📚 Nucleons and Electrons | 核子与电子
At the heart of all matter lie atoms, which are themselves composed of subatomic particles. The two classes of particles that define the structure and behaviour of atoms are nucleons (protons and neutrons) residing in the nucleus, and electrons orbiting the nucleus. Understanding their properties, interactions and the forces at play is fundamental to A-Level Physics.
所有物质的核心是原子，原子本身又由亚原子粒子组成。定义原子结构和行为的两类粒子是位于原子核中的核子（质子和中子），以及绕核运动的电子。理解它们的性质、相互作用以及涉及的力是A-Level物理的基础。
1. The Architecture of the Atom | 原子结构
An atom consists of a tiny, dense, positively charged nucleus surrounded by a cloud of electrons. The nucleus contains almost all the mass of the atom, while the electron cloud determines the atom's chemical properties and occupies most of its volume.
原子由一个微小、致密、带正电的原子核及环绕其周围的电子云组成。原子核几乎包含了原子的全部质量，而电子云决定了原子的化学性质并占据了原子的大部分体积。
The radius of a typical nucleus is about 10⁻¹⁴ m, whereas atomic radii are on the order of 10⁻¹⁰ m. This means the atom is mostly empty space; if the nucleus were the size of a marble, the atom would span several football pitches.
一个典型原子核的半径约为10⁻¹⁴米，而原子半径约为10⁻¹⁰米。这意味着原子绝大部分是空的；假如原子核像一颗弹珠那么大，原子将横跨几个足球场。
2. Nucleons: Protons and Neutrons | 核子：质子与中子
imately 1.673×10⁻²⁷ kg. The neutron is electrically neutral and has a mass slightly larger than that of the proton, about 1.675×10⁻²⁷ kg.
核子是构成原子核的粒子。质子带有一个单位的正电荷 +e（约1.60×10⁻¹⁹库仑），静止质量约为1.673×10⁻²⁷千克。中子呈电中性，质量比质子略大，约为1.675×10⁻²⁷千克。
Both protons and neutrons are not elementary; they are composed of smaller particles called quarks. A proton consists of two up quarks and one down quark (uud), whereas a neutron is made of one up quark and two down quarks (udd).
质子和中子都不是基本粒子；它们由更小的粒子——夸克组成。一个质子由两个上夸克和一个下夸克（uud）构成，而一个中子由一个上夸克和两个下夸克（udd）构成。
3. Nuclide Notation and Isotopes | 核素符号与同位素
for carbon-12 and ²³⁸₉₂U for uranium-238. The number of neutrons N is given by N = A − Z.
核素书写时，质量数A标在左上角，原子序数Z标在左下角，例如碳-12写作¹²₆C，铀-238写作²³⁸₉₂U。中子数 N = A − Z。
Isotopes are atoms of the same element (same Z) but with different numbers of neutrons (different A). For example, ¹²₆C and ¹⁴₆C are isotopes of carbon; both have 6 protons but 6 and 8 neutrons respectively.
同位素是同一元素（Z相同）但中子数不同（A不同）的原子。例如¹²₆C和¹⁴₆C是碳的同位素；两者都有6个质子，但分别有6个和8个中子。
4. The Strong Nuclear Force | 强核力
The nucleus contains protons that repel each other via the electromagnetic force. To remain bound, nucleons must experience an attractive force that overcomes this repulsion. This is the strong nuclear force, which acts between all nucleons (proton-proton, neutron-neutron, proton-neutron).
原子核内的质子之间会通过电磁力相互排斥。为了保持束缚，核子必须受到一种克服这种排斥的吸引力。这就是强核力，它作用于所有核子之间（质子-质子、中子-中子、质子-中子）。
ndependent but has a repulsive core at extremely short distances (below about 0.5 fm) to prevent nucleons from overlapping. Above 0.5 fm and out to roughly 3 fm it is strongly attractive, and beyond that it falls rapidly to zero.
强核力的作用范围非常短，仅在几飞米（约10⁻¹⁵米）内有效。它与电荷无关，但在极短距离处（约0.5 fm以下）有一个排斥核心以防止核子重叠。在0.5 fm以上到大约3 fm，它具有很强的吸引力，超出此范围，力迅速降为零。
5. Nuclear Stability and the N/Z Ratio | 核稳定性与中子-质子比
heavier nuclei, the number of neutrons must exceed the number of protons to counteract the increasing Coulomb repulsion, leading to N/Z ratios around 1.5 for the heaviest stable nuclei such as lead-208.
稳定的核素在核素图中占据一个区域。对于轻核（A < 20），当中子数N约等于质子数Z时达到稳定。对于较重的核，中子数必须超过质子数以抵消逐渐增大的库仑斥力，导致最重的稳定核（如铅-208）的N/Z比约为1.5。
dergo decay to move closer to stability. This can involve beta-minus decay (n → p + e⁻ + ν̄ re too many neutrons, or beta-plus decay/electron capture when there are too many protons.
位于稳定带之外的核素具有放射性，会发生衰变以接近稳定。当中子过多时，可能发生β⁻衰变（n → p + e⁻ + ν̄ₑ）；当质子过多时，可能发生β⁺衰变或电子俘获。
6. Mass Defect and Binding Energy | 质量亏损与结合能
individual nucleons. This difference is called the mass defect, Δm. According to Einstein's mass– gy equivalent of the mass defect is the binding energy of the nucleus, given by E = Δm c².
原子核的质量总是小于其各个核子质量之和。这个差值称为质量亏损Δm。根据爱因斯坦的质能等价关系，质量亏损对应的能量就是原子核的结合能，由E = Δm c²给出。
a more stable nucleus. For example, a helium-4 nucleus (⁴₂He) has a mass of 4.002603 u, while two protons and two neutrons have a total mass of 2×1.007276 u + 2×1.008665 u = 4.031882 u. The mass defect is 0.029279 u, corresponding to about 27.3 MeV.
结合能表示将一个原子核分离成它的组成质子和中子所需的能量。结合能越高，原子核越稳定。例如，一个氦-4核（⁴₂He）的质量为4.002603 u，而两个质子和两个中子的总质量为2×1.007276 u + 2×1.008665 u = 4.031882 u。质量亏损为0.029279 u，对应的结合能约为27.3 MeV。
7. Binding Energy per Nucleon Curve | 每个核子的结合能曲线
Dividing the total binding energy by the number of nucleons gives the binding energy per nucleon. A graph of average binding energy per nucleon against mass number A shows a peak around iron-56 (⁵⁶₂₆Fe), indicating the most tightly bound nuclei.
将总结合能除以核子数得到每个核子的结合能。每个核子的平均结合能随质量数A变化的曲线在铁-56（⁵⁶₂₆Fe）附近出现峰值，表明这些核结合得最紧密。
Light nuclei can release energy by fusion (moving towards higher binding energy per nucleon), while heavy nuclei can release energy by fission (also moving towards the peak). This is the basis of nuclear energy and explains why both fusion in stars and fission in reactors release energy.
轻核可以通过聚变释放能量（向每个核子结合能更高的方向移动），而重核可以通过裂变释放能量（同样向峰值方向移动）。这就是核能的基础，也解释了为什么恒星的聚变和反应堆的裂变都能释放能量。
8. Energy Levels of Electrons | 电子的能级
. Modern quantum mechanics describes electrons by probability clouds in orbitals defined by the principal quantum number n.
原子中的电子占据分立的能级。在玻尔模型中，这些是具有量子化角动量的稳定轨道，满足 mvr = nh/2π。现代量子力学用由主量子数n定义的轨道中的概率云来描述电子。
The ground state is the lowest energy level (n=1). Higher levels (n=2,3,… tween levels by absorbing or emitting a photon whose energy matches the energy difference.
基态是最低能级（n=1）。更高的能级（n=2,3,…）是激发态。电子可以通过吸收或发射能量匹配能级差的光子在能级间跃迁。
9. Electron Transitions and Photon Emission/Absorption | 电子跃迁与光子发射/吸收
gy level E₂ to a lower one E₁, it emits a photon of energy hf = E₂ − E₁, where h is Planck' wavelength of the emitted light are characteristic of the element, producing line spectra.
当电子从较高能级E₂跃迁到较低能级E₁时，会发射一个光子，能量为hf = E₂ − E₁，其中h是普朗克常数（6.63×10⁻³⁴ J s）。发射光的频率和波长是该元素的特征，产生线状光谱。
o n=1 (ultraviolet), Balmer series to n=2 (visible), and Paschen series to n=3 (infrared).
对于氢，能级由Eₙ = −13.6 eV / n²给出。莱曼系对应于跃迁到n=1（紫外），巴尔末系对应于跃迁到n=2（可见），帕申系对应于跃迁到n=3（红外）。
An electron can also absorb a photon and be excited to a higher level. If the photon energy is enough to overcome the ionisation energy (e.g. 13.6 eV for hydrogen in the ground state), the electron is ejected from the atom – this is ionisation.
电子也可以吸收一个光子并被激发到更高能级。如果光子能量足以克服电离能（例如基态氢为13.6 eV），电子就会从原子中被击出——这就是电离。
10. Quarks, Leptons and the Standard Model | 夸克、轻子与标准模型
+ ⅔ − ⅓ = +1, matching the elementary charge. The neutron (udd) has charge +⅔ − ⅓ − ⅓ = 0.
核子是强子，具体说是重子，意味着它们由三个夸克组成。质子（uud）的电荷为+⅔ + ⅔ − ⅓ = +1，与基元电荷相符。中子（udd）的电荷为+⅔ − ⅓ − ⅓ = 0。
Electrons belong to the lepton family; they are fundamental particles with no internal structure. The electron's antiparticle is the positron (e⁺), which has the same mass but opposite charge. Other leptons include the muon and tau, each with their own neutrino.
电子属于轻子家族；它们是基本粒子，没有内部结构。电子的反粒子是正电子（e⁺），质量相同但电荷相反。其他轻子包括μ子和τ子，各自有对应的中微子。
In beta-minus decay, a down quark inside a neutron changes into an up quark via the weak interaction, emitting a W⁻ boson. The W⁻ boson quickly decays into an electron and an electron antineutrino. This is how a nucleus converts a neutron into a proton, increasing Z by 1 while keeping A constant.
在β⁻衰变中，中子内的一个下夸克通过弱相互作用转变为上夸克，发射出一个W⁻玻色子。W⁻玻色子迅速衰变为一个电子和一个反电子中微子。这就是原子核将中子转化为质子的方式，使Z增加1而A保持不变
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