第 2 组元素：碱土金属

铍、镁、钙、钡、锶和镭是第 2 族碱土金属，它们是软银金属，金属质量低于第 1 族碱金属。尽管 Ca、Sr、Ba 和 Ra 等较重的金属与第 1 族碱金属具有许多共同特征，但它们实际上具有相同的反应性。第 2 族中所有元素的价壳层都有两个电子，使它们的氧化态为 +2。

第 2 组元素：碱土金属

碱土金属是元素周期表第 2 组中的一组六种化学元素。铍 (Be)、镁 (Mg)、钙 (Ca)、锶 (Sr)、钡 (Ba) 和镭 (Ra)涉及的元素 (Ra)。在标准温度和压力下，这些元素具有极其相似的特性：它们都是有光泽、银白色和中等反应性的金属。所有已知的碱土金属都存在于自然界中，尽管镭只是作为铀和钍衰变的副产品，而不是原始元素。

Because the alkaline earth metals’ valence shells have a completely full s-orbital, they quickly shed two electrons to form cations with a charge of +2. As a result, the alkaline earth metals’ most common oxidation state is +2. They have also known as group two metals since they are found in the second column of the periodic table.

这些元件的一般电子配置是ns ² 。

碱土金属的物理性质

• 原子和Ionic半径

由于电荷和将电子添加到相同的能级，离子和原子半径在元素周期表列中都会减小，从而使它们小于碱金属并大于同一周期的其他原子。在碱土元素中，两个 s 电子都可能丢失，使它们成为双正阳离子。阳离子原子的半径小于中性原子的半径。随着您沿着柱子前进，Ionic半径继续上升。

Be ⁺² ˂ Mg ⁺² ˂ Ca ⁺² ˂ Sr ⁺² ˂ Ba ⁺²

• 密度

因为原子的半径更小，所以原子的体积也更小。此外，由于存在两个价电子，原子具有更强的金属键合。因此，碱土金属比碱金属更致密和更坚韧。碱土金属的密度通常从镁增加到镭，其中钙的密度最低。

• 电离能

Alkaline earth elements can transfer both of their valence electrons to form an octet noble gas configuration. They have two ionization energies as a result:

碱土金属的第一电离能是从中性原子中除去第一个电子所需的能量。它比碱金属原子大，因为它的半径更小，电子被更高的核电荷紧紧抓住，以及电子从完全填充且如此稳定的亚壳中撤出。

来自碱土金属中阳离子的第二电子所需的第二电离能将高于原子的第一电离能，但低于任何碱金属的第二电离能。尽管电离能更大，但由于原子呈现惰性气体配置，两个电子都可以被移除，并且由于原子或离子在固体中的紧密堆积，更小的尺寸和更高的电荷有助于通过产生更高的晶格能量来克服更高的电离能。此外，由于较大的溶剂化作用，液体具有较高的水合能。

• 溶解度

铍离子是水溶性最强的碱土金属离子，其溶解度随着尺寸的增大而减小，使钡离子的水溶性最小。物质在水中的溶解度受其离子组成和大小的影响。较小的离子具有较高的电荷密度，可以被更多的水分子溶解。这增加了水合焓并使水合离子更稳定。

Be ⁺²的溶解度 > Mg ⁺²的溶解度 > Ca ⁺²的溶解度 > Sr ⁺²的溶解度 > Ba ⁺²的溶解度

• 反应性

电离能与还原能力成反比。从铍到钡，还原特性预计会随着电离能沿色谱柱的降低而增加。从铍到钡，还原电位下降，表明还原能力增强。碱土金属是比碱金属更差的还原剂，因为它们具有更高的电离能。

• 熔点和沸点

碱土金属的熔化和沸腾温度高于碱金属，因为它们的尺寸较小，并且在紧密堆积的结构中具有很强的金属结合力。除镁外，碱土金属的熔化和沸腾温度按从铍到钡的顺序下降。

碱土金属的化学性质

• 氢化物

铍不会立即与氢反应。用氢化铝锂还原氯化铍得到氢化铍。铍和镁产生共价氢化物，每个氢都有两个金属原子。香蕉键是具有三个中心仅共享两个电子的分子的一个例子。

• 与水反应

即使在更高的温度下，铍也不与水反应。镁仅在与热水反应时形成氢氧化物并释放氢气。镁接受其氧化物的保护性覆盖，保护它免受进一步的水分子攻击。其他碱土金属甚至在与冷水反应时也会产生氢气。

• 硫酸盐

与碱金属硫酸盐不同，硫酸铍是水溶性的。硫酸铍的水合能随着其尺寸和电荷密度的减小而增加，从而导致溶解度增加。当晶格能增加和水合能减少（由于尺寸增加）时，其他硫酸盐的溶解度从 BeSO ₄下降到 BaSO ₄ 。

Anomalous behaviour of Beryllium

Because of its small size, highest ionisation energy, high electropositive nature, and greatest polarising nature, Beryllium has a stronger covalent character. Beryllium’s characteristics set it apart from other alkaline earth metals.

• Among alkaline earth metals, it is the hardest.
• Even at extremely high temperatures, it does not react with water.
• Beryllium has the highest melting and boiling points.
• It does not create hydride when it comes into contact with hydrogen.
• Because of its higher electrode potential, it does not liberate hydrogen from acid like other alkaline earth metals. Concentrated nitric acid forms an oxide layer that renders it inactive.
• Oxide and hydroxide of amphoteric beryllium It forms salts when dissolved in acids and beryllate when dissolved in bases.
• Beryllium generates a carbide with a distinct formula when it reacts with water, yielding methane rather than acetylene like other metals.
• Beryllium nitride is a flammable material.
• It doesn’t react with nitrogen or oxygen in the air.

Diagonal Relationship of Beryllium with Aluminium

• Both are unaffected by ambient oxygen and nitrogen.
• Even at high temperatures, neither of them reacts with water.
• They don’t let hydrogen out of the acid. They become passive after being treated with strong nitric acid.
• Both produce polyvalent covalent bridged hydrides.
• Both have polyvalent, bridging halides with low melting points. Lewis acids are halides.
• Both nitrides are hydrolyzed by water, releasing ammonia.
• Be and Al oxides and hydroxides are amphoteric. As a result, they react with both acid and base.
• Both produce carbide, which is hydrolyzed to produce methane.
• Aluminum and beryllium carbonates are both unstable.

示例问题

问题1：命名称为碱土金属的元素？

回答：

Beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra) are alkaline earth metals (Ra).

问题2：为什么碱土金属比碱金属硬？

回答：

Because the radius of the atoms is smaller, the volume of the atoms is also less. Furthermore, atoms have stronger metallic bonding due to the existence of two valence electrons. As a result, alkaline earth metals are denser and tougher than alkali metals.

问题3：为什么碱土金属的熔点和沸点比碱金属高？

回答：

The melting and boiling temperatures of alkaline earth metals are higher than alkali metals due to their smaller size and strong metallic bonding in a close-packed structure.

问题4：碱土金属如何与水反应？

回答：

Even at higher temperatures, beryllium does not react with water. Magnesium only reacts with hot water to form hydroxides and emit hydrogen. Magnesium receives a protective coat of oxide, which prevents further attack by water molecules. Other alkaline earth metals release hydrogen when they react with even cold water.

问题 5：什么是香蕉债券？

回答：

Beryllium and magnesium combine to form covalent hydrides, in which each hydrogen is linked to two metal atoms. This is known as a banana bond.