases metallic reactivity. For nonmetals, a smaller radius often means the atom attracts additional electrons more strongly, making it a stronger oxidizer.
同族向下，新电子层距核更远，内层电子对外层电子的屏蔽作用增强，因此原子半径向下增大。对于金属，半径越大通常意味着外层电子越容易失去，金属反应活性越高；对于非金属，半径越小通常意味着原子越强烈地吸引额外电子，氧化能力越强。
Transition metals show only a slight decrease in atomic radius across each series because the added electrons enter inner d subshells, which do not greatly expand the atom. This explains why many transition metals have similar chemical reactivity and can form ions with different charges.
过渡金属在同一系列中原子半径仅轻微减小，因为新增电子进入内层d亚层，不会明显扩大原子体积。这就解释了为什么许多过渡金属具有相似的反应活性，并且能形成多种不同电荷的离子。
3. Ionization Energy | 电离能
Ionization energy is the energy required to remove one mole of electrons from one mole of gaseous atoms or ions. The first ionization energy is defined by the process:
电离能是指从一摩尔气态原子或离子中移除一摩尔电子所需的能量。第一电离能可用以下过程定义：
M(g) → M⁺(g) + e⁻
A low first ionization energy means the atom can easily form a cation, so the element is likely to be a reactive metal. Across a period, ionization energy generally increases because the same shell feels a stronger nuclear charge. Down a group, ionization energy decreases because the outer electron is farther from the nucleus and better shielded.
低的第一电离能意味着该原子容易形成阳离子，因此该元素很可能是活性金属。同周期内，电离能总体增大，因为核电荷对同一壳层的引力增强；同族向下，电离能减小，因为外层电子距核更远且屏蔽更强。
There are important exceptions. In Group 2 to Group 3, beryllium has a filled 2s subshell, while boron’s next electron enters a higher-energy 2p orbital, so boron has a lower first ionization energy than expected. Similarly, nitrogen has a half-filled 2p subshell, while oxygen has one doubly occupied p orbital; electron repulsion makes oxygen’s first ionization energy lower than nitrogen’s.
这里存在重要特例。从第2族到第3族，铍具有填满的2s亚层，而硼的下一个电子进入能量更高的2p轨道，因此硼的第一电离能低于预期。同理，氮具有半满的2p亚层，而氧的某个p轨道有两个电子；电子互斥使氧的第一电离能低于氮。
Successive ionization energies are also informative. When all valence electrons are removed, the next ionization energy jumps sharply because it must remove a core electron. For example, sodium has low IE₁ and IE₂ but a very large IE₃. This jump tells you that sodium has only one valence electron and that Na⁺ is its stable cation.
逐级电离能也具有诊断价值。当所有价电子都被移除后，下一级电离能会急剧跃升，因为需要移除内层电子。例如，钠的IE₁和IE₂较低，而IE₃非常大。这一跃升说明钠只有一个价电子，Na⁺是其稳定阳离子。
4. Electronegativity and Bond Polarity | 电负性与键的极性
Electronegativity, represented by the Greek letter chi (χ), measures the ability of an atom in a covalent bond to attract shared electrons. The most commonly used scale is the Pauling scale, in which fluorine has the highest value of about 4.0.
电负性用希腊字母chi（χ）表示，衡量的是共价键中原子吸引共享电子的能力。最常用的是Pauling标度，其中氟的电负性最高，约为4.0。
The general trends are the same as for ionization energy: electronegativity increases across a period and decreases down a group. Period 2 elements are usually more electronegative than Period 3 elements in the same group because smaller atoms hold electron density more tightly.
电负性的总体趋势与电离能一致：同周期从左向右增大，同族从上到下减小。第2周期元素通常比同族第3周期元素电负性更强，因为更小的原子将电子密度束缚得更紧。
The electronegativity difference between two bonded atoms predicts bond type. A difference close to zero gives a non-polar covalent bond. A difference between roughly 0.5 and 1.8 gives a polar covalent bond. A difference greater than about 1.8 often indicates an ionic bond. These values are only guides, but they are very useful when comparing oxides, halides, and acids.
两个成键原子之间的电负性差可预测键型。差值接近0形成非极性共价键；约0.5到1.8之间形成极性共价键；差值大于约1.8通常表示离子键。这些数值只是参考，但在比较氧化物、卤化物和酸时非常实用。
Bond polarity controls the electron-rich and electron-poor regions of a molecule. In carbonyl compounds, for example, carbon is electron-poor because oxygen is more electronegative, so nucleophiles attack carbon. In water, oxygen is electron-rich and acts as a Lewis base toward many acids. Thus electronegativity is not just a periodic trend; it is a guide to reaction mechanism.
键的极性决定分子中电子富集区和缺电子区。例如，羰基化合物中，氧的电负性更强，碳是缺电子中心，因此亲核试剂进攻碳；水中氧是富电子中心，在许多酸反应中充当路易斯碱。因此电负性不仅是周期趋势，更是反应机理的指南。
5. Metal Reactivity and the Activity Series | 金属反应活性与活动性序列
Metallic reactivity is the ease with which a metal loses electrons to form positive ions. This depends on ionization energy, atomization energy, hydration enthalpy, and lattice enthalpy in the final salt. In aqueous chemistry, the electrochemical series gives the most reliable ranking, but a simple activity series remains useful for displacement reactions and reactions with water or acid.
金属反应活性是金属失去电子形成阳离子的难易程度。它取决于电离能、原子化能、水合焓以及最终盐的晶格焓。在水溶液化学中，电化学序列给出最可靠排序，但简单的活动性序列对于置换反应以及与水和酸的反应仍然非常有用。
| Metal | Ion formed | Trend |
| Potassium K | K⁺ | Most reactive |
| Sodium Na | Na⁺ | ↑ |
| Calcium Ca | Ca²⁺ | ↑ |
| Magnesium Mg | Mg²⁺ | ↑ |
| Aluminium Al | Al³⁺ | ↑ |
| Zinc Zn | Zn²⁺ | ↑ |
| Iron Fe | Fe²⁺ | ↑ |
| Lead Pb | Pb²⁺ | ↑
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