📚 Reactions of Group 2 Elements | 第2族元素的反应
Group 2 elements, also called alkaline earth metals, are reducing agents that form +2 ions. This revision guide covers their reactions with oxygen, water and dilute acids, and explains key trends in reactivity, hydroxide and sulfate solubility, and thermal decomposition of carbonates and nitrates.
第2族元素又称碱土金属，是能形成+2离子的还原剂。本复习指南涵盖它们与氧气、水和稀酸的反应，并解释反应性、氢氧化物和硫酸盐溶解性以及碳酸盐和硝酸盐热分解的重要趋势。
1. Introduction to Group 2 | 第2族简介
The Group 2 metals in the periodic table are beryllium, magnesium, calcium, strontium, barium and radium. They are s-block elements with two electrons in their outermost s subshell. In most A-level exam questions, the typical metals studied are magnesium to barium, because beryllium is atypical and radium is radioactive.
周期表第2族金属包括铍、镁、钙、锶、钡和镭。它们是最外层s亚层有2个电子的s区元素。在大多数A-level考试题中，典型金属的研究范围是镁到钡，因为铍性质特殊，镭具有放射性。
Group 2 elements are often called alkaline earth metals because their oxides and hydroxides are basic and their oxides are found in the Earth’s crust. They are reactive metals, but less reactive than the Group 1 alkali metals.
第2族元素常被称为碱土金属，因为它们的氧化物和氢氧化物呈碱性，且氧化物存在于地壳中。它们是活泼金属，但活泼性低于第1族碱金属。
2. Electronic Configuration and Oxidation | 电子构型与氧化态
Every Group 2 atom has an outer electron configuration of ns². For example, magnesium is [Ne]3s² and calcium is [Ar]4s². These two outer electrons are relatively easy to remove, so all Group 2 metals form +2 ions in their compounds.
每个第2族原子的最外层电子构型均为ns²。例如，镁为[Ne]3s²，钙为[Ar]4s²。这两个外层电子较容易失去，因此所有第2族金属在化合物中均形成+2离子。
M(g) → M²⁺(g) + 2e⁻
The +2 ion has a stable noble gas electron configuration. The second ionisation energy is larger than the first because it is harder to remove a negative electron from a positive ion, but both values decrease down the group.
+2离子具有稳定的惰性气体电子构型。第二电离能大于第一电离能，因为从阳离子中移去电子更困难，但两者沿族向下均减小。
3. Reaction with Oxygen | 与氧气的反应
All Group 2 metals burn in oxygen or air to form white solid oxides with the general formula MO. The general equation is shown below:
所有第2族金属在氧气或空气中燃烧，生成通式为MO的白色固体氧化物。一般方程式如下：
2M(s) + O₂(g) → 2MO(s)
Magnesium burns with a brilliant white flame, calcium with a brick-red flame, strontium with a crimson flame and barium with an apple-green flame. These flame colours are useful in flame tests, although magnesium is normally not identified by this method.
镁燃烧时发出耀眼白光，钙为砖红色，锶为深红色，钡为苹果绿色。这些焰色可用于火焰测试，但镁通常不用此方法鉴定。
Down the group, the metals lose their two outer electrons more easily because atomic radius increases and ionisation energy decreases, so oxidation is more favourable. However, all of the metals from Mg to Ba burn readily once heated strongly enough.
沿族向下，金属更容易失去两个外层电子，因为原子半径增大、电离能减小，因此氧化反应更容易进行。但只要加热到足够高的温度，从Mg到Ba的所有金属都能燃烧。
4. Reaction with Water | 与水的反应
Beryllium is unusual: it does not react with water. Magnesium reacts very slowly with cold water to form a sparingly soluble hydroxide and hydrogen. The general reaction for the heavier Group 2 metals with cold water is:
铍性质特殊：它不与水反应。镁与冷水反应非常缓慢，生成微溶的氢氧化物和氢气。较重的第2族金属与冷水反应的一般方程式为：
M(s) + 2H₂O(l) → M(OH)₂(s) + H₂(g)
Calcium reacts readily with cold water to give a milky suspension of calcium hydroxide and hydrogen. Strontium and barium react even more vigorously, with barium producing a clear solution of barium hydroxide because this hydroxide is soluble.
钙与冷水容易反应，生成氢氧化钙的乳状悬浊液和氢气。锶和钡反应更剧烈，钡生成澄清的氢氧化钡溶液，因为该氢氧化物可溶。
Magnesium reacts much faster with steam than with cold water. The products are magnesium oxide and hydrogen, not the hydroxide:
镁与蒸汽的反应比与冷水快得多。产物是氧化镁和氢气，而不是氢氧化物：
Mg(s) + H₂O(g) → MgO(s) + H₂(g)
In all cases, the gas evolved is hydrogen, which gives a squeaky pop when tested with a lighted splint.
在所有反应中，产生的气体都是氢气，用点燃的木条检验会发出轻微的爆鸣声。
5. Reaction with Dilute Acids | 与稀酸的反应
Group 2 metals react with dilute acids such as hydrochloric acid to form a salt and hydrogen gas. The ionic equation is the same for every Group 2 metal:
第2族金属与稀酸（如稀盐酸）反应生成盐和氢气。每个第2族金属的离子方程式相同：
M(s) + 2H⁺(aq) → M²⁺(aq) + H₂(g)
For example, magnesium reacts with dilute hydrochloric acid according to:
例如，镁与稀盐酸反应如下：
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Reactivity with dilute acids increases down the group. Magnesium fizzes steadily, while calcium, strontium and barium react increasingly vigorously. Barium must be handled carefully because the reaction is strongly exothermic.
与稀酸的反应性沿族向下增强。镁稳定地产生气泡，钙、锶和钡的反应越来越剧烈。钡反应强烈放热，操作时必须小心。
Dilute sulfuric acid is not ideal for calcium, strontium and barium because the sulfate product is only sparingly soluble or insoluble and can form a protective layer on the metal surface, slowing or stopping the reaction.
稀硫酸不适合用于钙、锶和钡，因为生成的硫酸盐微溶或不溶，可能在金属表面形成保护层，减缓或阻止反应。
6. Explanation of Reactivity Trend | 反应性趋势解释
Down Group 2, the atoms gain an extra full electron shell. This increases atomic radius and gives more inner-shell shielding. The outer two s electrons are further from the nucleus and are less strongly attracted to it.
沿第2族向下，原子增加一个完整的电子层。这使原子半径增大，内层屏蔽增加。两个外层s电子离核更远，受到的核吸引力更弱。
As a result, the first and second ionisation energies decrease down the group. Less energy is required to remove the two outer electrons, so the metal becomes a stronger reducing agent and reactions become more vigorous.
因此，第一和第二电离能沿族向下减小。移去两个外层电子所需能量更少，金属变成更强的还原剂，反应更加剧烈。
M(g) → M⁺(g) + e⁻
M⁺(g) → M²⁺(g) + e⁻
This trend explains the increase in reactivity with oxygen, water and dilute acids from magnesium to barium.
这一趋势解释了从镁到钡与氧气、水和稀酸反应性增强的原因。
7. Hydroxides and Solubility Trends | 氢氧化物及溶解性趋势
Group 2 hydroxides have the general formula M(OH)₂. Their solubility in water increases down the group:
第2族氢氧化物的通式为M(OH)₂。它们在水中的溶解性沿族向下增大：
- Mg(OH)₂ – sparingly soluble | Mg(OH)₂ – 微溶
- Ca(OH)₂ – slightly soluble | Ca(OH)₂ – 略溶
- Sr(OH)₂ – more soluble | Sr(OH)₂ – 较易溶
- Ba(OH)₂ – soluble | Ba(OH)₂ – 易溶
Magnesium hydroxide is sold as milk of magnesia and is used as an antacid. Calcium hydroxide is used in limewater, a test for carbon dioxide. Barium hydroxide is alkaline enough to neutralise acids in the laboratory.
氢氧化镁以氧化镁乳剂形式出售，用作抗酸剂。氢氧化钙用于石灰水，可检验二氧化碳。氢氧化钡碱性较强，可在实验室中中和酸。
Because hydroxide solubility increases down the group, saturated solutions become more strongly alkaline from magnesium to barium. This means the pH of a saturated solution is higher for barium hydroxide than for magnesium hydroxide.
由于氢氧化物溶解度沿族向下增大，饱和溶液的碱性从镁到钡逐渐增强。因此氢氧化钡饱和溶液的pH高于氢氧化镁饱和溶液。
8. Sulfates and Solubility Trends | 硫酸盐及溶解性趋势
Group 2 sulfates have the general formula MSO₄. Their solubility in water decreases down the group:
第2族硫酸盐的通式为MSO₄。它们在水中的溶解性沿族向下减小：
- MgSO₄ – soluble | MgSO₄ – 可溶
- CaSO₄ – sparingly soluble | CaSO₄ – 微溶
- SrSO₄ – insoluble | SrSO₄ – 难溶
- BaSO₄ – insoluble | BaSO₄ – 难溶
Barium sulfate is so insoluble that it is used as a barium meal in medical X-ray imaging and as a white pigment in paint. Its low solubility also makes barium chloride an excellent reagent for detecting sulfate ions.
硫酸钡极难溶解，因此可用作医学X射线成像中的钡餐，
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