Physical Properties of Group 2 Elements | 第2族元素的物理性质

📚 Physical Properties of Group 2 Elements | 第2族元素的物理性质

The Group 2 elements – beryllium, magnesium, calcium, strontium, barium and radium – are reactive metals that form 2+ ions by losing their two outer s electrons. Their physical properties show systematic trends down the group, and these trends can be explained using atomic structure and metallic bonding.

第2族元素——铍、镁、钙、锶、钡和镭——是活泼金属，通过失去两个外层s电子形成2+离子。它们的物理性质沿族呈现系统性变化趋势，这些趋势可以用原子结构和金属键来解释。

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1. Position and Electronic Configuration | 位置与电子构型

Group 2 elements occupy the s-block of the Periodic Table. Each atom has two electrons in its outermost s orbital, giving the general outer electronic configuration ns². For example, magnesium is [Ne]3s² and calcium is [Ar]4s².

第2族元素位于周期表的s区。每个原子的最外层s轨道都有两个电子，因此它们的外层电子构型通式为ns²。例如，镁是[Ne]3s²，钙是[Ar]4s²。

Down the group, the principal quantum number n increases, so an extra filled electron shell is added from one element to the next. This is the key structural change that drives trends in physical properties.

沿族向下，主量子数n增大，因此从一个元素到下一个元素会增加一个填满的电子层。这是推动物理性质趋势的关键结构变化。

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2. Atomic Radius | 原子半径

The atomic radius of Group 2 elements increases down the group. Each successive element has one more electron shell, so the outermost electrons are further from the nucleus. Although nuclear charge also increases, the increased shielding by inner shells dominates, reducing the effective nuclear attraction on the outer electrons.

第2族元素的原子半径沿族向下增大。每后一个元素多一个电子层，因此最外层电子离核更远。尽管核电荷也增加，但内层电子屏蔽效应增强占主导地位，降低了对最外层电子的有效核吸引。

Typical metallic radii are: Be 112 pm, Mg 160 pm, Ca 197 pm, Sr 215 pm and Ba 222 pm. The steady increase confirms the addition of electron shells.

典型的金属半径是：Be 112 pm、Mg 160 pm、Ca 197 pm、Sr 215 pm和Ba 222 pm。半径稳步增大证实了电子层的增加。

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3. Ionic Radius | 离子半径

Group 2 metals form M²⁺ ions by losing their two outer electrons. The 2+ ion has one fewer electron shell than the parent atom, so the ionic radius is much smaller than the atomic radius. For example, a calcium atom has a metallic radius of 197 pm, but Ca²⁺ has an ionic radius of only 99 pm.

第2族金属通过失去两个外层电子形成M²⁺离子。2+离子比母原子少一个电子层，因此离子半径远小于原子半径。例如，钙原子的金属半径为197 pm，而Ca²⁺的离子半径只有99 pm。

Down the group, the ionic radius increases because each M²⁺ ion has one more inner electron shell. The values are Be²⁺ 31 pm, Mg²⁺ 65 pm, Ca²⁺ 99 pm, Sr²⁺ 113 pm and Ba²⁺ 135 pm.

沿族向下，离子半径增大，因为每个M²⁺离子多一个内层电子层。数值为Be²⁺ 31 pm、Mg²⁺ 65 pm、Ca²⁺ 99 pm、Sr²⁺ 113 pm和Ba²⁺ 135 pm。

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4. First Ionisation Energy | 第一电离能

The first ionisation energy decreases down Group 2. The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms: M(g) → M⁺(g) + e⁻. As the atomic radius increases and shielding increases, the outermost electron is held less strongly, so less energy is needed to remove it.

第一电离能沿第2族向下降低。第一电离能是从1摩尔气态原子中移走1摩尔电子所需的能量：M(g) → M⁺(g) + e⁻。随着原子半径增大和屏蔽效应增强，最外层电子被核吸引的强度减弱，因此移走它所需的能量减少。

Approximate first ionisation energies are: Be 900 kJ mol⁻¹, Mg 738 kJ mol⁻¹, Ca 590 kJ mol⁻¹, Sr 550 kJ mol⁻¹ and Ba 503 kJ mol⁻¹. The general fall is regular enough to be a reliable exam trend.

近似的第一电离能是：Be 900 kJ mol⁻¹、Mg 738 kJ mol⁻¹、Ca 590 kJ mol⁻¹、Sr 550 kJ mol⁻¹和Ba 503 kJ mol⁻¹。总体下降趋势足够有规律，是考试中可靠的规律。

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5. Melting Point | 熔点

The melting points of Group 2 metals generally decrease down the group. Group 2 metals have a metallic lattice of M²⁺ cations surrounded by a sea of delocalised electrons. The strength of the metallic bond depends on the charge density of the cation. Larger cations have lower charge density, so they attract the delocalised electrons less strongly, weakening the metallic bond and lowering the melting point.

第2族金属的熔点总体沿族向下降低。第2族金属具有由M²⁺阳离子和离域电子海组成的金属晶格。金属键的强度取决于阳离子的电荷密度。较大的阳离子电荷密度较低，因此对离域电子的吸引力较弱，金属键减弱，熔点降低。

Typical melting points are: Be 1287 °C, Mg 650 °C, Ca 842 °C, Sr 777 °C and Ba 727 °C. The trend is not perfectly smooth because the crystal structure changes between elements; magnesium is a particular anomaly having a lower melting point than calcium.

典型的熔点为：Be 1287 °C、Mg 650 °C、Ca 842 °C、Sr 777 °C和Ba 727 °C。这一趋势并不完全平滑，因为晶体结构在不同元素之间发生变化；镁是一个特殊异常，其熔点低于钙。

In exam answers, it is safest to state that melting points generally decrease down the group, and to explain this using increasing ionic radius and decreasing charge density.

在考试答案中，最稳妥的说法是熔点总体沿族向下降低，并用离子半径增大和电荷密度降低来解释。

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6. Density | 密度

Density generally increases down Group 2. Density depends on both atomic mass and atomic volume. Although atomic volume increases down the group, the relative atomic mass increases by a larger factor, so the mass per unit volume tends to rise.

密度总体沿第2族向下增大。密度取决于原子质量和原子体积。虽然原子体积沿族向下增大，但相对原子质量的增大比例更大，因此单位体积的质量趋于上升。

The experimental densities are: Be 1.85 g cm⁻³, Mg 1.74 g cm⁻³, Ca 1.55 g cm⁻³, Sr 2.63 g cm⁻³ and Ba

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