📚 Exploring the Internal Structure and Composition of Atoms | 探索原子内部结构与组成
The atom is the fundamental building block of all matter, yet its internal structure is remarkably intricate. In this article, we will explore the historical development of atomic theory, the properties of subatomic particles, and the way in which protons, neutrons and electrons are arranged within the atom. This topic is essential for CIE A-Level Physics, as it forms the foundation for nuclear physics, radioactivity and quantum mechanics.
原子是构成一切物质的基本单元，然而其内部结构却异常精巧。在本文中，我们将探索原子理论的历史发展、亚原子粒子的性质，以及质子、中子和电子在原子内部的排列方式。该主题是 CIE A-Level 物理的重要组成部分，为核物理、放射性和量子力学奠定基础。
1. The Historical Emergence of the Atomic Model | 原子模型的历史演进
The idea that matter is composed of indivisible particles dates back to ancient Greek philosophers such as Democritus. However, it was not until the 19th and early 20th centuries that scientists developed a credible model of the atom based on experimental evidence. John Dalton proposed that atoms were solid, indivisible spheres, while J.J. Thomson’s discovery of the electron in 1897 proved that atoms themselves contain smaller charged particles.
物质由不可分割的粒子组成的想法可以追溯到古希腊哲学家，如德谟克利特。然而，直到19世纪末和20世纪初，科学家们才基于实验证据建立了可信的原子模型。约翰·道尔顿提出原子是实心的、不可分割的球体，而 J.J. 汤姆孙在1897年发现电子，证明了原子本身含有更小的带电粒子。
Ernest Rutherford’s famous gold-foil experiment in 1911 revealed that most of the atom is empty space, with a tiny, dense, positively charged nucleus at the centre. Later, James Chadwick discovered the neutron in 1932, completing the picture of the nucleus as containing both protons and neutrons. The modern model, refined by Niels Bohr and subsequent quantum physicists, describes electrons as occupying quantised energy levels around the nucleus.
欧内斯特·卢瑟福在1911年著名的金箔实验揭示了原子的大部分区域是空的空间，在中心存在一个微小、致密、带正电的原子核。后来，詹姆斯·查德威克于1932年发现了中子，完善了原子核包含质子和中子的图景。由尼尔斯·玻尔及后来的量子物理学家完善的现代模型，将电子描述为占据原子核周围量子化的能级。
2. The Subatomic Particles and Their Properties | 亚原子粒子及其性质
The atom is composed of three primary subatomic particles: protons, neutrons and electrons. Each has distinct physical properties that determine the behaviour of the atom. In CIE A-Level Physics, you are expected to recall the charge and mass of each particle and understand their roles within the atom.
原子由三种主要的亚原子粒子组成：质子、中子和电子。每一种都具有不同的物理性质，这些性质决定了原子的行为。在 CIE A-Level 物理中，你应当记住每种粒子的电荷与质量，并理解它们在原子中的作用。
| Particle | Relative Charge | Relative Mass | Location in Atom |
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1836 (negligible) | Electron shells/orbitals |
In SI units, the mass of a proton is approximately 1.673 × 10⁻²⁷ kg, and the charge on an electron is -1.602 × 10⁻¹⁹ C. In A-Level calculations, we often use relative charges and relative masses for simplicity, where the proton and neutron each have a relative mass of 1 atomic mass unit (u).
在国际单位制中，质子的质量约为 1.673 × 10⁻²⁷ kg，电子的电荷为 -1.602 × 10⁻¹⁹ C。在 A-Level 计算中，为简便起见，我们常使用相对电荷和相对质量，其中质子和中子的相对质量均为 1 原子质量单位（u）。
3. Atomic Number and Mass Number | 原子序数与质量数
The identity of an element is determined by the number of protons in the nucleus, called the atomic number (Z). The total number of protons and neutrons in the nucleus is called the mass number (A), also known as the nucleon number. In a neutral atom, the number of electrons equals the number of protons.
元素的身份由原子核中的质子数决定，该数目称为原子序数（Z）。原子核中质子与中子的总数称为质量数（A），也称为核子数。在中性原子中，电子数等于质子数。
Z = number of protons = number of electrons (in a neutral atom)
A = number of protons + number of neutrons
For example, the nuclide notation for carbon is written as ¹²₆C. Here, the lower number 6 is the atomic number, and the upper number 12 is the mass number. The number of neutrons is therefore 12 − 6 = 6. This notation is widely used in nuclear equations and radioactivity questions.
例如，碳的核素符号写作 ¹²₆C。其中下方的数字 6 是原子序数，上方的数字 12 是质量数。因此，中子数目为 12 − 6 = 6。该符号广泛应用于核反应方程和放射性相关问题中。
4. Isotopes and Their Significance | 同位素及其意义
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers. For example, carbon-12 (¹²₆C) and carbon-14 (¹⁴₆C) are both isotopes of carbon; they have 6 protons but 6 and 8 neutrons respectively.
同位素是同一元素中质子数相同但中子数不同的原子。这意味着它们具有相同的原子序数但不同的质量数。例如，碳-12（¹²₆C）和碳-14（¹⁴₆C）都是碳的同位素；它们都有 6 个质子，但分别有 6 个和 8 个中子。
Isotopes of an element exhibit identical chemical behaviour because chemical properties are determined primarily by the electron configuration. However, physical properties — such as mass and density — may differ. Some isotopes are unstable and undergo radioactive decay, which has important applications in medicine, archaeology (carbon dating) and nuclear power generation.
同一元素的同位素表现出相同的化学行为，因为化学性质主要由电子排布决定。然而，物理性质（如质量和密度）可能有所不同。一些同位素不稳定并会发生放射性衰变，这在医学、考古学（碳定年法）和核能发电中具有重要应用。
5. The Nucleus and the Strong Nuclear Force | 原子核与强核力
The nucleus contains protons and neutrons, collectively called nucleons. Since protons are positively charged, they experience a strong electrostatic repulsion from one another. If this were the only force acting, the nucleus would fly apart. The fact that the nucleus remains stable suggests the existence of an attractive force that overcomes this repulsion — this is the strong nuclear force.
原子核包含质子和中子，统称为核子。由于质子带正电荷，它们彼此之间存在强烈的静电排斥力。如果这是唯一的作用力，原子核将会飞散。原子核保持稳定的事实表明存在一种克服这种排斥力的吸引力——这就是强核力。
The strong nuclear force is one of the four fundamental forces of nature. It is a short-range force, acting only over distances of approximately 1–3 femtometres (1 fm = 10⁻¹⁵ m). Within this range, the strong nuclear force is much stronger than the electrostatic repulsion between protons, binding the nucleus tightly together. Beyond this range, the force falls off extremely rapidly.
强核力是自然界四种基本力之一。它是一种短程力，仅在约 1–3 飞米（1 fm = 10⁻¹⁵ m）的距离内起作用。在此范围内，强核力远强于质子间的静电斥力，将原子核紧密结合在一起。超出此范围后，该力迅速衰减至几乎为零。
Electrostatic repulsion (repulsive, long range) vs Strong nuclear force (attractive, very short range)
6. Electron Shells and Energy Levels | 电子壳层与能级
Electrons do not orbit the nucleus in random paths; they occupy discrete energy levels or shells. Each shell can hold a maximum number of electrons given by 2n², where n is the principal quantum number. The first shell (n = 1) holds up to 2 electrons, the second (n = 2) holds up to 8, and the third (n = 3) holds up to 18, although in many introductory models only the first 8 are considered for the third shell.
电子并非沿随机路径绕核旋转；它们占据离散的能级或壳层。每个壳层所能容纳的最大电子数目由 2n² 给出，其中 n 为主量子数。第一壳层（n = 1）最多容纳 2 个电子，第二壳层（n = 2）最多容纳 8 个，第三壳层（n = 3）最多容纳 18 个，尽管在许多入门模型中只考虑第三壳层的前 8 个电子。
When an electron absorbs energy, it jumps from a lower energy level to a higher one, entering an excited state. When it falls back to a lower level, it emits energy in the form of electromagnetic radiation, often visible light. This principle is the basis for understanding atomic emission spectra, which is another important topic in A-Level physics.
当电子吸收能量时，它会从较低能级跃迁到较高能级，进入激发态。当它回落到较低能级时，会以电磁辐射（通常为可见光）的形式释放能量。这一原理是理解原子发射光谱的基础，也是 A-Level 物理中的另一个重要主题。
7. The Bohr Model and its Limitations | 玻尔模型及其局限性
Niels Bohr proposed a model of the hydrogen atom in 1913, in which electrons orbit the nucleus in fixed circular paths without emitting radiation. According to Bohr, angular momentum is quantised, so only certain orbits are allowed. The energy of each orbit is fixed, and radiation is emitted or absorbed only when an electron transitions between orbits.
尼尔斯·玻尔于1913年提出了氢原子模型，在该模型中，电子沿固定的圆形轨道绕核运动而不辐射能量。根据玻尔的观点，角动量是量子化的，因此只有某些轨道是允许的。每个轨道的能量是固定的，只有当电子在轨道之间跃迁时才会发射或吸收辐射。
While the Bohr model successfully explains the hydrogen spectrum, it fails for multi-electron atoms. It also violates the Heisenberg uncertainty principle, which states that we cannot simultaneously know both the exact position and exact momentum of an electron. Modern quantum mechanics replaces the idea of precise orbits with probability distributions called orbitals, which describe where an electron is likely to be found.
尽管玻尔模型成功解释了氢原子光谱，但它无法解释多电子原子。它还违背了海森堡不确定性原理，该原理指出我们无法同时精确知道电子的位置和动量。现代量子力学用称为轨道的概率分布取代了精确轨道的概念，这些轨道描述电子可能被发现的区域。
8. Nucleons and Nuclide Notation | 核子与核素符号
A nucleon is any particle found in the nucleus — either a proton or a neutron. The term “nuclide” refers to a specific nucleus characterised by its number of protons and neutrons. The standard nuclide notation places the mass number as a superscript and the atomic number as a subscript, both to the left of the element symbol.
核子是原子核中任何粒子的统称——无论是质子还是中子。“核素”一词指的是由质子和中子数目具体表征的某个核。标准核素符号将质量数作为上标、原子序数作为下标，均置于元素符号的左侧。
^(A)_(Z)X
Here, X is the chemical symbol of the element, A is the mass number and Z is the atomic number. For example, ^{235}_{92}U represents a uranium nucleus with 92 protons and 143 neutrons (235 − 92 = 143). This notation is indispensable when writing nuclear equations and balancing alpha or beta decay reactions.
其中 X 是元素的化学符号，A 是质量数，Z 是原子序数。例如，^{235}_{92}U 表示一个含有 92 个质子和 143 个中子（235 − 92 = 143）的铀核。在书写核反应方程以及配平 α 衰变或 β 衰变反应时，这种符号是必不可少的。
9. Protons and Neutrons: Similarities and Differences | 质子与中子的异同
Protons and neutrons are both nucleons and both have approximately the same mass, roughly 1 atomic mass unit. However, they differ in charge: the proton carries a positive charge of +1, while the neutron is electrically neutral. Because neutrons have no charge, they can approach the nucleus more easily and are therefore highly effective projectiles in nuclear fission reactions.
质子和中子都是核子，二者质量大致相同，均约为 1 原子质量单位。然而它们的电荷不同：质子带 +1 的正电荷，而中子呈电中性。由于中子不带电荷，它们更容易接近原子核，因此在核裂变反应中是极为有效的轰击粒子。
Within the nucleus, the ratio of neutrons to protons affects stability. Light stable nuclei tend to have approximately equal numbers of protons and neutrons, whereas heavy stable nuclei contain more neutrons than protons to help dilute the electrostatic repulsion among protons.
在原子核内部，中子与质子的比例影响核的稳定性。轻的稳定核倾向于质子数与中子数大致相等，而重的稳定核含有比质子更多的中子，以帮助稀释质子之间的静电排斥力。
10. The Electron: Charge, Mass and Role | 电子：电荷、质量与作用
The electron is a negatively charged particle with a charge of -1.602 × 10⁻¹⁹ C and a rest mass of 9.11 × 10⁻³¹ kg, which is approximately 1/1836 that of a proton. Because its mass is so small, the electron contributes negligibly to the total mass of the atom, but it determines the atom’s chemical reactivity and its interactions with electromagnetic fields.
电子是一种带负电荷的粒子，电荷为 -1.602 × 10⁻¹⁹ C，静质量为 9.11 × 10⁻³¹ kg，约为质子质量的 1/1836。由于质量极小，电子对原子总质量的贡献可以忽略不计，但它决定了原子的化学反应活性及其与电磁场的相互作用。
In a neutral atom, the number of electrons equals the number of protons. If an atom gains or loses electrons, it becomes a charged ion — either negative (anion) or positive (cation). Ions play a crucial role in electric conduction in gases and solutions, as well as in many physical phenomena studied in A-Level physics.
在中性原子中，电子数等于质子数。如果原子获得或失去电子，它便成为带电离子——负离子（阴离子）或正离子（阳离子）。离子在气体和溶液中的导电以及 A-Level 物理中研究的许多物理现象中起着至关重要的作用。
11. Atomic Structure and the Periodic Table | 原子结构与元素周期表
The periodic table organises elements by increasing atomic number and arranges them in periods and groups. Elements in the same group have the same number of electrons in their outermost shell, which is why they exhibit similar chemical properties. For example, all elements in Group 1 have one outer electron, such as sodium and potassium.
元素周期表按原子序数递增排列元素，并将其分为周期和族。同一族的元素在最外层壳层具有相同数目的电子，因此它们表现出相似的化学性质。例如，第 1 族的所有元素都有一个外层电子，如钠和钾。
Atomic radius generally decreases across a period due to increasing nuclear charge pulling electrons closer, while it increases down a group as additional electron shells are added. Ionisation energy — the energy required to remove an electron from a gaseous atom — follows similar trends and is closely linked to atomic structure.
原子半径在同一周期内通常减小，因为核电荷增加而将电子拉得更近；而在同一族中随壳层增加而增大。电离能——从气态原子中移除一个电子所需的能量——遵循类似的趋势，并且与原子结构密切相关。
12. Experimental Evidence for the Nuclear Model | 核模型的实验证据
Rutherford’s alpha-particle scattering experiment remains the most compelling evidence for the nuclear model of the atom. A beam of alpha particles was directed at a thin gold foil, and the angles at which the particles were deflected were observed using a fluorescent screen. Most alpha particles passed straight through, a small number were deflected through large angles, and a very few (about 1 in 8000) rebounded almost directly backwards.
卢瑟福的α粒子散射实验仍然是支持原子核模型的最有力证据。将一束α粒子射向薄金箔，并用荧光屏观测粒子被偏转的角度。大多数α粒子直接穿过，少数被大角度偏转，而极少数（约 1/8000）几乎直接反弹回来。
These observations led to the following conclusions: atoms are mostly empty space; the positive charge of an atom is concentrated in a tiny, massive central nucleus; and the nucleus is surrounded by electrons. This experiment was a turning point in atomic physics and replaced Thomson’s “plum pudding” model.
这些观察结果得出以下结论：原子大部分是空的；原子的正电荷集中在微小而致密的中心原子核上；原子核周围环绕着电子。该实验是原子物理的转折点，取代了汤姆孙的“葡萄干布丁”模型。
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