Ionic lattices, covalent molecules, metallic seas of electrons, and everything in between. This unit explains why substances have the properties they do — from melting points to conductivity to solubility — by connecting structure to bonding.离子晶格（lattice）、共价分子（covalent bond）、金属"电子海"（metallic bond），以及介于它们之间的各种情况。本单元通过把结构与化学键联系起来，解释物质的熔点、导电性、溶解度等性质从何而来。
Ionic bonding occurs between metals and non-metals. Metal atoms lose electrons to form cations (positive ions); non-metal atoms gain electrons to form anions (negative ions). The ionic bond is the electrostatic attraction between these oppositely charged ions.
Predicting Ion Charges
Group 1 metals form +1 ions, Group 2 form +2, Group 13 form +3 (for main group). Group 17 non-metals form −1, Group 16 form −2, Group 15 form −3. Transition metals can form multiple charges (e.g., Fe$^{2+}$ and Fe$^{3+}$) — the charge must be specified in the name using Roman numerals: iron(II) chloride, iron(III) chloride.
Binary ionic compounds are named with the cation first, then the anion with the suffix "-ide" (e.g., sodium chloride). For compounds containing polyatomic ions, you must know these by name and formula: ammonium (NH$_4^+$), hydroxide (OH$^-$), nitrate (NO$_3^-$), hydrogencarbonate (HCO$_3^-$), carbonate (CO$_3^{2-}$), sulfate (SO$_4^{2-}$), phosphate (PO$_4^{3-}$).
Ionic compounds form three-dimensional lattice structures represented by empirical formulas (not molecular formulas, since discrete molecules do not exist). Their properties follow directly from this structure:
Properties of Ionic CompoundsHigh melting/boiling points: Strong electrostatic attractions between ions require a lot of energy to overcome.Electrical conductivity: Do not conduct when solid (ions fixed in lattice). Conduct when molten or dissolved in water (ions free to move).Solubility: Many ionic compounds dissolve in water because polar water molecules can surround and stabilize the ions (hydration).Brittleness: When layers shift, like charges align and repel, shattering the crystal.
Lattice Enthalpy
Lattice enthalpy measures the strength of the ionic bond. It increases with higher ion charges and smaller ion radii (both increase the electrostatic attraction). For example, MgO has a much higher lattice enthalpy than NaCl because Mg$^{2+}$ and O$^{2-}$ have greater charges and smaller radii than Na$^+$ and Cl$^-$.
Which compound would be expected to have the highest melting point?下列哪种化合物预计熔点最高？
NaCl
KBr
MgO
CsI
Correct! MgO has the highest charges (2+ and 2−) and the smallest ions, giving it the strongest lattice enthalpy and highest melting point.正确！MgO 的电荷最高（2+ 与 2−）、离子半径又最小，因此晶格焓最大、熔点最高。
Lattice enthalpy (and melting point) increases with higher ion charges and smaller ion radii. MgO (Mg$^{2+}$, O$^{2-}$) wins on both counts. Answer: (C).晶格焓（以及熔点）随离子电荷增大、半径减小而升高。MgO（Mg$^{2+}$、O$^{2-}$）在两方面都占优。答案：(C)。
Structure 2.2
The Covalent Model共价模型
A covalent bond forms when two atoms share a pair of electrons. The electrostatic attraction between the shared electron pair and both positively charged nuclei holds the atoms together. The octet rule states that atoms tend to gain, lose, or share electrons to achieve 8 valence electrons (with exceptions).
Lewis formulas show all valence electrons — both bonding pairs and lone (non-bonding) pairs. You must be able to draw these for molecules and ions with up to four electron domains on each atom. Remember: some atoms can have fewer than an octet (e.g., BF$_3$ has only 6 electrons around boron).
Single bonds share one pair, double bonds share two pairs, triple bonds share three pairs. As bond order increases: bond strength increases and bond length decreases. A coordination bond (dative bond) is a covalent bond where both electrons come from the same atom (e.g., in NH$_4^+$, the N donates a lone pair to H$^+$).
The Valence Shell Electron Pair Repulsion model predicts molecular shapes by minimizing repulsion between electron domains around a central atom. Lone pairs repel more than bonding pairs, reducing bond angles.
Bond polarity arises from a difference in electronegativity between bonded atoms. The greater the difference, the more polar the bond. Molecular polarity depends on both bond polarity and molecular geometry — symmetrical molecules can have polar bonds but be non-polar overall (e.g., CO$_2$, CCl$_4$) because the dipoles cancel.
Carbon and silicon form covalent network structures — giant lattices of covalently bonded atoms. Diamond (each C bonded to 4 others tetrahedrally) is extremely hard and does not conduct. Graphite (layers of hexagonal C with delocalized electrons between layers) conducts electricity and is a lubricant. Graphene is a single layer of graphite. Fullerenes (C$_{60}$) are spherical molecules. Silicon and SiO$_2$ also form network structures.
碳和硅可以形成共价网状结构——由共价键连接的巨型晶格（lattice）。金刚石（每个 C 与 4 个 C 以正四面体方式成键）极硬、不导电。石墨（六元环碳层之间夹着可离域电子）能导电、可作润滑剂。石墨烯是单层石墨。富勒烯（C$_{60}$）是球形分子。硅与 SiO$_2$ 同样形成网状结构。
Intermolecular Forces分子间作用力
The forces between molecules (not within them) determine physical properties like boiling point, volatility, and solubility. From weakest to strongest:
Intermolecular Forces (Ranked by Strength)London (dispersion) forces: Present in ALL molecules. Caused by temporary dipoles from fluctuating electron density. Strength increases with molecular size (more electrons = more polarizable).Dipole–dipole forces: Between polar molecules. The δ+ end of one molecule attracts the δ− end of another.Hydrogen bonding: A special, strong dipole–dipole force. Occurs when H is bonded to N, O, or F and interacts with a lone pair on a neighbouring N, O, or F atom. Explains the anomalously high boiling points of H$_2$O, NH$_3$, and HF.
分子间作用力（按强度排序）伦敦色散力（London dispersion force）：存在于所有分子之中。电子密度瞬时涨落产生瞬时偶极，互相诱导而产生吸引。分子越大、电子越多、越易极化，色散力就越强。这也是范德华力（van der Waals force）的主要组成之一。偶极-偶极作用力（dipole-dipole）：极性分子之间。一个分子的 δ+ 端吸引另一个分子的 δ− 端。氢键（hydrogen bond）：一种特殊、较强的偶极-偶极作用。当 H 与 N、O 或 F 直接成键，并与相邻分子上 N、O、F 的孤对电子相互作用时形成。它解释了 H$_2$O、NH$_3$、HF 异常偏高的沸点。
Properties of Covalent SubstancesSimple molecular: Low melting/boiling points (weak IMFs), do not conduct electricity, many are insoluble in water (non-polar) but dissolve in organic solvents.Covalent network: Very high melting points (strong covalent bonds throughout), generally do not conduct (exception: graphite), insoluble.
Chromatography separates mixture components based on their different affinities (via intermolecular forces) for a mobile phase and a stationary phase. The retardation factor $R_f$ quantifies how far a substance moves relative to the solvent front:
$$R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}}$$
比移值 $R_f$
$$R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}}$$
HL Only — Additional Topics in 2.2Resonance (2.2.11–12): When multiple valid Lewis structures exist. The real structure is a weighted average. Benzene is the key example — all C–C bonds are equal in length (between single and double).Expanded octets (2.2.13): Atoms in period 3+ can have 5 or 6 electron domains (e.g., PCl$_5$, SF$_6$). Use VSEPR for trigonal bipyramidal and octahedral geometries.Formal charge (2.2.14): Used to determine the preferred Lewis structure. Formal charge = valence electrons − lone pair electrons − 1/2(bonding electrons). Structures with formal charges closest to zero are preferred.Sigma and pi bonds (2.2.15): Sigma ($\sigma$) bonds form by head-on orbital overlap along the bond axis. Pi ($\pi$) bonds form by lateral overlap of p-orbitals above and below the bond axis. A single bond = 1$\sigma$. A double bond = 1$\sigma$ + 1$\pi$. A triple bond = 1$\sigma$ + 2$\pi$.Hybridization (2.2.16): sp$^3$ = tetrahedral (4 domains). sp$^2$ = trigonal planar (3 domains). sp = linear (2 domains). The hybridization matches the electron domain geometry.
Determine the shape, bond angle, and polarity of SCl$_2$.
Step 1 — Lewis structure
S has 6 valence electrons. Each Cl shares one pair. S has 2 bonding pairs + 2 lone pairs = 4 electron domains.
Step 2 — VSEPR
4 electron domains = tetrahedral electron domain geometry. 2 bonding + 2 lone pairs = bent (V-shaped) molecular geometry. Bond angle approximately 104–105 degrees (less than 109.5 due to lone pair repulsion).
Step 3 — Polarity
S–Cl bonds are polar (Cl is more electronegative). The bent shape means the dipoles do NOT cancel. SCl$_2$ is a polar molecule.
例题 — 预测 SCl$_2$ 的形状和极性
判断 SCl$_2$ 的分子几何、键角和极性。
第 1 步 — 路易斯结构
S 有 6 个价电子；每个 Cl 与 S 共用一对电子。S 周围共有 2 个成键对 + 2 个孤对 = 4 个电子域。
Which molecule is non-polar despite having polar bonds?下列哪个分子虽含有极性键，整体却是非极性？
H$_2$O
CCl$_4$
NH$_3$
HCl
Correct! CCl$_4$ has 4 polar C–Cl bonds, but the tetrahedral symmetry means all bond dipoles cancel perfectly, making the molecule non-polar overall.正确！CCl$_4$ 有 4 条极性 C–Cl 键，但正四面体的对称性让所有键偶极完全相消，整体表现为非极性。
CCl$_4$ is tetrahedral — the 4 identical polar bonds cancel by symmetry. H$_2$O and NH$_3$ are bent/pyramidal so dipoles do not cancel. Answer: (B).CCl$_4$ 是正四面体——4 条相同的极性键因对称而相消。H$_2$O 与 NH$_3$ 是 V 形/三角锥形，偶极无法抵消。答案：(B)。
Structure 2.3
The Metallic Model金属模型
A metallic bond is the electrostatic attraction between a lattice of metal cations and a "sea" of delocalized electrons. This model elegantly explains all characteristic metallic properties.
Properties Explained by the Metallic ModelElectrical conductivity: Delocalized electrons are free to move and carry charge.Thermal conductivity: Delocalized electrons transfer kinetic energy rapidly through the lattice.Malleability and ductility: Layers of cations can slide past each other without breaking the bond — the electron sea simply adjusts. (Contrast with ionic compounds, which shatter.)
Factors Affecting Metallic Bond Strength影响金属键强度的因素
The strength of the metallic bond (and therefore the melting point) depends on the charge of the cation and the radius of the metal ion. Higher charge and smaller radius both increase the attraction between cations and the electron sea. This explains why Na (1+ charge, larger) has a lower melting point than Mg (2+ charge, smaller).
HL Only — Transition Elements (2.3.3)
Transition elements have delocalized d-electrons in addition to s-electrons, giving them more delocalized electrons per atom. This explains their characteristically high melting points and good electrical conductivity compared to s- and p-block metals.
HL 专属 — 过渡元素（2.3.3）
过渡元素除 s 电子外，还有可离域的 d 电子，因此每个原子贡献的离域电子更多。这解释了过渡金属为何普遍比 s 区、p 区金属熔点更高、导电性更好。
Why is aluminium a better electrical conductor than sodium?为什么铝的导电性比钠好？
Al has a higher cation charge (3+) and more delocalized electrons per atomAl 阳离子电荷更高（+3），每个原子提供的离域电子更多
Al has a lower melting point than NaAl 的熔点比 Na 低
Al is more electronegative than NaAl 的电负性比 Na 大
Al has fewer protons than NaAl 的质子数比 Na 少
Correct! Al$^{3+}$ contributes 3 delocalized electrons per atom compared to Na$^+$'s 1. More mobile electrons and stronger metallic bonding both contribute to better conductivity.正确！每个 Al 原子贡献 3 个离域电子（形成 Al$^{3+}$），而 Na 只贡献 1 个。可移动电子更多、金属键更强，都让导电性更好。
Al forms Al$^{3+}$ ions with 3 delocalized electrons per atom vs Na$^+$ with 1. More free electrons means better conductivity. Answer: (A).Al 形成 Al$^{3+}$，每原子贡献 3 个离域电子；Na 形成 Na$^+$，只有 1 个。自由电子越多导电性越好。答案：(A)。
Structure 2.4
From Models to Materials从模型到材料
The Bonding Triangle成键三角形（bonding triangle）
Real bonding is not purely ionic, covalent, or metallic — it exists on a continuum. The bonding triangle (provided in the data booklet) places compounds according to the relative contributions of all three bonding types. A compound's position is determined by electronegativity differences and average electronegativities of the atoms involved. The position predicts physical properties.
Alloys are mixtures of a metal with other metals or non-metals. The different-sized atoms disrupt the regular lattice, preventing layers from sliding as easily. This makes alloys harder and stronger than pure metals. Examples include bronze (Cu + Sn), brass (Cu + Zn), and stainless steel (Fe + Cr + Ni). Alloys are mixtures, not compounds — they have no fixed ratio.
Addition vs. Condensation PolymersAddition polymers form when alkene monomers break their double bonds and link together. No atoms are lost — atom economy is 100%. Example: polyethene from ethene.Condensation polymers (HL) form when monomers with two functional groups react, releasing a small molecule (usually H$_2$O) with each linkage. Polyesters form from diols + dicarboxylic acids. Polyamides form from diamines + dicarboxylic acids. All biological macromolecules (proteins, polysaccharides, nucleic acids) are condensation polymers.
Why are alloys generally harder than pure metals?为什么合金通常比纯金属更硬？
Alloys have stronger covalent bonds合金的共价键更强
Alloys conduct electricity better合金导电性更好
Alloys have higher electronegativity合金的电负性更高
Different-sized atoms prevent layers from sliding easily大小不同的原子让晶格层不易滑动
Correct! The presence of atoms with different radii disrupts the regular arrangement of the lattice, making it more difficult for layers of cations to slide past each other.正确！半径不同的原子破坏了晶格的规则排列，阳离子层之间更难滑动。
In alloys, different-sized atoms disrupt the regular lattice, preventing layers from sliding — increasing hardness. Answer: (D).合金中大小不同的原子打乱规则晶格，使各层不能轻易滑动——硬度因此升高。答案：(D)。
Preparation
Exam Strategy考试策略
Paper 1 (Multiple Choice)Paper 1（多项选择）
VSEPR questions are very common — memorize the shapes table and practise drawing Lewis structures quickly. Polarity questions require two steps: identify polar bonds, then check if geometry cancels them. Know which intermolecular forces are present from the molecular structure (is there H bonded to N/O/F? is it polar? what is the molar mass?).
VSEPR 题非常常见——把分子几何表背熟，路易斯结构要画得又快又准。判断分子极性分两步：先识别极性键，再看几何是否让偶极相消。学会从分子结构直接判断分子间作用力：是否有 H 直接连 N/O/F？分子是否极性？摩尔质量多大？
Paper 2 (Extended Response)Paper 2（论述题）
When asked to "explain" physical properties, always link to the type of bonding or intermolecular force AND the strength of that interaction. "High melting point because of strong ionic bonds" is not enough — say "strong electrostatic attractions between oppositely charged ions in the lattice require a large amount of energy to overcome." Be specific.
遇到 "explain" 物理性质的题目，必须同时给出作用力类型和该作用力的强度。只写 "high melting point because of strong ionic bonds" 不够分，要写 "strong electrostatic attractions between oppositely charged ions in the lattice require a large amount of energy to overcome"，越具体越好。
Data Booklet数据手册
Key items: electronegativity values (for bond polarity), the bonding triangle, bond enthalpy data (for comparing bond strengths). For HL: IR spectroscopy data, NMR chemical shifts.
必查内容：电负性（electronegativity）数值（判断键的极性）、成键三角形、键能（bond enthalpy）数据（比较键强度）。HL 另需 IR 光谱数据、NMR 化学位移表。
Watch Out
Common Mistakes常见错误
Mistake 1 — Confusing Electron Domain Geometry with Molecular Geometry
Electron domain geometry includes lone pairs. Molecular geometry describes only the atom positions. NH$_3$ has tetrahedral electron domain geometry but trigonal pyramidal molecular geometry. Always state both if asked.
Mistake 2 — Saying "Ionic Compounds Have Molecules"
Ionic compounds exist as lattices, not discrete molecules. They are represented by empirical formulas. Never refer to "a molecule of NaCl."
错误 2 — 说 "离子化合物有分子"
离子化合物以晶格（lattice）形式存在，没有独立分子，用实验式表示。绝对不要写 "a molecule of NaCl"。
Mistake 3 — Confusing Intermolecular Forces with Intramolecular Bonds
Boiling a covalent substance breaks intermolecular forces, NOT covalent bonds. Water boils at 100 degrees C because hydrogen bonds between molecules break. The O–H covalent bonds within each molecule remain intact.
Mistake 4 — Forgetting London Forces Exist in ALL Molecules
London forces are not exclusive to non-polar molecules. They are present in every substance. In small polar molecules, dipole–dipole or hydrogen bonding may dominate, but London forces still contribute.
The electrostatic attraction between oppositely charged ions in a lattice.晶格中带相反电荷的离子之间的静电吸引。
Shape of a molecule with 4 electron domains, 2 lone pairs?4 个电子域、2 个孤对的分子是什么形状？
Bent (V-shaped), approximately 104.5 degrees. Example: H$_2$O.V 形（bent），约 104.5°。例子：H$_2$O。
When does hydrogen bonding occur?氢键（hydrogen bond）在什么条件下形成？
When H is covalently bonded to N, O, or F, and interacts with a lone pair on a neighbouring N, O, or F atom.H 与 N、O 或 F 共价成键，并与相邻分子上 N、O、F 的孤对电子发生相互作用时。
Why does graphite conduct electricity?石墨为什么能导电？
Each carbon bonds to only 3 others (sp$^2$), leaving one delocalized electron per atom free to move between layers.每个 C 只与 3 个 C 成键（sp$^2$ 杂化），剩下一个离域电子，可在层与层之间自由移动。
What is a coordination bond?什么是 coordinate bond？
A covalent bond where both electrons in the shared pair come from the same atom. Also called a dative bond.配位键：共用电子对的两个电子全部来自同一个原子。亦称 dative bond。
Why are metals malleable but ionic compounds brittle?为什么金属可锻而离子化合物脆？
In metals, the electron sea adjusts when layers slide. In ionic compounds, shifting layers brings like charges together, causing repulsion and shattering.金属中电子海会随层的滑动而调整；离子化合物中层错位后，同号电荷相对、彼此排斥，晶体破碎。
How do you determine if a molecule is polar?如何判断一个分子是否极性？
1) Identify polar bonds (electronegativity difference). 2) Check if geometry allows dipoles to cancel. If they do not cancel, the molecule is polar.① 看是否存在极性键（依电负性差判断）；② 看几何能否让偶极相消。不能相消则分子为极性。
HL: How many sigma and pi bonds in a triple bond?HL：一个三键中有几个 σ 键、几个 π 键？
1 sigma bond + 2 pi bonds. The sigma forms first by head-on overlap; the two pi bonds form by lateral overlap of p-orbitals.1 σ + 2 π。σ 键由轨道正面重叠先形成；两个 π 键由 p 轨道侧面重叠形成。
Assessment
Unit Quiz单元测验
1. Which substance conducts electricity in the solid state?1. 下列哪种物质在固态下导电？
GraphiteGraphite（石墨）
NaCl
DiamondDiamond（金刚石）
SiO$_2$
Correct! Graphite has delocalized electrons between its carbon layers that are free to move and carry charge, even in the solid state. NaCl only conducts when molten or dissolved.正确！石墨的碳层之间有离域电子，可以自由移动、携带电荷，即便固态也能导电。NaCl 只在熔融或溶解后才导电。
Graphite has delocalized electrons free to move between layers. Ionic solids like NaCl have ions locked in place. Diamond and SiO$_2$ have no mobile charge carriers. Answer: (A).石墨的离域电子可在层间自由移动。像 NaCl 这样的离子固体，离子被锁在晶格中。金刚石和 SiO$_2$ 没有可移动的载流子。答案：(A)。
2. What intermolecular force is primarily responsible for the high boiling point of water?2. 水沸点偏高，主要是因为哪种分子间作用力？
London dispersion forcesLondon dispersion forces（伦敦色散力）
Covalent bondingCovalent bonding（共价键）
Hydrogen bondingHydrogen bonding（氢键）
Ionic bondingIonic bonding（离子键）
Correct! Water molecules form extensive hydrogen bonds (H bonded to the highly electronegative O, interacting with lone pairs on neighbouring O atoms). This requires significant energy to overcome, giving water its anomalously high boiling point.正确！水分子之间形成大量氢键（H 直接连在电负性很大的 O 上，并与相邻 O 的孤对相互作用），破坏这些氢键需要大量能量，因此水的沸点异常偏高。
Water's high boiling point is due to strong hydrogen bonds between molecules (not covalent bonds, which hold atoms within molecules). Answer: (C).水的高沸点来自分子之间的氢键（共价键是把分子内部原子结合在一起的）。答案：(C)。
3. (HL) How many sigma and pi bonds are present in ethyne (C$_2$H$_2$)?3.（HL）乙炔 C$_2$H$_2$ 中含有多少 σ 键和 π 键？
3 sigma, 3 pi
3 sigma, 2 pi
5 sigma, 0 pi
2 sigma, 3 pi
Correct! Ethyne has a triple bond between the two carbons (1$\sigma$ + 2$\pi$) plus one C–H $\sigma$ bond on each side. Total: 3$\sigma$ + 2$\pi$.正确！乙炔中两个 C 之间是三键（1$\sigma$ + 2$\pi$），两侧各有一个 C–H $\sigma$ 键。合计：3$\sigma$ + 2$\pi$。
H–C triple bond C–H: each C–H is 1$\sigma$, the triple bond is 1$\sigma$ + 2$\pi$. Total: 3$\sigma$ + 2$\pi$. Answer: (B).H–C≡C–H：每个 C–H 为 1$\sigma$，三键为 1$\sigma$ + 2$\pi$。合计：3$\sigma$ + 2$\pi$。答案：(B)。