Atomic Structure: Inside the Atom | 原子内部结构解析

📚 Atomic Structure: Inside the Atom | 原子内部结构解析

The atom is the fundamental building block of matter, yet its internal structure is surprisingly complex. Understanding how protons, neutrons and electrons are arranged — and how these arrangements give rise to the chemical behaviour of elements — is the bedrock of the entire A-Level Chemistry syllabus. This article breaks down the internal structure of the atom in a clear, exam-focused way.

原子是物质的基本组成单位，但它的内部结构却远比想象中复杂。理解质子、中子和电子如何排列，以及这些排列如何决定元素的化学性质，是整个A-Level化学考纲的基石。本文将以紧扣考点的方式，系统解析原子的内部结构。

1. The Subatomic Particles | 亚原子粒子

Every atom is composed of three key subatomic particles: protons, neutrons and electrons. Protons carry a relative charge of +1 and a relative mass of 1; neutrons carry no charge and also have a relative mass of 1; electrons carry a relative charge of −1 and a negligible relative mass of approximately 1/1840.

每个原子都由三种关键亚原子粒子组成：质子、中子和电子。质子带+1相对电荷，相对质量为1；中子不带电荷，相对质量也为1；电子带−1相对电荷，相对质量约为1/1840，可忽略不计。

Particle	Relative Charge	Relative Mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	1/1840	Electron shells

You must memorise these values precisely — examiners frequently test them in multiple-choice and short-answer questions. Note that the ‘relative’ scale is based on carbon-12, where one atom of carbon-12 is defined as exactly 12 atomic mass units.

这些数值必须精确记忆——考官经常在选择题和简答题中考查。请注意，”相对”标度以碳-12为基准，即一个碳-12原子的质量被精确地定义为12个原子质量单位。

2. Nuclear Structure: Protons and Neutrons | 核结构：质子与中子

The nucleus sits at the centre of the atom and contains virtually all of its mass. It is composed of protons and neutrons, collectively called nucleons. The nucleus is incredibly small — its diameter is roughly 10⁻¹⁵ m, whereas the atom as a whole has a diameter of about 10⁻¹⁰ m. This means the nucleus occupies only about 10⁻⁵ of the atom’s total volume, yet contains more than 99.9% of its mass.

原子核位于原子中心，几乎承载了原子的全部质量。它由质子和中子组成，统称为核子。原子核极小——直径约10⁻¹⁵ m，而整个原子的直径约10⁻¹⁰ m。这意味着原子核仅占原子总体积的约10⁻⁵，却包含了超过99.9%的质量。

The positive charge of the nucleus arises from the protons. The number of protons determines which element the atom belongs to; changing the number of neutrons creates different isotopes of the same element, while changing the number of electrons affects only the charge state of the atom (ionisation).

原子核的正电荷来源于质子。质子数决定了原子属于哪种元素；改变中子数会产生同一元素的不同同位素，而改变电子数只会影响原子的带电状态（电离）。

3. 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 uniquely identifies an element. The mass number (A) is the total number of protons plus neutrons in the nucleus.

原子序数（Z）是原子核中的质子数，它唯一地确定一种元素。质量数（A）是原子核中质子数与中子数之和。

A = Z + N

where A is the mass number, Z is the atomic number and N is the number of neutrons. For example, sodium has Z = 11 and A = 23, so it contains 11 protons, 11 electrons (in a neutral atom) and 23 − 11 = 12 neutrons.

其中A为质量数，Z为原子序数，N为中子数。例如，钠的Z = 11，A = 23，因此它含有11个质子、11个电子（中性原子状态下）和23 − 11 = 12个中子。

The standard notation for a nuclide is ^AZ X, where X is the chemical symbol. For instance, ²³Na means a sodium atom with mass number 23. The number of neutrons can be calculated using the formula N = A − Z.

核素的标淮表示法为^AZ X，其中X是元素符号。例如，²³Na表示质量数为23的钠原子。中子数可用公式N = A − Z计算。

4. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Consequently, isotopes of an element have identical atomic numbers but different mass numbers.

同位素是同一元素中具有相同质子数但不同中子数的原子。因此，同一元素的同位素具有相同的原子序数，但质量数不同。

For example, carbon exists naturally as three isotopes: ¹²C (98.9%), ¹³C (1.1%) and ¹⁴C (trace, radioactive). All three have 6 protons, but they contain 6, 7 and 8 neutrons respectively.

例如，碳天然存在三种同位素：¹²C（98.9%）、¹³C（1.1%）和¹⁴C（痕量，具有放射性）。三者都有6个质子，但分别含有6、7和8个中子。

Isotopes of the same element have identical chemical properties because chemical behaviour depends on the electron configuration, which is the same.
Isotopes may have different physical properties, such as density, melting point and diffusion rate, because these depend on mass.
Isotopes can be separated by physical methods such as gaseous diffusion or centrifugation, which exploit small mass differences.

同一元素的同位素具有完全相同的化学性质，因为化学行为取决于电子排布，而同位素的电子排布相同。
同位素可能具有不同的物理性质，如密度、熔点和扩散速率，因为这些性质取决于质量。
同位素可通过气体扩散或离心等物理方法分离，这些方法利用微小的质量差异。

5. Relative Atomic Mass and Relative Isotopic Mass | 相对原子质量与相对同位素质量

The relative isotopic mass is the mass of one atom of a particular isotope relative to 1/12 of the mass of one atom of carbon-12. The relative atomic mass (Aᵣ) is the weighted mean mass of an atom of an element relative to 1/12 of the mass of an atom of carbon-12, taking into account the natural abundances of all isotopes.

相对同位素质量是某一特定同位素的一个原子的质量相对于碳-12原子质量的1/12的比值。相对原子质量（Aᵣ）是元素的一个原子的加权平均质量相对于碳-12原子质量的1/12的比值，需考虑所有同位素的自然丰度。

Aᵣ = Σ (isotopic mass × fractional abundance)

For example, chlorine consists of ³⁵Cl (75%) and ³⁷Cl (25%). Its relative atomic mass is calculated as:

例如，氯由³⁵Cl（75%）和³⁷Cl（25%）组成。其相对原子质量计算如下：

Aᵣ(Cl) = (35 × 0.75) + (37 × 0.25) = 26.25 + 9.25 = 35.5

This explains why chlorine’s relative atomic mass appears as 35.5 on the periodic table rather than a whole number.

这解释了为什么氯在元素周期表上的相对原子质量显示为35.5而非整数。

6. Mass Spectrometry: Evidence for Isotopes | 质谱法：同位素存在的证据

The mass spectrometer is a powerful analytical instrument that measures the mass-to-charge ratio (m/z) of ions. It provides direct experimental evidence for the existence of isotopes and enables the precise determination of relative atomic mass.

质谱仪是一种强大的分析仪器，用于测量离子的质荷比（m/z）。它为同位素的存在提供了直接实验证据，并能精确测定相对原子质量。

The key stages of mass spectrometry are:

质谱法的主要阶段如下：

Ionisation: The sample is vaporised and bombarded with high-energy electrons, knocking off electrons to form positive ions: X(g) + e⁻ → X⁺(g) + 2e⁻
Acceleration: The positive ions are accelerated by an electric field to give them the same kinetic energy.
Deflection: Ions are deflected by a magnetic field. The amount of deflection depends on their mass-to-charge ratio — lighter ions are deflected more than heavier ions; ions with higher charge are deflected more than those with lower charge.
Detection: Ions strike a detector, generating a current proportional to the abundance of each ion. The results are plotted as a mass spectrum.

电离：样品被气化并用高能电子轰击，轰掉电子形成正离子：X(g) + e⁻ → X⁺(g) + 2e⁻
加速：正离子在电场中被加速，获得相同的动能。
偏转：离子在磁场中发生偏转。偏转程度取决于质荷比——较轻的离子比重的离子偏转更多；电荷较高的离子比电荷较低的离子偏转更多。
检测：离子撞击检测器，产生与各离子丰度成正比的电流。结果以质谱图形式呈现。

In the mass spectrum of chlorine, two peaks appear at m/z = 35 and m/z = 37, with heights in the ratio 3:1. This confirms the presence of two isotopes and allows Aᵣ to be calculated from the peak intensities.

在氯的质谱图中，m/z = 35和m/z = 37处出现两个峰，峰高比为3:1。这证实了两种同位素的存在，并可通过峰强度计算Aᵣ。

7. Electronic Structure: Shells and Sub-shells | 电子结构：电子层与亚层

Electrons occupy regions of space around the nucleus called energy levels or electron shells. Each shell is labelled by a principal quantum number n (n = 1, 2, 3, 4…), corresponding to shells K, L, M, N. Electrons in shells closer to the nucleus have lower energy.

电子占据原子核周围的空间区域，称为能级或电子层。每个电子层用主量子数n标记（n = 1, 2, 3, 4…），对应K、L、M、N层。越靠近原子核的电子层能量越低。

Each shell is further divided into sub-shells. The number of sub-shells in a shell equals the value of n. The sub-shells are designated s, p, d and f:

每个电子层进一步分为亚层。一个电子层中亚层的数目等于n的值。亚层分别标记为s、p、d和f：

Principal quantum number n	Sub-shells present	Maximum number of electrons
1	1s	2
2	2s, 2p	8
3	3s, 3p, 3d	18
4	4s, 4p, 4d, 4f	32

The maximum number of electrons in a shell is given by 2n². An s sub-shell holds 2 electrons, a p sub-shell holds 6, a d sub-shell holds 10 and an f sub-shell holds 14.

一个电子层的最大电子数由2n²给出。s亚层可容纳2个电子，p亚层可容纳6个，d亚层可容纳10个，f亚层可容纳14个。

8. Atomic Orbitals | 原子轨道

An atomic orbital is a region of space within an atom where the probability of finding an electron is highest (about 90%). Each orbital can hold a maximum of two electrons with opposite spins. The shapes and orientations of orbitals are characteristic of their type.

原子轨道是原子内部找到电子概率最高（约90%）的空间区域。每个轨道最多容纳两个自旋相反的电子。轨道的形状和取向由其类型决定。

An s orbital is spherical in shape. There is one s orbital per energy level. The 1s orbital has the lowest energy; 2s is higher than 1s but lower than 2p.
A p orbital is dumbbell-shaped, consisting of two lobes. There are three p orbitals in each p sub-shell (pₓ, pᵧ, p_z), oriented along the x, y and z axes respectively.
d orbitals have more complex shapes: four of them are clover-leaf shaped and the fifth has a distinctive shape with a ring. There are five d orbitals per d sub-shell.
f orbitals are even more complex; there are seven f orbitals per f sub-shell, but these are not required in detail at A-Level.

s轨道呈球形。每个能级有一个s轨道。1s轨道能量最低；2s高于1s但低于2p。
p轨道呈哑铃形，由两瓣组成。每个p亚层中有三个p轨道（pₓ、pᵧ、p_z），分别沿x、y、z轴取向。
d轨道形状更复杂：其中四个呈四叶草形，第五个具有带环的特殊形状。每个d亚层有五个d轨道。
f轨道更加复杂；每个f亚层有七个f轨道，但A-Level阶段不要求掌握细节。

9. Electron Configuration: Filling Order | 电子排布：填充顺序

The electron configuration of an atom describes how electrons are distributed among the atomic orbitals. Electrons fill orbitals according to three fundamental rules:

原子的电子排布描述了电子在原子轨道中的分布方式。电子按照三条基本规则填充轨道：

1. Aufbau Principle: Electrons fill the lowest energy orbitals first before occupying higher energy orbitals.

1. 构造原理：电子首先填入最低能量的轨道，然后才占据更高能量的轨道。

2. Pauli Exclusion Principle: Each orbital can hold a maximum of two electrons, and these two electrons must have opposite spins.

2. 泡利不相容原理：每个轨道最多容纳两个电子，且这两个电子的自旋方向必须相反。

3. Hund’s Rule: When electrons occupy orbitals of the same energy (degenerate orbitals), they fill each orbital singly with parallel spins before pairing up.

3. 洪德规则：当电子占据相同能量（简并轨道）的轨道时，它们先以平行自旋的方式单独填入每个轨道，然后才配对。

The filling order of orbitals follows the sequence: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p…

轨道的填充顺序为：1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p…

Note the important anomaly: the 4s orbital is lower in energy than the 3d orbital, so 4s fills first. However, when transition metal atoms form cations, electrons are removed from the 4s orbital before the 3d orbital. For example, iron (Fe, Z = 26) has the configuration [Ar] 3d⁶4s², but the Fe²⁺ ion is [Ar] 3d⁶.

注意一个重要的特殊情况：4s轨道的能量低于3d轨道，因此4s先填充。但是，当过渡金属原子形成阳离子时，电子先从4s轨道移除，然后才是3d轨道。例如，铁（Fe，Z = 26）的排布为[Ar] 3d⁶4s²，但Fe²⁺离子为[Ar] 3d⁶。

10. Writing Electron Configurations | 书写电子排布

Electron configurations can be written in two common forms: full notation and the abbreviated (noble gas core) notation. The full notation lists every occupied orbital, while the abbreviated form uses the preceding noble gas as a shorthand.

电子排布有两种常见书写形式：完整写法和简写（稀有气体核心）写法。完整写法列出每个被占据的轨道，简写形式则用前一个稀有气体作简写。

Examples:

示例：

Sodium (Z = 11): Full: 1s²2s²2p⁶3s¹; Abbreviated: [Ne] 3s¹
Chlorine (Z = 17): Full: 1s²2s²2p⁶3s²3p⁵; Abbreviated: [Ne] 3s²3p⁵
Potassium (Z = 19): Full: 1s²2s²2p⁶3s²3p⁶4s¹; Abbreviated: [Ar] 4s¹
Chromium (Z = 24): [Ar] 3d⁵4s¹ (exception — half-filled d sub-shell is more stable than 3d⁴4s²)
Copper (Z = 29): [Ar] 3d¹⁰4s¹ (exception — fully filled d sub-shell is more stable than 3d⁹4s²)

钠（Z = 11）：完整：1s²2s²2p⁶3s¹；简写：[Ne] 3s¹
氯（Z = 17）：完整：1s²2s²2p⁶3s²3p⁵；简写：[Ne] 3s²3p⁵
钾（Z = 19）：完整：1s²2s²2p⁶3s²3p⁶4s¹；简写：[Ar] 4s¹
铬（Z = 24）：[Ar] 3d⁵4s¹（例外——半充满d亚层比3d⁴4s²更稳定）
铜（Z = 29）：[Ar] 3d¹⁰4s¹（例外——全充满d亚层比3d⁹4s²更稳定）

Two notable exceptions are chromium and copper. In these cases, a half-filled or fully filled d sub-shell confers extra stability, and one electron is promoted from the 4s orbital to achieve this configuration. You must memorise these exceptions as they are frequently examined.

两个著名的例外是铬和铜。在这两种情况下，半充满或全充满的d亚层赋予额外稳定性，因此一个电子从4s轨道被激发以达成该排布。这两个例外必须牢记，因为它们是高频考点。

11. Ionisation Energy: Evidence for Shells | 电离能：电子层存在的证据

First ionisation energy is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous unipositive ions. The equation for the first ionisation energy of sodium is:

第一电离能的定义是：从一摩尔气态原子中移走一摩尔电子，形成一摩尔气态一价正离子所需的能量。钠的第一电离能方程式为：

Na(g) → Na⁺(g) + e⁻

The successive ionisation energies of an element provide powerful evidence for the existence of electron shells. As electrons are removed one by one, the ionisation energy generally increases because the remaining electrons are held more tightly by the increasing nuclear charge. However, a dramatic jump occurs when an electron is removed from a shell closer to the nucleus — this indicates that a complete shell has been removed.

元素的逐级电离能为电子层的存在提供了有力证据。随着电子逐个被移走，电离能总体上不断增加，因为剩余电子被增强的核电荷束缚得更紧。然而，当从一个更靠近原子核的电子层中移走电子时，会出现一个急剧跳跃——这表明一个完整的电子层已被移除。

For example, the ionisation energies of sodium (Z = 11) show a small increase from IE₁ to IE₁₀, but IE₁₁ is drastically larger — roughly ten times greater than IE₁₀. This confirms that sodium has 11 electrons arranged in three shells: the first two shells are fully occupied, and the third shell contains just one electron.

例如，钠（Z = 11）的电离能从IE₁到IE₁₀增长缓慢，但IE₁₁急剧增大——大约是IE₁₀的十倍。这证实了钠的11个电子排列在三个电子层中：前两层完全占满，第三层只有一个电子。

12. Ionisation Energy Trends in the Periodic Table | 周期表中电离能的变化趋势

Understanding ionisation energy trends is essential for explaining periodic behaviour. Two major trends are examined in detail at A-Level:

理解电离能的变化趋势对于解释周期律至关重要。A-Level阶段详细考查两大趋势：

Down a group: First ionisation energy decreases. As the atomic radius increases, the outer electron is further from the nucleus and is more strongly shielded by inner electrons. Therefore, the attractive force on the outer electron decreases, making it easier to remove.

同族自上而下：第一电离能减小。随着原子半径增大，外层电子离核更远，受到内层电子的屏蔽效应更强。因此，外层电子受到的吸引力减小，更容易被移走。

Across a period: First ionisation energy generally increases. The nuclear charge increases while the shielding effect stays approximately constant (electrons are added to the same shell). Hence the outer electrons experience a stronger attraction, and more energy is required to remove them.

同周期自左向右：第一电离能总体增大。核电荷增加而屏蔽效应大致不变（电子加到同一电子层中）。因此外层电子受到更强的吸引力，移走它们需要更多能量。

Two subtle deviations from this trend must be noted:

该趋势有两个细微偏差需要特别注意：

Group 2 to Group 3 (e.g., Be to B): IE decreases slightly. The outer electron in Group 3 enters a p orbital, which is slightly higher in energy than the s orbital, making it easier to remove.
Group 5 to Group 6 (e.g., N to O): IE decreases slightly. In Group 5, the three 2p electrons each occupy separate orbitals. In Group 6, the fourth 2p electron must pair up with an electron in an already-occupied orbital, and the electron-electron repulsion facilitates its removal.

第2族到第3族（如Be到B）：IE略有下降。第3族的外层电子进入p轨道，p轨道能量略高于s轨道，因此更容易移走。
第5族到第6族（如N到O）：IE略有下降。第5族的三个2p电子各占一个独立轨道。第6族中，第四个2p电子必须与已占据轨道中的电子配成对，电子-电子排斥作用使它更容易被移走。

In summary, the internal structure of the atom is defined by the arrangement of protons, neutrons and electrons. The nucleus contains the protons and neutrons, while electrons occupy quantised energy levels around it. Mastery of subatomic particle properties, isotope calculations, mass spectrometry, orbital theory and ionisation energy trends is essential for success in the CIE A-Level Chemistry examination. These concepts form the foundation for every subsequent topic in the syllabus.

总而言之，原子的内部结构由质子、中子和电子的排列决定。原子核包含质子和中子，而电子占据原子核周围量子化的能级。熟练掌握亚原子粒子性质、同位素计算、质谱法、轨道理论和电离能趋势，是CIE A-Level化学考试取得成功的必要条件。这些概念构成了考纲中所有后续专题的基础。

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Atomic Structure: Inside the Atom | 原子内部结构解析