📚 Looking Inside the Atom | 原子内部探秘
All matter is built from atoms, but the atom itself has an intricate internal structure. For CIE A-Level Physics, 'looking inside the atom' means understanding the nucleus, the particles that make it up, the forces holding it together, and the decays and reactions that release nuclear energy. This revision guide works through the whole topic in the order it is usually taught: evidence for the nuclear model, the nucleons and electrons, nuclides and isotopes, the strong nuclear force, nuclear size and density, mass defect and binding energy, radioactivity, exponential decay, fission and fusion, particle physics and finally exam technique.
所有物质都由原子构成，但原子本身具有复杂的内部结构。在 CIE A-Level 物理中，'原子内部探秘'意味着理解原子核、组成它的粒子、维持它稳定的作用力，以及释放核能的衰变和反应。本文按照通常的教学顺序完整梳理这一专题：核式模型的实验证据、核子与电子、核素与同位素、强核力、核的大小与密度、质量亏损与结合能、放射性、指数衰变、裂变与聚变、粒子物理，最后是考试技巧。
1. The Nuclear Model of the Atom | 原子的核式模型
Rutherford's alpha-particle scattering experiment showed that most of the atom is empty space, with almost all mass and all positive charge concentrated in a tiny central nucleus. Electrons orbit this nucleus at relatively large distances, held in their paths by the electrostatic (Coulomb) attraction between their negative charge and the positive charge of the nucleus.
卢瑟福的 α 粒子散射实验表明，原子内部绝大部分是空的，几乎所有质量和全部正电荷都集中在一个微小的中心原子核中。电子在离核较远的地方绕核运动，靠自身负电荷与核内正电荷之间的静电（库仑）引力维持轨道。
In the experiment, alpha particles were fired at an extremely thin gold foil. The vast majority passed straight through with little or no deflection, which is only possible if the atom is mostly empty space. A small number were deflected through large angles, and roughly 1 in 8000 was reflected back towards the source, which requires an extremely concentrated region of positive charge and mass - the nucleus.
实验中，α 粒子射向极薄的金箔。绝大多数 α 粒子几乎不偏转地直穿而过，这只有在原子内部大部分为空的情况下才可能发生。少数粒子发生大角度偏转，约八千分之一被反弹回放射源方向，这要求存在一个正电荷与质量高度集中的区域——原子核。
The nuclear diameter is about 10⁻¹⁵ m to 10⁻¹⁴ m, while the atomic diameter is about 10⁻¹⁰ m. This means the nucleus is about 10,000 to 100,000 times smaller than the atom. If the nucleus were scaled up to a marble of 1 cm diameter, the whole atom would be roughly 1 km across, which shows just how much of the atom is empty space.
原子核的直径约为 10⁻¹⁵ m 到 10⁻¹⁴ m，而原子直径约为 10⁻¹⁰ m。这意味着原子核比原子小约 1 万到 10 万倍。若把原子核放大到直径 1 cm 的弹珠，整个原子的直径大约有 1 km，可见原子内部有多么空旷。
2. Protons, Neutrons and Electrons | 质子、中子与电子
neutral, and an electron carries charge −e and orbits the nucleus. An atom is electrically neutral overall because the number of orbiting electrons equals the number of protons in the nucleus.
原子核包含质子和中子，统称为核子。质子带电荷 +e（e = 1.60 × 10⁻¹⁹ C），中子不带电。电子带电荷 −e，绕原子核运动。原子整体呈电中性，因为绕核运动的电子数等于核内质子数。
equal (about 1.67 × 10⁻²⁷ kg), whereas the electron mass is only about 1/1836 of a proton' in its nucleus, and why the mass number of an atom is effectively the number of nucleons.
质子和中子的质量几乎相等（约为 1.67 × 10⁻²⁷ kg），而电子质量仅为质子质量的约 1/1836。这正是原子几乎全部质量都集中在原子核内的原因，也是原子质量数实际上等于核子数的原因。
| Particle 粒子 | Charge 电荷 | Rest mass 静质量 / kg | Relative mass 相对质量 |
| Proton 质子 | +1.60 × 10⁻¹⁹ C | 1.673 × 10⁻²⁷ | 1.000 |
| Neutron 中子 | 0 | 1.675 × 10⁻²⁷ | 1.001 |
| Electron 电子 | −1.60 × 10⁻¹⁹ C | 9.11 × 10⁻³¹ | 1/1836 ≈ 0.00055 |
Specific charge is the ratio of charge to mass
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