Group 2, the Alkaline Earth Metals | 第二族碱土金属

📚 Group 2, the Alkaline Earth Metals | 第二族碱土金属

The Group 2 elements — beryllium, magnesium, calcium, strontium, barium and radium — are known as the alkaline earth metals. They share characteristic properties arising from their outer shell of two s-electrons, and their behaviour illustrates key trends in ionisation energy, atomic radius, reactivity and compound solubility. This article provides a complete revision of the AQA A-Level specification for this topic, with exam-focused explanations and worked patterns.

第二族元素——铍、镁、钙、锶、钡和镭——被称为碱土金属。它们因最外层具有两个 s 电子而表现出共同的性质，其行为清晰地展示了电离能、原子半径、反应活性及化合物溶解度的变化趋势。本文针对 AQA A-Level 考纲对这一主题进行全面复习，提供紧扣考点的解释与规律总结。

1. Electronic Configuration and Ion Formation | 电子构型与离子形成

All Group 2 metals have the general outer electronic configuration ns². For example, magnesium is 1s² 2s² 2p⁶ 3s², written as [Ne] 3s², and calcium is [Ar] 4s². The two outer electrons are relatively easily lost to form the stable +2 cation, M²⁺.

所有第二族金属的最外层电子构型均为 ns²。例如，镁为 1s² 2s² 2p⁶ 3s²，可写为 [Ne] 3s²；钙为 [Ar] 4s²。这两个外层电子较易失去，形成稳定的 +2 阳离子 M²⁺。

The loss of two electrons produces an ion with a noble gas configuration. This explains why the chemistry of Group 2 metals is dominated by redox reactions in which the metal acts as a reducing agent, losing two electrons: M → M²⁺ + 2e⁻.

失去两个电子后形成的离子具有稀有气体构型。这解释了第二族金属的化学性质为何以氧化还原反应为主——金属作为还原剂失去两个电子：M → M²⁺ + 2e⁻。

2. Atomic Radius and Ionisation Energy Trends | 原子半径与电离能趋势

Going down Group 2 from Be to Ba, the atomic radius increases because each successive element has an additional electron shell. The increased shielding effect outweighs the increase in nuclear charge, so the outer electrons are held less tightly.

从铍到钡沿第二族向下，原子半径逐渐增大，因为每个后续元素都增加了一个电子壳层。增大的屏蔽效应超过了核电荷的增加，使外层电子被束缚得更不紧密。

Consequently, the first ionisation energy decreases down the group. For example, the first ionisation energy of magnesium is 736 kJ mol⁻¹ while that of barium is only 503 kJ mol⁻¹. The second ionisation energy also decreases down the group, although it is always larger than the first because the second electron is removed from a now-positively-charged ion.

因此，第一电离能沿族向下递减。例如，镁的第一电离能为 736 kJ mol⁻¹，而钡仅为 503 kJ mol⁻¹。第二电离能同样沿族向下递减，但始终高于第一电离能，因为第二个电子是从已带正电荷的离子中移除的。

Property	Trend down Group 2	Reason
Atomic radius	Increases	Extra electron shells, increased shielding
First ionisation energy	Decreases	Outer electron further from nucleus, more shielded
Melting point	Decreases (irregular)	Metallic bonding weakens as ionic radius increases

3. Reactivity Increases Down the Group | 反应活性沿族向下增强

As ionisation energies decrease down the group, the metals lose their two outer electrons more readily. Therefore, the reactivity of the Group 2 metals increases from beryllium to barium. This is evident in their reactions with oxygen, water and dilute acids.

由于电离能沿族向下降低，金属更容易失去两个外层电子。因此，第二族金属的反应活性从铍到钡逐渐增强。这一点在其与氧气、水和稀酸的反应中表现得很明显。

All Group 2 metals burn in oxygen to form the metal oxide. For example:

2Mg(s) + O₂(g) → 2MgO(s)

2Ca(s) + O₂(g) → 2CaO(s)

Magnesium burns with a brilliant white flame, producing a white solid of magnesium oxide. Calcium burns with a brick-red flame. These observations are commonly tested in practical-based exam questions.

镁燃烧时发出耀眼白光，生成白色固体氧化镁。钙燃烧时产生砖红色火焰。这些实验现象是实验类考题中的常见考点。

4. Reactions with Water | 与水反应

The reaction of Group 2 metals with water is an important redox reaction that demonstrates the trend in reactivity. Magnesium reacts very slowly with cold water, producing magnesium hydroxide and hydrogen gas:

第二族金属与水的反应是展现反应活性趋势的重要氧化还原反应。镁与冷水反应非常缓慢，生成氢氧化镁和氢气：

Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)

However, magnesium reacts much more rapidly with steam to form magnesium oxide and hydrogen:

然而，镁与蒸汽反应要快得多，生成氧化镁和氢气：

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Calcium, strontium and barium react vigorously with cold water. Calcium reacts noticeably with a steady fizzing, while barium reacts most violently. The general equation is:

钙、锶和钡与冷水反应剧烈。钙以稳定冒泡的方式明显反应，而钡反应最为猛烈。通式如下：

M(s) + 2H₂O(l) → M(OH)₂(aq) + H₂(g)

The hydroxide formed dissolves in water, and the resulting solution is alkaline. This is a key reason why these metals are called “alkaline earth” metals — their oxides and hydroxides form alkaline solutions.

生成的氢氧化物溶于水，所得溶液呈碱性。这正是这些金属被称为“碱土”金属的关键原因——它们的氧化物和氢氧化物形成碱性溶液。

5. Reactions with Acids | 与酸反应

Group 2 metals react with dilute acids to form a salt and hydrogen gas. For example, the reaction of magnesium with hydrochloric acid:

第二族金属与稀酸反应生成盐和氢气。例如，镁与盐酸的反应：

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Ca(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂(g)

These reactions are redox reactions in which the metal is oxidised (loses electrons) and the H⁺ ion from the acid is reduced (gains electrons). The metal acts as a reducing agent. As reactivity increases down the group, the rate of reaction with acids increases accordingly.

这些反应均为氧化还原反应，其中金属被氧化（失去电子），酸中的 H⁺ 被还原（获得电子）。金属充当还原剂。随着反应活性沿族向下增强，与酸的反应速率也相应加快。

6. Solubility of Hydroxides | 氢氧化物的溶解度

The solubility of Group 2 hydroxides increases down the group. Magnesium hydroxide is sparingly soluble; calcium hydroxide is slightly soluble; strontium and barium hydroxides are significantly more soluble.

第二族氢氧化物的溶解度沿族向下增大。氢氧化镁微溶；氢氧化钙略微可溶；氢氧化锶和氢氧化钡的溶解度明显更高。

This trend is explained by lattice enthalpy and hydration enthalpy. As the M²⁺ ion becomes larger down the group, its charge density decreases, reducing the lattice enthalpy of the hydroxide by a greater extent than the hydration enthalpy decreases. Since the enthalpy of solution becomes more exothermic down the group, solubility increases.

这一趋势可通过晶格焓与水合焓来解释。随 M²⁺ 沿族向下半径增大，其电荷密度减小，晶格焓的降低幅度大于水合焓的降低幅度。由于溶解焓沿族向下变得更放热，溶解度随之增大。

M(OH)₂(s) ⇌ M²⁺(aq) + 2OH⁻(aq)

A suspension of calcium hydroxide in water, known as limewater, is used to test for carbon dioxide gas. When CO₂ is bubbled through, it turns milky due to the precipitation of calcium carbonate.

氢氧化钙在水中的悬浮液称为石灰水，用于检验二氧化碳气体。当通入 CO₂ 时，溶液变浑浊，原因是生成了碳酸钙沉淀。

7. Solubility of Sulfates | 硫酸盐的溶解度

The solubility of Group 2 sulfates decreases down the group — the opposite trend to the hydroxides. Magnesium sulfate is very soluble; calcium sulfate is sparingly soluble; barium sulfate is essentially insoluble.

第二族硫酸盐的溶解度沿族向下减小——与氢氧化物的趋势相反。硫酸镁极易溶；硫酸钙微溶；硫酸钡几乎不溶。

As the cation radius increases down the group, the lattice enthalpy of the sulfate decreases less rapidly than the hydration enthalpy, so the enthalpy of solution becomes more endothermic and solubility falls. Barium sulfate’s insolubility is exploited medically: a “barium meal” is administered before X-ray imaging of the digestive tract because BaSO₄ absorbs X-rays while being harmless, as no toxic Ba²⁺ ions are released into the body.

随阳离子半径沿族向下增大，硫酸盐的晶格焓降幅小于水合焓降幅，溶解焓变得更加吸热，溶解度下降。硫酸钡的不溶性被应用于医学：在进行消化道 X 射线成像前服用“钡餐”，因为 BaSO₄ 能吸收 X 射线且对人体无害——不会释放有毒的 Ba²⁺ 进入体内。

Compound	Solubility trend	Key exam application
M(OH)₂	Increases down group	Mg(OH)₂ used as antacid
MSO₄	Decreases down group	BaSO₄ used in medical imaging

8. Thermal Decomposition of Carbonates and Nitrates | 碳酸盐与硝酸盐的热分解

Group 2 carbonates decompose on heating to form the metal oxide and carbon dioxide:

第二族碳酸盐受热分解生成金属氧化物和二氧化碳：

MCO₃(s) → MO(s) + CO₂(g)

Group 2 nitrates decompose on heating to form the metal oxide, nitrogen dioxide and oxygen:

第二族硝酸盐受热分解生成金属氧化物、二氧化氮和氧气：

2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g)

The thermal stability of both carbonates and nitrates increases down the group. As the cation size increases, the charge density decreases, so the cation has less polarising power. The large, polarisable CO₃²⁻ and NO₃⁻ ions are therefore less distorted by the cation, and more energy is required to break them apart thermally.

碳酸盐和硝酸盐的热稳定性都沿族向下增强。随着阳离子尺寸增大，电荷密度减小，阳离子的极化能力减弱。因此，大的、可极化的 CO₃²⁻ 和 NO₃⁻ 离子被阳离子扭曲的程度降低，需要更多能量才能使其热分解。

This is a classic AQA exam question: you must be able to explain the thermal stability trend using the polarisation argument, and to write balanced equations for the decomposition of specific carbonates and nitrates.

这是 AQA 考试中的经典题目：你必须能够用极化理论解释热稳定性趋势，并写出特定碳酸盐和硝酸盐分解的配平方程式。

9. Uses of Group 2 Compounds | 第二族化合物的用途

Several Group 2 compounds have important real-world applications that are frequently examined:

几种第二族化合物具有重要的现实应用，是考题中的常见内容：

Mg(OH)₂ and CaCO₃ are used as antacids to neutralise excess stomach acid. They react with HCl in the stomach: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.
Mg(OH)₂ 和 CaCO₃ 用作抗酸剂以中和过量胃酸。它们与胃中的 HCl 反应：Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O。
Ca(OH)₂ (slaked lime) is used in agriculture to neutralise acidic soils. It is cheaper than other bases and is applied as a powder or a slurry.
Ca(OH)₂（熟石灰）用于农业中中和酸性土壤。它比其他碱更便宜，以粉末或浆液形式施用。
BaSO₄ is used as a contrast medium in X-ray imaging because it is dense, insoluble and non-toxic.
BaSO₄ 用作 X 射线成像中的造影剂，因为它密度大、不溶且无毒。
CaO (quicklime) is used in the steel industry to remove acidic impurities such as SiO₂ from molten iron.
CaO（生石灰）用于钢铁工业中去除铁水中的酸性杂质如 SiO₂。

In each case, the use connects directly to a chemical property discussed above — alkalinity, solubility, or thermal behaviour. Exam questions often ask you to justify a use with reference to these properties.

每种用途都直接与上述化学性质相关——碱性、溶解度或热行为。考题常要求你结合这些性质来论证某种用途的合理性。

10. Key Exam Calculations and Half-Equations | 关键考点计算与半反应式

You should be able to write full and ionic equations for all the reactions of Group 2 metals and their compounds, and to perform mole calculations involving these reactions. For example, the redox equation for calcium with water as an ionic equation is:

你应该能够为第二族金属及其化合物的所有反应写出完整方程式和离子方程式，并能进行涉及这些反应的摩尔计算。例如，钙与水的氧化还原反应离子方程式为：

Ca(s) + 2H₂O(l) → Ca²⁺(aq) + 2OH⁻(aq) + H₂(g)

The two half-equations are:

两个半反应式为：

Oxidation: Ca(s) → Ca²⁺(aq) + 2e⁻

Reduction: 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq)

Titration calculations using Group 2 hydroxides, for example determining the concentration of a calcium hydroxide solution by titration with hydrochloric acid, are a standard quantitative exercise. The stoichiometry is 1:2 for M(OH)₂ : HCl.

使用第二族氢氧化物进行的滴定计算，例如用盐酸滴定氢氧化钙溶液以测定其浓度，是标准的定量练习。M(OH)₂ 与 HCl 的化学计量比为 1:2。

11. Common Misconceptions and Exam Pitfalls | 常见误区与考试陷阱

Students frequently confuse the direction of the solubility trends. Remember: hydroxides become more soluble down the group, while sulfates become less soluble. These opposing trends are a favourite target of examiners.

学生经常混淆溶解度趋势的方向。请记住：氢氧化物沿族向下溶解度增大，而硫酸盐沿族向下溶解度减小。这两种相反趋势是出题人最爱考查的内容。

Another common error is writing MOH instead of M(OH)₂. Since Group 2 metals form +2 ions, the hydroxide formula must be M(OH)₂ to balance the two positive charges. Similarly, the nitrate is M(NO₃)₂ and the sulfate is MSO₄.

另一个常见错误是将 M(OH)₂ 误写为 MOH。由于第二族金属形成 +2 离子，氢氧化物通式必须为 M(OH)₂ 以平衡两个正电荷。同理，硝酸盐通式为 M(NO₃)₂，硫酸盐通式为 MSO₄。

Finally, when explaining thermal stability, always mention the polarising power of the cation and how it decreases with increasing ionic radius. The anion distortion argument is essential for full marks.

最后，在解释热稳定性时，务必提及阳离子的极化能力及其如何随离子半径增大而减弱。阴离子扭曲的论述是获得满分的必要条件。

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Group 2, the Alkaline Earth Metals | 第二族碱土金属