Atomic Structure and Electron Configuration | 原子结构与电子排布

📚 Atomic Structure and Electron Configuration | 原子结构与电子排布

Atomic structure forms the very foundation of A-level Chemistry. Understanding how protons, neutrons and electrons are arranged within an atom, and how electrons occupy energy levels, is essential for explaining periodic trends, chemical bonding and reactivity. This chapter covers everything you need for the AS syllabus: subatomic particles, atomic number, mass number, isotopes, mass spectrometry, and the rules governing electron configuration.

原子结构是 A-level 化学的基石。理解质子、中子和电子在原子内部的排列方式，以及电子如何占据能级，是解释元素周期律、化学键和反应活性的关键。本章完整覆盖 AS 考纲要求：亚原子粒子、原子序数、质量数、同位素、质谱分析，以及电子排布的基本规则。

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1. The Nuclear Model of the Atom | 原子的核模型

Modern atomic theory describes the atom as a very small, dense nucleus containing positively charged protons and neutral neutrons, surrounded by a cloud of negatively charged electrons. The nucleus occupies only a tiny fraction of the atom’s volume but accounts for almost all of its mass.

现代原子理论认为，原子由一个极小且致密的原子核构成，核内含有带正电的质子和不带电的中子，核外散布着带负电的电子云。原子核仅占原子体积的极小部分，却几乎承载了原子的全部质量。

The key properties of subatomic particles are summarised below:

亚原子粒子的关键性质总结如下：

Particle | 粒子	Relative Charge | 相对电荷	Relative Mass | 相对质量	Location | 位置
Proton | 质子	+1	1	Nucleus | 原子核
Neutron | 中子	0	1	Nucleus | 原子核
Electron | 电子	-1	1/1840 (≈0)	Electron shells | 电子层

Electrons are so light that their mass is usually ignored when calculating the mass of an atom. An atom is electrically neutral because the number of protons equals the number of electrons.

电子的质量极轻，在计算原子质量时通常忽略不计。原子呈电中性，因为质子数等于电子数。

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2. Atomic Number and Mass Number | 原子序数与质量数

The atomic number (Z) is the number of protons in the nucleus of an atom. It defines the identity of an element. The mass number (A) is the total number of protons and neutrons in the nucleus.

原子序数（Z）是原子核中的质子数，它决定了元素的种类。质量数（A）是原子核中质子数和中子数的总和。

The standard notation for an atom is:

原子的标准表示法为：

ᵃₓX

where X is the element symbol, a is the mass number, and z is the atomic number. For a neutral atom:

其中 X 是元素符号，a 是质量数，z 是原子序数。对于中性原子：

Number of electrons = Number of protons = Z

电子数 = 质子数 = Z

Number of neutrons = A – Z

中子数 = A – Z

For example, a sodium atom, ²³₁₁Na, contains 11 protons, 11 electrons, and 23 – 11 = 12 neutrons.

例如，钠原子 ²³₁₁Na 含有 11 个质子、11 个电子，和 23 – 11 = 12 个中子。

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3. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because they have the same number of protons and electrons, isotopes of an element have identical chemical properties. However, they differ in physical properties such as density and rate of diffusion.

同位素是同一元素中质子数相同但中子数不同的原子。由于质子数和电子数相同，同一元素的同位素具有完全相同的化学性质。但在物理性质上，如密度和扩散速率，会存在差异。

The chemical behaviour of an element is determined by its electron arrangement, which depends solely on the number of protons (and hence electrons). Since all isotopes of an element have the same electron configuration, they react in exactly the same way chemically.

元素的化学行为由其电子排布决定，而电子排布仅取决于质子数（即电子数）。由于同一元素的所有同位素具有相同的电子排布，它们的化学反应方式完全相同。

Common examples include:

常见的例子包括：

• Hydrogen has three isotopes: ¹H, ²H (deuterium), ³H (tritium). | 氢有三种同位素：¹H（氕）、²H（氘）、³H（氚）。

• Carbon has isotopes ¹²C and ¹⁴C; the latter is used in radiocarbon dating. | 碳有 ¹²C 和 ¹⁴C 两种同位素，后者用于放射性碳定年法。

• Chlorine has two common isotopes: ³⁵Cl and ³⁷Cl. | 氯有两种常见同位素：³⁵Cl 和 ³⁷Cl。

Notice that the relative atomic mass of an element is the weighted mean mass of all its isotopes compared with 1/12 of the mass of one atom of ¹²C.

请注意，元素的相对原子质量是该元素所有同位素的加权平均质量，以 ¹²C 原子质量的 1/12 为标准。

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4. Mass Spectrometry | 质谱法

A mass spectrometer is an instrument used to measure the precise masses and relative abundances of ions. It is a key tool in determining relative atomic mass, identifying isotopes, and analysing molecular structures.

质谱仪是一种用于精确测量离子质量和相对丰度的仪器，是计算相对原子质量、识别同位素和分析分子结构的重要工具。

The five key stages of mass spectrometry are:

质谱法的五个关键步骤是：

1. Vaporisation | 气化：The sample is turned into a gas. | 样品被转化为气体。

2. Ionisation | 离子化：The gas is bombarded with high-energy electrons to form positively charged ions. | 用高能电子轰击气体，形成带正电的离子。

3. Acceleration | 加速：Ions are accelerated by an electric field so they all have the same kinetic energy. | 离子在电场中加速，获得相同的动能。

4. Deflection | 偏转：Ions pass through a magnetic field and are deflected by different amounts depending on their charge-to-mass ratio (m/z). | 离子通过磁场时，根据其质荷比（m/z）发生不同程度的偏转。

5. Detection | 检测：Ions strike a detector, producing a current proportional to their abundance. | 离子撞击检测器，产生与丰度成正比的电流信号。

Lighter ions or ions with a higher charge are deflected more. The output is a mass spectrum showing peaks at different m/z values, with the height of each peak indicating relative abundance.

质量更小的离子或电荷更高的离子偏转更大。输出的质谱图在不同 m/z 值处显示峰，峰的高度代表相对丰度。

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5. Calculating Relative Atomic Mass from Mass Spectra | 由质谱计算相对原子质量

To calculate the relative atomic mass (Ar) of an element from its mass spectrum, multiply each isotopic mass by its relative abundance (as a percentage), sum the products, and divide by 100.

由质谱计算元素的相对原子质量（Ar），需要用每个同位素的质量乘以它的相对丰度（百分比），将所有乘积相加后除以 100。

Consider chlorine, which has two isotopes: ³⁵Cl (75%) and ³⁷Cl (25%). The calculation is:

以氯为例，它有两种同位素：³⁵Cl（75%）和 ³⁷Cl（25%）。计算如下：

Ar = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5

This matches the value on the periodic table. For molecules containing multiple atoms, the mass spectrum also shows molecular ion peaks that help determine relative molecular mass.

这与周期表上的数值一致。对于含多个原子的分子，质谱中的分子离子峰还可用于确定相对分子质量。

Worked example: A sample of rubidium contains ⁸⁵Rb (72%) and ⁸⁷Rb (28%). Calculate Ar.

例题：某铷样品含 ⁸⁵Rb（72%）和 ⁸⁷Rb（28%），计算 Ar。

Ar = (85 × 72 + 87 × 28) ÷ 100 = (6120 + 2436) ÷ 100 = 85.56

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6. Evidence for Energy Levels: Ionisation Energies | 能级存在的证据：电离能

First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous singly positive ions. It is a periodic trend that directly reflects the electronic structure of atoms.

第一电离能是指从一摩尔气态原子中移除一摩尔电子，形成一摩尔气态一价正离子所需的能量。该周期性趋势直接反映了原子的电子结构。

For example, the first ionisation energy of sodium is:

例如，钠的第一电离能为：

Na(g) → Na⁺(g) + e⁻ ΔH = +496 kJ mol⁻¹

Successive ionisation energies provide clear evidence for electron shells. Consider the successive ionisation energies of sodium:

逐级电离能提供了电子层存在的直接证据。以钠的逐级电离能为例：

Electron removed | 移除的电子	Ionisation energy / kJ mol⁻¹ | 电离能 / kJ mol⁻¹
1st | 第一	496
2nd | 第二	4560
3rd | 第三	6910
4th | 第四	9540
5th | 第五	13400
6th | 第六	16600
7th | 第七	20100
8th | 第八	25500
9th | 第九	35600
10th | 第十	141000
11th | 第十一	159000

A huge jump occurs between the first and second ionisation energies, and again between the tenth and eleventh. This tells us that sodium has one electron in its outermost shell, eight electrons in the second shell, and two electrons in the first shell. Each large jump corresponds to removing an electron from a closer, much more tightly held shell.

在第一和第二电离能之间，以及第十和第十一之间，出现了巨大的跳跃。这告诉我们钠的最外层有 1 个电子，第二层有 8 个电子，第一层有 2 个电子。每一次大的跳跃都对应着从更内层、结合更紧密的电子层中移除电子。

This pattern demonstrates that electrons are arranged in distinct shells at different distances from the nucleus, with inner-shell electrons experiencing much stronger attraction to the nucleus.

这一模式证明电子在距核不同距离的分离壳层中排列，内层电子受到原子核强得多的吸引。

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7. Atomic Orbitals and Sub-shells | 原子轨道与亚层

Within each principal energy level (shell), electrons occupy sub-shells known as s, p, d and f. Each sub-shell contains one or more orbitals, which are regions of space where there is a high probability of finding an electron. Each orbital can hold a maximum of two electrons.

在每个主能级（电子层）内，电子占据称为 s、p、d、f 的亚层。每个亚层包含一个或多个轨道；轨道是找到电子概率最高的空间区域。每个轨道最多容纳两个电子。

• s sub-shell | s 亚层：Contains 1 orbital; holds up to 2 electrons. Spherical in shape. | 含 1 个轨道，最多容纳 2 个电子，呈球形。

• p sub-shell | p 亚层：Contains 3 orbitals; holds up to 6 electrons. Dumbbell-shaped, oriented along the x, y and z axes. | 含 3 个轨道，最多容纳 6 个电子，呈哑铃形，沿 x、y、z 轴取向。

• d sub-shell | d 亚层：Contains 5 orbitals; holds up to 10 electrons. More complex shapes. | 含 5 个轨道，最多容纳 10 个电子，形状更复杂。

• f sub-shell | f 亚层：Contains 7 orbitals; holds up to 14 electrons. | 含 7 个轨道，最多容纳 14 个电子。

The maximum number of electrons in each shell is 2n², where n is the principal quantum number. For the first shell (n=1): 2 electrons; second shell (n=2): 8 electrons; third shell (n=3): 18 electrons.

每个电子层的最大电子数为 2n²，其中 n 是主量子数。第一层（n=1）最多 2 个电子；第二层（n=2）最多 8 个电子；第三层（n=3）最多 18 个电子。

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8. The Order of Filling Sub-shells | 亚层的填充顺序

Electrons fill sub-shells in order of increasing energy. The Aufbau principle states that electrons occupy the lowest available energy level first. The order of filling is:

电子按能量升序填充亚层。构造原理指出，电子首先占据能量最低的可用能级。填充顺序为：

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s…

Note the important anomaly: 4s has lower energy than 3d and therefore fills before 3d. This applies to potassium and calcium, where the 4s sub-shell fills before 3d.

请注意一个重要的特殊之处：4s 的能量低于 3d，因此 4s 先于 3d 填充。钾和钙就属于这种情况，4s 亚层先于 3d 填充。

The energy order within a period follows the (n + l) rule: the lower the sum of the principal quantum number and azimuthal quantum number, the lower the energy. When the sums are equal, the sub-shell with the smaller n fills first.

能级在同一周期内的顺序遵循（n + l）规则：n + l 之和越小，能量越低；当和相等时，n 较小的亚层先填充。

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9. Rules for Electron Configuration | 电子排布规则

Three fundamental rules govern how electrons are arranged in orbitals:

三个基本规则决定了电子在轨道中的排列方式：

• Aufbau Principle | 构造原理：Electrons fill orbitals in order of increasing energy. | 电子按能量递增的顺序填充轨道。

• Pauli Exclusion Principle | 泡利不相容原理：Each orbital can hold a maximum of two electrons, and they must have opposite spins. | 每个轨道最多容纳两个电子，且自旋方向必须相反。

• Hund’s Rule | 洪特规则：When filling sub-shells with multiple orbitals of equal energy (e.g., 2p), electrons occupy each orbital singly with parallel spins before pairing up. | 当填充多个能量相同的轨道时（如 2p 亚层的三个轨道），电子先以相同自旋方向单独占据每个轨道，然后才配对。

For example, nitrogen (Z=7) has the configuration 1s² 2s² 2p³. The three 2p electrons occupy three separate p orbitals, each with the same spin, rather than pairing in one orbital.

例如，氮（Z=7）的排布为 1s² 2s² 2p³。三个 2p 电子分别占据三个不同的 p 轨道，且自旋方向相同，而不是在同一轨道中配对。

These rules ensure the most stable arrangement of electrons by minimising electron-electron repulsion.

这些规则通过最小化电子间的排斥作用，确保了最稳定的电子排布。

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10. Writing Electron Configurations | 书写电子排布

Electron configurations can be written in three ways: full notation, shorthand (using the noble gas core), and orbital diagrams. All three appear in AS examination papers.

电子排布可以用三种方式书写：完整写法、简写（稀有气体核）、以及轨道图。这三种形式在 AS 考试中都会出现。

Full notation | 完整写法：The sub-shells are listed in order of increasing energy with the number of electrons as a superscript.

完整写法：按能量递增顺序列出亚层，用上标数字表示该亚层中的电子数。

Oxygen (Z=8): 1s² 2s² 2p⁴

氧（Z=8）：1s² 2s² 2p⁴

Iron (Z=26): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

铁（Z=26）：1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

Shorthand notation | 简写：The noble gas core in square brackets represents all completed shells, followed by the remaining electrons.

简写：用方括号中的稀有气体代表已完成的内层电子排布，之后写出剩余电子。

Iron: [Ar] 4s² 3d⁶

铁：[Ar] 4s² 3d⁶

Orbital diagrams | 轨道图：Each orbital is shown as a box, with arrows representing electrons. An upward arrow (↑) and downward arrow (↓) represent electrons with opposite spins.

轨道图：每个轨道用方框表示，箭头代表电子，向上（↑）和向下（↓）代表自旋方向相反的电子。

When writing configurations for ions, remember: electrons are removed from the outermost shell first. For transition metals, the 4s electrons are removed before the 3d electrons.

书写离子的电子排布时，请记住：电子最先从最外层移走。对过渡金属而言，4s 电子先于 3d 电子被移除。

Fe²⁺: [Ar] 3d⁶

Fe³⁺: [Ar] 3d⁵

铁离子 Fe²⁺：[Ar] 3d⁶；Fe³⁺：[Ar] 3d⁵

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11. Electron Configurations of the First 20 Elements | 前 20 号元素的电子排布

The first 20 elements show a clear pattern in their electron configurations:

前 20 号元素的电子排布呈现出清晰的规律：

Element | 元素	Atomic Number | 原子序数	Configuration | 电子排布	Shorthand | 简写
H | 氢	1	1s¹	1s¹
He | 氦	2	1s²	1s²
Li | 锂	3	1s² 2s¹	[He] 2s¹
C | 碳	6	1s² 2s² 2p²	[He] 2s² 2p²
Ne | 氖	10	1s² 2s² 2p⁶	[He] 2s² 2p⁶
Na | 钠	11	1s² 2s² 2p⁶ 3s¹	[Ne] 3s¹
K | 钾	19	1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹	[Ar] 4s¹
Ca | 钙	20	1s² 2s² 2p⁶ 3s² 3p⁶ 4s²	[Ar] 4s²

Notice how the fourth period begins with K and Ca filling the 4s sub-shell before the 3d sub-shell is used, starting from scandium (Z=21). This is because the 4s orbital has lower energy than the 3d orbital.

注意第四周期从钾和钙开始，先填充 4s 亚层，之后从钪（Z=21）开始才填充 3d 亚层。这是因为 4s 轨道能量低于 3d 轨道。

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12. Common Mistakes and Exam Tips | 常见错误与考试要点

Students often lose unnecessary marks on this topic due to a few recurring mistakes. Here are the most important points to remember:

学生常因几个反复出现的错误在本题上丢掉不必要的分数。以下是需要记住的最重要考点：

• Do not confuse the order of filling with the order of removing electrons. Although 4s fills before 3d, when forming transition metal ions, 4s electrons are removed first: Fe → Fe²⁺ removes the 4s electrons. | 不要混淆填充顺序与移除顺序。虽然 4s 先于 3d 填充，但形成过渡金属离子时，4s 电子最先被移除：Fe → Fe²⁺ 移除的是 4s 电子。

• Chromium and copper exceptions: Chromium is [Ar] 3d⁵ 4s¹ and copper is [Ar] 3d¹⁰ 4s¹. The half-filled and fully-filled d sub-shells are extra stable. | 铬和铜的例外：铬是 [Ar] 3d⁵ 4s¹，铜是 [Ar] 3d¹⁰ 4s¹。半充满和全充满的 d 亚层具有额外稳定性。

• Check your arithmetic when counting electrons. The sum of superscripts must equal the atomic number. | 检查电子数的计算——所有上标之和必须等于原子序数。

• When writing ion configurations for anions, the extra electrons go into the next available orbital following the Aufbau order. For O²⁻: 1s² 2s² 2p⁶. | 书写阴离子排布时，电子按照构造原理填入下一个可用轨道。O²⁻ 为 1s² 2s² 2p⁶。

• Understand successive ionisation energy graphs. Each sharp increase in ionisation energy indicates that an electron is being removed from a closer, inner shell. | 学会分析逐级电离能曲线——每次电离能的急剧跃升都代表从更内层移除了电子。

In the examination, always show the full working when calculating relative atomic mass, and write configurations using the correct superscripts. Be careful with the distinction between ‘shell’, ‘sub-shell’ and ‘orbital’ — these terms are defined precisely and are not interchangeable.

考试中，计算相对原子质量时要写完整步骤，书写电子排布要注意正确的上标。同时，务必区分“电子层（shell）”“亚层（sub-shell）”和“轨道（orbital）”这三个术语——它们的定义是精确的，不能互换使用。

Mastering atomic structure and electron configuration is not only important for its own sake, but is a prerequisite for understanding ionisation energy trends, bonding theories, and the chemistry of transition elements in later topics.

掌握原子结构与电子排布不仅本身重要，更是后续理解电离能趋势、化学键理论及过渡元素化学的必备基础。

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