📚 Polarity | 极性
Polarity is a fundamental concept in Edexcel A-Level Chemistry that describes how electron density is distributed unevenly within a bond or across a whole molecule. It explains why some molecules have partially charged ends and how these charges influence physical properties such as boiling point, melting point and solubility. Mastering polarity requires combining three ideas: electronegativity, molecular shape and symmetry.
极性是 Edexcel A-Level 化学中的核心概念，描述电子密度在一根化学键中或整个分子中分布不均匀的方式。它可以解释为什么一些分子的两端带有部分电荷，以及这些电荷如何影响沸点、熔点和溶解度等物理性质。掌握极性需要把三个要点结合起来：电负性、分子形状和对称性。
1. What is Polarity? | 什么是极性？
Polarity arises when a bond or a molecule has an uneven distribution of electron density. In a covalent bond, electrons are shared between two atoms, but this sharing is not always equal. If one atom attracts the bonding pair more strongly, one end becomes slightly negative (δ⁻) and the other slightly positive (δ⁺).
当化学键或分子中电子密度分布不均匀时，就会产生极性。在共价键中，两个原子共享电子，但这种共享并不总是均等。如果一个原子对成键电子对的吸引更强，一端就会带上部分负电荷（δ⁻），另一端带上部分正电荷（δ⁺）。
Polarity can occur at two levels: bond polarity and molecular polarity. Bond polarity looks at a single bond and asks whether the two atoms have different electronegativities. Molecular polarity looks at the whole molecule and asks whether the individual bond dipoles add together to give a net dipole, or cancel out because of symmetry.
极性可以出现在两个层面：键的极性和分子的极性。键的极性只看一根键，判断成键的两个原子电负性是否不同。分子的极性则看整个分子，判断各个键偶极是叠加成一个净偶极，还是因为对称性而相互抵消。
A molecule may contain polar bonds but still be non-polar overall if the bond dipoles cancel by symmetry. This distinction is examined almost every year, so always separate the two ideas clearly in your answer.
一个分子可能含有极性键，但如果键的偶极因对称性而相互抵消，分子整体仍可能是非极性的。这一区分几乎每年都会考到，所以在答题时一定要把这两个层面清楚分开。
2. Electronegativity and Bond Polarity | 电负性与键的极性
Electronegativity is the power of an atom to attract the bonding pair of electrons in a covalent bond. It is a relative property measured on a scale, not an absolute energy value, so it should not be confused with electron affinity or ionisation energy.
电负性是原子在共价键中吸引成键电子对的能力。它是一种在标度上测量的相对性质，不是绝对的能量数值，因此不要把它和电子亲和能或电离能混淆。
On the Pauling scale, fluorine has the highest value of 4.0, while caesium has one of the lowest values at about 0.7. A difference in electronegativity (Δχ) between two bonded atoms creates bond polarity. The bigger the difference, the more polar the bond is.
在鲍林标度上，氟的电负性最高，为 4.0，而铯是最低值之一，约为 0.7。两个成键原子之间的电负性差值（Δχ）会产生键的极性。差值越大，键的极性越强。
There is a useful approximate guide that links Δχ to bond type. It is only a guide, and examiners accept it as a rough rule rather than a strict law.
有一个常用的近似规则把 Δχ 与键的类型联系起来。它只是参考规则，考试中把它当作粗略判断即可，而不是严格的定律。
→ non-polar covalent bond; 0.4 ≤ Δχ ≤ 1.7 → polar covalent bond; Δχ > 1.7 → ionic bond
.4, which sits exactly on the borderline. In practice, C-H bonds are treated as essentially non-polar in organic molecules, because the difference is so small that the bond dipole is negligible.
注意 C-H 键的 Δχ = 2.5 − 2.1 = 0.4，正好落在临界点上。实际处理有机分子时，C-H 键通常被视为基本非极性，因为差值太小，键偶极可以忽略。
3. The Pauling Scale and Common Values | 鲍林标度与常见数值
Learning a small set of Pauling values makes polarity questions much faster, because you can estimate Δχ without being given data. The most useful values for A-Level are shown below.
记住一小组鲍林电负性数值会让极性题目快很多，因为你可以直接估算 Δχ，不必等题目给数据。A-Level 中最常用的数值如下表。
| Element | 元素 | Pauling electronegativity | 鲍林电负性 |
| F | 4.0 |
| O | 3.5 |
| N, Cl | 3.0 |
| C, S, I | 2.5 |
| H | 2.1 |
| Na, Li | 0.9 to 1.0 |
Electronegativity increases across a period and decreases down a group, because nuclear charge rises across a period while atomic radius and shielding increase down a group. This trend mirrors the periodic trends you already know for ionisation energy.
电负性在同一周期中从左到右增大，在同一族中从上到下减小，因为同周期核电荷增加，而同族中原子半径和屏蔽效应随周期向下而增大。这一趋势与你已经掌握的电离能周期性规律一致。
lent with chlorine as the δ⁻ end. In Na-Cl, Δχ = 3.0 − 0.9 = 2.1, so the bonding is ionic.
快速估算通常就够了。例如在 H-Cl 中，Δχ = 3.0 − 2.1 = 0.9，所以这是极性共价键，氯是 δ⁻ 的一端。在 Na-Cl 中，Δχ = 3.0 − 0.9 = 2.1，因此属于离子键。
4. Bond Dipoles and Dipole Moments | 键偶极与偶极矩
A polar bond can be represented by an arrow with a small cross at the positive end. The arrow points from δ⁺ to δ⁻, which is the direction in which the bonding electrons are pulled. This arrow is called a bond dipole.
极性键可以用一个带箭头、正电端标小十字的符号表示。箭头从 δ⁺ 指向 δ⁻，也就是成键电子被拉向的方向。这个箭头称为键偶极。
The size of a bond dipole is measured by the dipole moment, symbol μ, which depends on both the size of the partial charges and the distance between them. Dipole moments are measured in debye (D), and a larger μ means a more polar bond.
键偶极的大小用偶极矩来量度，符号为 μ，它取决于部分电荷的大小和正负电荷中心之间的距离。偶极矩的单位是德拜（D），μ 越大表示键的极性越强。
μ = Q × r
Here Q is the magnitude of the partial charge and r is the distance between the two charge centres. Because dipole moment is a vector quantity, bond dipoles must be added with direction in mind, not simply added as numbers.
式中 Q 是部分电荷的大小，r 是两个电荷中心之间的距离。由于偶极矩是矢量，键偶极相加时必须考虑方向，不能简单地把数值相加。
When two bond dipoles point in opposite directions along the same line, they cancel exactly and the net dipole moment is zero. When they point in directions that do not cancel, a net dipole remains and the molecule is polar.
当两个键偶极沿同一直线指向相反方向时，它们完全抵消，净偶极矩为零。当它们的方向不能相互抵消时，就留下净偶极，分子就是极性的。
5. Molecular Shape and VSEPR | 分子形状与 VSEPR
Molecular polarity depends on shape, and shape is predicted using VSEPR theory. Electron pairs around the central atom repel each other and arrange themselves as far apart as possible, so the number of bonding pairs and lone pairs determines the geometry.
分子的极性取决于形状，而形状可以用 VSEPR 理论预测。中心原子周围的电子对相互排斥，会尽可能彼此远离，因此成键电子对和孤对电子的数目决定了几何形状。
Lone pairs repel more strongly than bonding pairs, so they compress the bond angles. For example, CH₄ is a perfect tetrahedron with 109.5°, but NH₃ is trigonal pyramidal with about 107°, and H₂O is bent with about 104.5°.
孤对电子的排斥力比成键电子对大，因此会压缩键角。例如 CH₄ 是标准四面体，键角 109.5°；NH₃ 是三角锥形，键角约 107°；H₂O 是 V 形（角形），键角约 104.5°。
Linear (2 electron regions): CO₂, BeCl₂ | 直线形（2 个电子域）：CO₂、BeCl₂
Trigonal planar (3): BF₃, SO₃ | 平面三角形（3 个）：BF₃、SO₃
Tetrahedral (4): CH₄, CCl₄ | 四面体（4 个）：CH₄、CCl₄
Trigonal pyramidal (3 bonds + 1 lone pair): NH₃ | 三角锥形（3 键 + 1 孤对）：NH₃
Bent (2 bonds + 2 lone pairs): H₂O, SO₂ | V 形（2 键 + 2 孤对）：H₂O、SO₂
Shape alone does not decide polarity, but it is the first step. Once you know the shape, you can draw the bond dipoles in three dimensions and check whether they cancel.
形状本身并不能决定极性，但它是第一步。知道形状之后，你就能在三维空间中画出键偶极，并判断它们是否抵消。
6. Symmetry and Dipole Cancellation | 对称性与偶极抵消
Symmetry is the fastest way to decide whether a molecule is polar. If a molecule has a centre of symmetry, or a plane of symmetry that mirrors the bond dipoles, the dipoles cancel and the molecule is non-polar.
判断分子是否具有极性，最快的方法是看对称性。如果分子有对称中心，或存在能镜像键偶极的对称面，偶极就会抵消，分子就是非极性的。
oles are equal in size and opposite in direction, so the net dipole moment is zero, even though each C=O bond is strongly polar.
以 CO₂ 为例，它是直线形分子，两个极性相同的 C=O 键沿着分子轴指向正好相反的方向。两个键偶极大小相等、方向相反，因此净偶极矩为零，尽管每一根 C=O 键的极性都很强。
Now consider H₂O. It is bent, with a bond angle of about 104.5°, so the two O-H bond dipoles point in directions that do not lie on the same line. The components perpendicular to the bisector add together, giving a net dipole pointing from the hydrogen side towards the oxygen side. Water is therefore polar.
再看 H₂O。它是 V 形分子，键角约 104.5°，两个 O-H 键偶极的方向不在同一直线上。垂直于角平分线的分量会叠加，形成一个从氢一侧指向氧一侧的净偶极。因此水是极性分子。
A simple rule follows: linear and symmetrical molecules such as CO₂ are non-polar, whereas bent molecules such as H₂O and SO₂ are polar. This single contrast appears repeatedly in exam papers.
由此可以总结出一条简单规律：像 CO₂ 这样的直线形对称分子是非极性的，而像 H₂O 和 SO₂ 这样的 V 形分子是极性的。这一组对比在考试中反复出现。
7. Worked Examples: Polar or Non-polar? | 实例分析：极性还是非极性？
Working through a set of standard molecules builds the pattern recognition that exam questions reward. For each example, first identify the bonds, then the shape, then check for cancellation.
分析一组标准分子，可以培养考试题目非常看重的模式识别能力。对每个例子，先判断键的类型，再判断形状，最后检查偶极是否抵消。
HCl is a simple diatomic molecule with a single polar H-Cl bond, so it is polar. Its permanent dipole allows dipole-dipole forces between molecules.
HCl 是简单的双原子分子，只有一根极性 H-Cl 键，因此是极性分子。它的永久偶极使分子之间可以形成偶极-偶极作用力。
NH₃ has three polar N-H bonds and a lone pair, giving a trigonal pyramidal shape. The three bond dipoles point towards nitrogen and do not cancel, so ammonia is polar and also shows hydrogen bonding.
NH₃ 含有三根极性 N-H 键和一对孤对电子，形状为三角锥形。三个键偶极都指向氮原子，不能相互抵消，因此氨是极性分子，并且能形成氢键。
CH₄ and CCl₄ have four identical polar bonds arranged tetrahedrally. Although each individual bond is polar, the four dipoles cancel by symmetry, so both molecules are non-polar overall.
CH₄ 和 CCl₄ 各有四根相同的极性键，呈四面体排布。虽然每根键本身都是极性的，但四个偶极因对称性相互抵消，所以两个分子整体都是非极性的。
BF₃ is trigonal planar with three identical B-F bond dipoles separated by 120°. The three vectors cancel, so BF₃ is non-polar despite the large Δχ of about 2.0 in each B-F bond.
BF₃ 是平面三角形，三根相同的 B-F 键偶极彼此间隔 120°。三个矢量相互抵消，所以即使每根 B-F 键的 Δχ 约为 2.0，BF₃ 整体仍是非极性的。
SO₂ is a counterexample worth remembering. It is bent because of the lone pair on sulfur, so the two S-O dipoles do not cancel and SO₂ is polar, unlike the linear CO₂.
SO₂ 是一个值得记住的反例。由于硫上有一对孤对电子，SO₂ 呈 V 形，两根 S-O 偶极不能抵消，因此 SO₂ 是极性的，与直线形的 CO₂ 不同。
8. Polar Bonds with No Net Dipole | 含极性键但无净偶极的分子
Molecules with polar bonds but no net dipole are a classic exam trap. The key is that symmetry can cancel bond dipoles completely, so bond polarity does not automatically mean molecular polarity.
含极性键但没有净偶极的分子是经典的考试陷阱。关键在于对称性可以完全抵消键偶极，所以键有极性并不自动意味着分子有极性。
Typical examples are CO₂, CH₄, CCl₄, BF₃ and SF₆. In each case the terminal atoms are identical and arranged symmetrically around the central atom, so the vector sum of the bond dipoles is zero.
典型例子包括 CO₂、CH₄、CCl₄、BF₃ 和 SF₆。在这些分子中，端基原子相同，并且围绕中心原子对称排布，因此键偶极的矢量和为零。
Replacing even one of those identical atoms usually breaks the symmetry and makes the molecule polar. CH₃Cl, for example, keeps roughly the tetrahedral shape of CH₄, but the C-Cl dipole no longer has three identical partners to cancel it, so CH₃Cl is polar.
只要替换掉其中一个相同的原子，通常就会破坏对称性，使分子变为极性。例如 CH₃Cl 大体保持 CH₄ 的四面体形状，但 C-Cl 偶极不再有另外三根完全相同的偶极与之抵消，因此 CH₃Cl 是极性的。
A useful check is to ask two questions. Are all the terminal atoms the same? Is the arrangement symmetrical with respect to the central atom? If the answer to both is yes, the molecule is likely non-polar.
一个实用的检查方法是问两个问题。所有端基原子是否相同？排布相对于中心原子是否对称？如果两个答案都是肯定的，该分子很可能是非极性的。
9. How Polarity Affects Boiling Point | 极性对沸点的影响
Polarity affects physical properties because polar molecules experience permanent dipole-dipole forces in addition to induced dipole-dipole (van der Waals) forces. These extra attractions must be overcome during boiling, so polar molecules generally have higher boiling points than non-polar molecules of similar mass.
极性会影响物理性质，因为极性分子除了有诱导偶极-诱导偶极（范德华）作用力之外，还存在永久偶极-偶极作用力。沸腾时必须克服这些额外的吸引作用，因此在相对分子质量相近时，极性分子的沸点通常高于非极性分子。
This is why the boiling points of the hydrogen halides rise from HCl to HI: the number of electrons, and therefore the strength of van der Waals forces, increases down the group. HF is the exception and has the highest boiling point of all, because it forms hydrogen bonds rather than only dipole-dipole interactions.
这就是为什么卤化氢的沸点从 HCl 到 HI 逐渐升高：随着族向下，电子数增加，范德华力增强。HF 是例外，它的沸点最高，因为它形成的是氢键，而不仅仅是偶极-偶极作用。
Hydrogen bonding is a special, much stronger case of dipole-dipole attraction. It requires a hydrogen atom bonded to a highly electronegative atom (N, O or F) and a lone pair on a neighbouring N, O or F atom.
氢键是偶极-偶极吸引中一种特别强的情况。它需要氢原子与电负性很强的原子（N、O 或 F）成键，同时邻近的 N、O 或 F 原子上要有孤对电子。
The order of strength for intermolecular forces is: van der Waals < permanent dipole-dipole < hydrogen bonding. Comparing molecules always starts by identifying which of these forces each one can form.
分子间作用力由弱到强的顺序是：范德华力 < 永久偶极-偶极作用 < 氢键。比较分子性质时，第一步永远是判断每种分子能形成其中的哪一种作用力。
10. Solubility: Like Dissolves Like | 溶解度：相似相溶
Polarity also explains solubility through the rule that like dissolves like. Polar solvents dissolve polar solutes, and non-polar solvents dissolve non-polar solutes, because the intermolecular forces between solvent and solute must be similar in strength to those being broken.
极性也通过相似相溶规律解释溶解度。极性溶剂溶解极性溶质，非极性溶剂溶解非极性溶质，因为溶剂与溶质之间的分子间作用力必须与被破坏的作用力强度相近。
Water is a polar solvent and dissolves ionic compounds such as NaCl as well as polar covalent substances such as ethanol and glucose. The δ⁺ hydrogen atoms of water are attracted to anions and the δ⁻ oxygen atoms to cations, which is why ionic solids hydrate and dissolve.
水是极性溶剂，可以溶解 NaCl 这类离子化合物，也可以溶解乙醇、葡萄糖这类极性共价物质。水中带 δ⁺ 的氢原子吸引阴离子，带 δ⁻ 的氧原子吸引阳离子，这就是离子晶体能够水合并溶解的原因。
Iodine is non-polar and dissolves readily in non-polar solvents such as hexane or tetrachloromethane, giving a purple solution, but it is only very slightly soluble in water. This contrast is a standard test of the like dissolves like principle.
碘是非极性的，很容易溶于己烷或四氯化碳等非极性溶剂，形成紫色溶液，但在水中溶解度极低。这一对比是检验相似相溶原理的经典实例。
Alcohols show the intermediate case well. Ethanol has a polar O-H group and a non-polar hydrocarbon chain, so it dissolves in both water and hexane. As the hydrocarbon chain gets longer, solubility in water decreases sharply.
醇类很好地体现了中间情况。乙醇既有极性的 O-H 基团，又有非极性的烃链，因此既能溶于水又能溶于己烷。随着烃链变长，在水中的溶解度会急剧下降。
11. Exam Technique and Common Mistakes | 考试技巧与常见错误
Polarity questions are usually worth several marks and are marked on reasoning, not just on the final answer. Always structure your answer as: identify the bonds, state the shape, then comment on symmetry or cancellation.
极性题目通常分值较高，评分看的是推理过程，而不只是最终答案。答题时务必按这个结构展开：判断键的极性、说明分子形状、再评论对称性或偶极抵消。
Common mistakes to avoid:
需要避免的常见错误：
Assuming a molecule is polar just because it contains polar bonds. | 仅因为分子含有极性键就断定它是极性分子。
Forgetting that lone pairs change the shape and therefore change the polarity. | 忘记孤对电子会改变形状，从而改变极性。
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