Enthalpy of Atomisation and Electron Affinity | 原子化焓变与电子亲和能

📚 Enthalpy of Atomisation and Electron Affinity | 原子化焓变与电子亲和能

Enthalpy of atomisation and electron affinity are two cornerstone thermodynamic concepts in the CIE A-Level Chemistry syllabus. They appear repeatedly in Born-Haber cycles, lattice-energy calculations, and periodicity questions. Mastering their definitions, sign conventions, and periodic trends is essential for securing full marks in energetics.

原子化焓变与电子亲和能是CIE A-Level化学考纲中的两大核心热力学概念，频繁出现在Born-Haber循环、晶格能计算以及元素周期律题中。熟练掌握它们的定义、符号约定和周期趋势，是在能量学部分拿满分的关键。

1. Enthalpy of Atomisation: Definition | 原子化焓变的定义

The standard enthalpy of atomisation (ΔH°ₐₜ) is defined as the enthalpy change when one mole of gaseous atoms is formed from one mole of the element in its standard state, under standard conditions (298 K and 1 atm). The unit is kJ mol⁻¹.

标准原子化焓变（ΔH°ₐₜ）定义为：在标准条件（298 K和1 atm）下，一摩尔处于标准状态的元素转化为一摩尔气态原子时的焓变，单位为 kJ mol⁻¹。

For a metal such as sodium, the atomisation process involves converting the solid metal lattice into isolated gaseous atoms:

以金属钠为例，原子化过程是将固态金属晶格转化为孤立气态原子：

Na(s) → Na(g)　ΔH°ₐₜ = +107 kJ mol⁻¹

Since bonds must be broken to separate atoms, atomisation of an element is always endothermic, giving a positive ΔH°ₐₜ.

由于必须断裂化学键才能将原子分离，元素的原子化总是吸热的，因此ΔH°ₐₜ为正值。

2. Atomisation of Non-Metals | 非金属的原子化焓变

For non-metals that exist as diatomic molecules, the definition refers to half a mole of the molecule, so that exactly one mole of gaseous atoms is produced. For chlorine:

对于以双原子分子存在的非金属，定义取半摩尔分子，以便恰好生成一摩尔气态原子。以氯为例：

½Cl₂(g) → Cl(g)　ΔH°ₐₜ = +121.7 kJ mol⁻¹

For graphite (carbon):

对石墨（碳）而言：

C(s, graphite) → C(g)　ΔH°ₐₜ = +715 kJ mol⁻¹

The large value for carbon reflects the enormous strength of the covalent network in graphite. This value is much larger than those of most metals because metallic bonds are generally weaker than the covalent bonds in a giant covalent lattice.

碳的数值很大，反映石墨中共价网络结构极其牢固。该值比多数金属大得多，因为一般情况下金属键弱于巨型共价晶格中的共价键。

3. Factors Affecting Enthalpy of Atomisation | 影响原子化焓变的因素

The magnitude of ΔH°ₐₜ for a metal depends on the strength of its metallic bonding. Metallic bond strength increases with:

金属的ΔH°ₐₜ大小取决于金属键强度，而金属键强度受以下因素影响而增强：

Higher charge on the metal ion — more electrons are released into the delocalised ‘sea of electrons’, bonding more strongly.
Smaller ionic radius — the nucleus attracts the delocalised electrons more effectively.

金属离子电荷越高——释放到离域“电子海”中的电子越多，键合越强。
离子半径越小——原子核对离域电子的吸引力越有效。

For example, ΔH°ₐₜ increases from Na (+107 kJ mol⁻¹) to Mg (+148 kJ mol⁻¹) to Al (+326 kJ mol⁻¹), matching the trend in ionic charge across Period 3.

例如，ΔH°ₐₜ从Na（+107 kJ mol⁻¹）到Mg（+148 kJ mol⁻¹）再到Al（+326 kJ mol⁻¹）依次增大，正好符合第三周期离子电荷递增的趋势。

4. Electron Affinity: First Definition | 电子亲和能：第一定义

The first electron affinity (ΔH°ₑₐ₁ or EA₁) is the enthalpy change when one mole of gaseous atoms each gains one electron to form one mole of gaseous uninegative ions, under standard conditions.

第一电子亲和能（ΔH°ₑₐ₁ 或 EA₁）定义为：在标准条件下，一摩尔气态原子各获得一个电子，形成一摩尔气态一价负离子时的焓变。

Cl(g) + e⁻ → Cl⁻(g)　ΔH°ₑₐ₁ = −349 kJ mol⁻¹

For most atoms, the first electron affinity is negative (exothermic), because the incoming electron is attracted to the nucleus and is placed in a lower energy orbital than infinity. Energy is released.

对大多数原子而言，第一电子亲和能为负值（放热），因为新进入的电子受核吸引，占据比无穷远处能量更低的轨道，因而释放能量。

5. The Second Electron Affinity | 第二电子亲和能

The second electron affinity (ΔH°ₑₐ₂ or EA₂) is the enthalpy change when one mole of gaseous uninegative ions each gains one more electron to form one mole of gaseous dinegative ions.

第二电子亲和能（ΔH°ₑₐ₂ 或 EA₂）定义为：一摩尔气态一价负离子各再获得一个电子，形成一摩尔气态二价负离子时的焓变。

O⁻(g) + e⁻ → O²⁻(g)　ΔH°ₑₐ₂ = +753 kJ mol⁻¹

The second electron affinity is always endothermic (positive). The incoming electron must be forced onto an already negatively charged ion, and the electron–electron repulsion far outweighs the attraction of the nucleus. Energy must be supplied to overcome this repulsion.

第二电子亲和能总是吸热的（正值）。新电子必须被强制加到已经带负电的离子上，电子间的排斥力远大于原子核对它的吸引。必须输入能量以克服这种排斥。

6. Why the Overall O²⁻ Formation Is Exothermic | 为什么O²⁻的整体生成仍是放热

When considering oxygen in a Born-Haber cycle, we must add EA₁ and EA₂ together:

在Born-Haber循环中考虑氧时，须将EA₁与EA₂相加：

O(g) → O²⁻(g)　ΔH°ₑₐ₁ + ΔH°ₑₐ₂ = (−141) + (+753) = +612 kJ mol⁻¹

Overall, forming O²⁻(g) is strongly endothermic. This energy cost is compensated in ionic compounds like MgO by the very large lattice energy released when Mg²⁺ and O²⁻ condense into a crystal. This explains why MgO is thermodynamically stable despite the unfavourable electron affinity.

总体上，形成O²⁻(g)是强烈吸热的。这一能量代价在MgO等离子化合物中，由Mg²⁺与O²⁻凝聚成晶体时释放的巨大晶格能所补偿。这解释了为什么MgO尽管电子亲和能不利，仍然热力学稳定。

7. Factors Affecting Electron Affinity | 影响电子亲和能的因素

The magnitude of the first electron affinity is governed by three factors:

第一电子亲和能的大小受三个因素支配：

Atomic charge (nuclear charge): a greater nuclear charge attracts the incoming electron more strongly, making the electron affinity more negative.
Atomic radius: a larger distance between the nucleus and the incoming electron weakens the attraction, making the electron affinity less negative.
Electron shielding: inner-shell electrons repel the incoming electron, reducing the effective nuclear attraction.

核电荷：核电荷越大，对新进入电子的吸引越强，电子亲和能越负。
原子半径：核与新进入电子之间的距离越大，吸引越弱，电子亲和能负值越小。
电子屏蔽：内层电子排斥外来电子，削弱有效核吸引。

The combination of increasing nuclear charge and decreasing radius explains why electron affinity becomes more negative from left to right across a period.

核电荷增加与半径减小共同作用，解释了为什么从左到右穿越一个周期时电子亲和能越来越负。

8. Periodic Trends in Enthalpy of Atomisation | 原子化焓变的周期趋势

Across Period 3 (Na → Ar), the enthalpy of atomisation shows a characteristic pattern:

纵观第三周期（Na → Ar），原子化焓变呈现特有的分布模式：

Element	Na	Mg	Al	Si	P	S	Cl	Ar
ΔH°ₐₜ / kJ mol⁻¹	+107	+148	+326	+456	+315	+279	+122	0

The values rise from Na to Si as metallic bonding strengthens (increasing charge and decreasing radius). A sharp drop from Si to P occurs because P₄ contains weaker van der Waals forces between P₄ molecules rather than a giant covalent or metallic lattice. A further decline to Cl and Ar reflects decreasing intermolecular forces and, for Ar, no bonds at all — argon exists as monatomic gas, so its atomisation enthalpy is zero.

从Na到Si数值持续上升，因为金属键增强（电荷增加、半径减小）。从Si到P出现骤降，因为P₄分子间仅靠较弱的范德华力结合，而非巨型共价或金属晶格。到Cl和Ar进一步下降，反映分子间作用力递减；而Ar本身为单原子气体，无需原子化，其原子化焓变为零。

9. Periodic Trends in Electron Affinity | 电子亲和能的周期趋势

Down a group, electron affinity generally becomes less negative. For example:

同族自上而下，电子亲和能通常负值变小。例如：

F: −328 kJ mol⁻¹　Cl: −349 kJ mol⁻¹　Br: −325 kJ mol⁻¹　I: −295 kJ mol⁻¹

Despite fluorine having the smallest radius and greatest electronegativity, its electron affinity is less negative than chlorine’s. This anomaly arises because fluorine’s 2p subshell is compact and electron-rich, so the incoming electron experiences significant repulsion. Chlorine has a larger 3p orbital and better accommodates the additional electron.

尽管氟半径最小、电负性最大，其电子亲和能却不如氯的负值大。这个异常源于氟的2p亚层紧致且电子密度高，新进入的电子受到显著排斥。氯的3p轨道更大，能更好地容纳额外电子。

Across a period, electron affinity becomes more negative as effective nuclear charge increases:

同一周期从左到右，有效核电荷增大，电子亲和能越来越负：

Li: −60 kJ mol⁻¹　N: −7 kJ mol⁻¹　O: −141 kJ mol⁻¹　F: −328 kJ mol⁻¹

Nitrogen has an unusually small (close-to-zero) electron affinity because its 2p subshell is half-filled and thus stable; the added electron must pair with an existing electron, incurring extra electron–electron repulsion.

氮的电子亲和能异常小（接近零），因为其2p亚层处于半满稳定状态，外来电子须与已有电子成对，产生额外的电子排斥。

10. Application in Born-Haber Cycles | 在Born-Haber循环中的应用

Born-Haber cycles for ionic compounds use both atomisation and electron affinity values to calculate lattice energy. For sodium chloride:

离子化合物的Born-Haber循环利用原子化焓变和电子亲和能来计算晶格能。以氯化钠为例：

ΔH°f(NaCl) = ΔH°ₐₜ(Na) + ½ΔH°ₐₜ(Cl₂) + IE₁(Na) + EA₁(Cl) + LE

All terms are summed with their signs. The lattice energy (LE) is negative (exothermic), reflecting the energy released when gaseous ions form an ionic lattice. Rearranging the equation allows LE to be determined when ΔH°f is known experimentally.

所有项按符号相加。晶格能（LE）为负值（放热），表示气态离子形成离子晶格时释放的能量。重排方程后，当ΔH°f可由实验测得时，就能求出LE。

For magnesium oxide, two ionisation energies of Mg and the sum of both electron affinities of O must be included:

对氧化镁，必须计入Mg的两级电离能以及O的两级电子亲和能之和：

ΔH°f(MgO) = ΔH°ₐₜ(Mg) + ½ΔH°ₐₜ(O₂) + IE₁(Mg) + IE₂(Mg) + EA₁(O) + EA₂(O) + LE

Since EA₂(O) is positive, it imposes an energy penalty; nevertheless, the large lattice energy of MgO (dipositive and dinegative ions attract strongly) ensures the overall ΔH°f remains negative, explaining the stability of MgO.

由于EA₂(O)为正值，会带来能量代价；但MgO巨大的晶格能（正二价与负二价离子强烈吸引）确保了整体ΔH°f仍为负值，从而解释MgO的稳定性。

11. Common Errors in Exams | 考试中的常见错误

Students frequently lose marks on this topic for the following reasons:

学生常因以下原因在此专题失分：

Writing ΔH°ₐₜ for molecules incorrectly — always remember it refers to one mole of gaseous atoms, not molecules.
Forgetting that atomisation of an element is positive; examiners may ask you to predict the sign.
Using EA₂ with the wrong sign in Born-Haber cycles — since it is endothermic, it must be added as a positive value.
Confusing “electron affinity” with “electronegativity” — electron affinity is a measurable enthalpy change, whereas electronegativity is a dimensionless relative property.

错误地书写分子的ΔH°ₐₜ——切记它指的是生成一摩尔气态原子，而非分子。
忘记元素的原子化焓变为正值；考官常要求判断符号。
在Born-Haber循环中用错EA₂的符号——由于它吸热，必须以正值加入。
混淆”电子亲和能”与”电负性”——电子亲和能是可测量的焓变，而电负性是无量纲的相对性质。

Another frequent error is stating that “atomic radius increases from left to right across a period” — the opposite is true, and this directly affects trends in electron affinity.

另一个常见错误是声称“同一周期从左到右原子半径增大”——事实正好相反，而这直接影响电子亲和能的趋势。

12. Summary and Key Equations | 总结与关键方程式

The following equations summarise the core ideas of this article:

以下方程总结本文核心要点：

M(s) → M(g)　ΔH°ₐₜ > 0 (endothermic)

X(g) + e⁻ → X⁻(g)　ΔH°ₑₐ₁ <

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Enthalpy of Atomisation and Electron Affinity | 原子化焓变与电子亲和能