📚 What is a Transition Element? | 什么是过渡元素？
In A-Level chemistry, the term “transition element” has a specific meaning that is often tested. It is not simply another name for the d-block elements, although the two are closely related. A transition element is defined as an element that forms one or more stable ions with a partially filled d subshell. This definition is precise, and it excludes scandium and zinc, which are d-block elements but do not meet this criterion.
在A-Level化学中，“过渡元素”一词有其特定含义，经常在考试中考查。它并不只是d区元素的同义词，尽管两者关系密切。过渡元素的定义是：能形成一种或多种稳定离子，并且这些离子具有部分填满的d亚层的元素。这一定义非常精确，排除了钪和锌，它们虽然属于d区元素，但不符合这个标准。
1. Electron Configuration and the d-Block | 电子构型与d区
The d-block elements are found in the middle of the periodic table, between Groups 2 and 13. Their characteristic feature is that the highest-energy electrons are in a d subshell. For the first series, from scandium to zinc, the general outer configuration is 3d¹⁻¹⁰4s¹⁻². Two exceptions are chromium and copper, where the 4s electron is promoted to achieve a more stable d configuration.
d区元素位于周期表中部，介于第2族和第13族之间。其特征是最高能量电子位于d亚层。对于第一系列，从钪到锌，外层电子构型一般为3d¹⁻¹⁰4s¹⁻²。但铬和铜是例外，它们的4s电子被提升到d轨道以获得更稳定的d构型。
Sc: [Ar] 3d¹ 4s² Cr: [Ar] 3d⁵ 4s¹ Cu: [Ar] 3d¹⁰ 4s¹ Zn: [Ar] 3d¹⁰ 4s²
When transition metals form ions, the 4s electrons are lost first, before the 3d electrons. This is because the 4s orbital is actually slightly lower in energy than the 3d once the 3d orbitals are occupied. Therefore, iron(II) forms by losing the two 4s electrons, giving [Ar] 3d⁶, not by losing 3d electrons first.
当过渡金属形成离子时，首先失去的是4s电子，然后才是3d电子。这是因为一旦3d轨道被占据，4s轨道的能量实际上略低于3d。因此，铁(II)是通过失去两个4s电子形成的，其构型为[Ar] 3d⁶，而不是先失去3d电子。
2. Definition of a Transition Element | 过渡元素的定义
The IUPAC definition, as used by Cambridge, states that a transition element is an element whose atoms have a partially filled d subshell, or which can form stable ions with a partially filled d subshell. In practice, for the first-row d-block, we examine the ions that are stable in aqueous solution and in common compounds.
剑桥考试所采用的IUPAC定义指出：过渡元素是原子具有部分填满的d亚层，或能形成具有部分填满d亚层的稳定离子的元素。在实际中，对于第一行d区元素，我们考察的是在水溶液和常见化合物中稳定的离子。
Scandium forms only one stable ion, Sc³⁺, whose electron configuration is [Ar] 3d⁰. Because the d subshell is empty, scandium is not a transition element. Zinc forms Zn²⁺, which is [Ar] 3d¹⁰. Because the d subshell is full, zinc is also not a transition element. However, other elements such as iron (Fe²⁺, Fe³⁺) and copper (Cu²⁺) have partially filled d orbitals in their stable ions.
钪仅形成一种稳定离子Sc³⁺，其电子构型为[Ar] 3d⁰。由于d亚层是空的，钪不是过渡元素。锌形成Zn²⁺，其构型为[Ar] 3d¹⁰。由于d亚层是全满的，锌也不是过渡元素。而其他元素如铁（Fe²⁺、Fe³⁺）和铜（Cu²⁺）在其稳定离子中具有部分填满的d轨道。
Transition elements: d-block elements excluding Sc and Zn (for the first series).
过渡元素：第一系列d区中除钪和锌之外的元素。
3. Variable Oxidation States | 可变氧化态
A key characteristic of transition elements is their ability to exhibit multiple oxidation states. This arises because the 3d and 4s electrons have similar energies, so varying numbers of electrons can be lost without requiring a huge amount of energy. For example, iron can exist as Fe²⁺ and Fe³⁺; copper as Cu⁺ and Cu²⁺; manganese as Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁶⁺, and Mn⁷⁺.
过渡元素的一个关键特性是能够呈现多种氧化态。这是因为3d和4s电子能量相近，因此失去不同数量的电子不需要巨大的能量。例如，铁可以以Fe²⁺和Fe³⁺存在；铜可以是Cu⁺和Cu²⁺；锰可以是Mn²⁺、Mn³⁺、Mn⁴⁺、Mn⁶⁺和Mn⁷⁺。
The maximum oxidation state increases as the number of available d electrons increases, up to manganese (which can lose all five 3d electrons and both 4s electrons, giving +7). After manganese, the maximum oxidation state decreases because the 3d electrons become more tightly held by the increasing nuclear charge.
最高氧化态随着可用的d电子数量增加而增加，直到锰（锰可以失去全部五个3d电子和两个4s电子，达到+7）。在锰之后，最高氧化态下降，因为3d电子被增大的核电荷更紧密地吸引。
| Element | Common oxidation states |
| Ti | +2, +3, +4 |
| V | +2, +3, +4, +5 |
| Mn | +2, +3, +4, +6, +7 |
| Fe | +2, +3 |
| Cu | +1, +2 |
4. Formation of Coloured Ions | 形成有色离子
Most transition metal ions in aqueous solution are coloured. This is due to the splitting of the d orbitals in the presence of ligands (such as water molecules). In an isolated atom or ion, the five d orbitals have the same energy. When ligands approach, the d orbitals split into two groups of different energy: the lower-energy set (t₂g) and the higher-energy set (e_g for octahedral complexes).
大多数过渡金属离子在水溶液中呈现颜色。这是因为在配体（如水分子）存在时，d轨道会发生分裂。在孤立原子或离子中，五个d轨道能量相同。当配体靠近时，d轨道分裂为两组不同能量的轨道：较低能组（t₂g）和较高能组（e_g，对于八面体配合物）。
Electrons can absorb visible light and jump from a lower d orbital to a higher d orbital. The energy gap (Δ) corresponds to a specific wavelength of visible light. The observed colour is the complementary colour of the absorbed light. For example, [Cu(H₂O)₆]²⁺ absorbs orange-red light and appears blue.
电子可以吸收可见光，从较低d轨道跃迁到较高d轨道。能隙（Δ）对应特定波长的可见光。观察到的颜色是吸收光的互补色。例如，[Cu(H₂O)₆]²⁺吸收橙红色光，因此呈现蓝色。
Colour arises from d–d electron transitions.
颜色源于d–d电子跃迁。
Zinc and scandium ions are not coloured because their d subshells are either full (d¹⁰) or empty (d⁰), so no d–d transitions are possible.
锌和钪离子没有颜色，因为它们的d亚层要么全满（d¹⁰）要么全空（d⁰），因此不可能发生d–d跃迁。
5. Magnetic Properties | 磁性
Transition elements and their ions often show magnetic properties because of unpaired electrons. Substances with unpaired electrons are paramagnetic; they are attracted into a magnetic field. The greater the number of unpaired electrons, the stronger the paramagnetic effect. Ions such as Fe³⁺ (3d⁵) have five unpaired electrons, giving a strong magnetic moment.
过渡元素及其离子通常因存在未成对电子而具有磁性。具有未成对电子的物质是顺磁性的，它们会被磁场吸引。未成对电子越多，顺磁效应越强。例如，Fe³⁺（3d⁵）有五个未成对电子，产生强磁矩。
Zinc, with a full 3d¹⁰ configuration, has no unpaired electrons and is diamagnetic (slightly repelled by a magnetic field). In chemistry exams, you may be asked to predict whether a given ion is paramagnetic by writing its electron configuration and counting unpaired electrons.
锌具有全满的3d¹⁰构型，没有未成对电子，因而是抗磁性的（轻微被磁场排斥）。在化学考试中，可能会要求你通过写出电子构型并计算未成对电子来判断给定离子是否顺磁性。
6. Catalytic Activity | 催化活性
Transition metals and their compounds are widely used as catalysts in industrial and biological processes. Their catalytic activity is linked to their ability to adopt multiple oxidation states and to provide a surface on which reactants can adsorb. For example, iron is used in the Haber process for ammonia synthesis, and vanadium(V) oxide is used in the Contact process for sulfuric acid manufacture.
过渡金属及其化合物广泛用作工业和生物过程中的催化剂。它们的催化活性与其能呈现多种氧化态以及提供反应物吸附表面有关。例如，铁用于哈伯法合成氨，五氧化二钒用于接触法制硫酸。
In homogeneous catalysis, the transition metal ion in solution changes oxidation state during the reaction and then returns to its original state. A classic example is the catalysis of the reaction between iodide ions and peroxodisulfate ions by Fe²⁺/Fe³⁺. The Fe²⁺ reduces S₂O₈²⁻ to 2SO₄²⁻ and is oxidised to Fe³⁺; the Fe³⁺ then oxidises I⁻ to I₂ and is reduced back to Fe²⁺.
在均相催化中，溶液中的过渡金属离子在反应中改变氧化态，然后回到原始状态。一个经典例子是Fe²⁺/Fe³⁺对碘离子与过二硫酸根离子反应的催化作用。Fe²⁺将S₂O₈²⁻还原为2SO₄²⁻，本身被氧化为Fe³⁺；随后Fe³⁺将I⁻氧化为I₂，本身被还原回Fe²⁺。
7. Complex Ion Formation | 配离子形成
Transition metal ions have empty or partially filled d orbitals that can accept lone pairs from ligands to form coordinate (dative covalent) bonds. The resulting species is called a complex ion. Common ligands include H₂O, NH₃, Cl⁻, and CN⁻. The number of coordinate bonds is the coordination number, commonly 6 for octahedral complexes and 4 for tetrahedral or square planar complexes.
过渡金属离子具有空或部分填满的d轨道，可以接受配体的孤对电子形成配位键（配位共价键）。形成的物种称为配离子。常见配体包括H₂O、NH₃、Cl⁻和CN⁻。配位键的数量称为配位数，通常八面体配合物为6，四面体或平面正方形配合物为4。
For example, when excess ammonia is added to aqueous copper(II) sulfate, the pale blue [Cu(H₂O)₆]²⁺ is converted to the deep blue [Cu(NH₃)₄(H₂O)₂]²⁺. Similarly, iron(III) forms [FeCl₄]⁻ in concentrated hydrochloric acid. The ability to form complexes has practical uses in qualitative analysis and metal extraction.
例如，当过量氨水加入硫酸铜水溶液时，浅蓝色的[Cu(H₂O)₆]²⁺转化为深蓝色的[Cu(NH₃)₄(H₂O)₂]²⁺。类似地，铁(III)在浓盐酸中形成[FeCl₄]⁻。形成配合物的能力在定性分析和金属提取中有实际应用。
8. Distinction Between d-Block and Transition Elements | d区元素与过渡元素的区别
It is essential to distinguish these two terms. The d-block elements are those in which the highest-energy electron is in a d orbital. This includes all elements from Sc to Zn in period 4. Transition elements are a subset of the d-block: they are d-block elements that also form stable ions with a partially filled d subshell. Scandium and zinc are d-block elements but not transition elements.
将这两个术语区分开来至关重要。d区元素是指最高能量电子位于d轨道的元素，包括第四周期从钪到锌的所有元素。过渡元素是d区的一个子集：它们是能够形成具有部分填满d亚层的稳定离子的d区元素。钪和锌是d区元素，但不是过渡元素。
| d-block element | Transition element? | Stable ion | d configuration in ion |
| Sc | No | Sc³⁺ | 3d⁰ |
| Zn | No | Zn²⁺ | 3d¹⁰ |
| Fe | Yes | Fe²⁺ / Fe³⁺ | 3d⁶ / 3d⁵ |
9. Typical Exam Questions and Common Mistakes | 典型考题与常见错误
A frequent question is: “State the electronic configuration of Sc³⁺ and explain why scandium is not a transition element.” The correct answer is that Sc³⁺ has the configuration [Ar] 3d⁰, so its d subshell is empty, not partially filled. Another common trap is zinc: Zn²⁺ has [Ar] 3d¹⁰, so its d subshell is full. Students often forget that transition elements must have a partially filled d subshell in a stable ion, not just in the atom.
一个常见问题是：“写出Sc³⁺的电子构型，并解释为什么钪不是过渡元素。”正确答案是Sc³⁺的构型为[Ar] 3d⁰，其d亚层是空的，而不是部分填满。另一个常见陷阱是锌：Zn²⁺构型为[Ar] 3d¹⁰，其d亚层是全满的。学生经常忘记过渡元素必须在稳定离子中具有部分填满的d亚层，而不仅仅是在原子中。
Another mistake is to say that copper is not a transition element because Cu⁺ has a full d¹⁰ configuration. However, copper also forms Cu²⁺, which has a d⁹ configuration, and Cu²⁺ is stable. Thus copper does meet the definition, because it forms at least one stable ion with a partially filled d subshell.
另一个错误是认为铜不是过渡元素，因为Cu⁺具有全满的d¹⁰构型。然而，铜还形成Cu²⁺，其具有d⁹构型，且Cu²⁺是稳定的。因此铜确实符合定义，因为它至少形成一种具有部分填满d亚层的稳定离子。
10. Summary of Key Properties | 关键性质总结
Transition elements share several characteristic properties that can be used to identify them:
过渡元素具有若干可用于识别它们的特征性质：
- Variable oxidation states / 可变氧化态
- Coloured ions in aqueous solution / 水溶液中形成有色离子
- Catalytic activity / 催化活性
- Formation of complex ions / 形成配离子
- Magnetic properties due to unpaired electrons / 因未成对电子而具有磁性
All of these properties stem from the partially filled d subshell in their stable ions. By contrast, scandium and zinc lack this feature and therefore do not show these properties to the same extent.
所有这些性质都源于它们稳定离子中部分填满的d亚层。相比之下，钪和锌缺乏这一特征，因此不表现出同样的性质。
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