📚 Nucleons and Electrons | 核子与电子
Atoms are often described as the smallest units of chemical identity, but A-Level physics looks deeper: every atom is built from a central nucleus containing protons and neutrons, surrounded by electrons. Understanding the properties of nucleons and electrons is essential for explaining nuclear stability, radioactivity, and the fundamental interactions that govern matter.
原子常被描述为化学特性的最小单位，但在 A-Level 物理中我们需要看得更深：每个原子都由包含质子和中子的中心原子核以及绕核运动的电子组成。理解核子与电子的性质对于解释核稳定性、放射性以及支配物质的基本相互作用至关重要。
1. Inside the Atom | 原子内部
A neutral atom consists of a small, dense nucleus made of protons and neutrons, with electrons occupying the surrounding space. The nucleus carries almost all of the mass, while the electrons determine chemical behaviour and occupy a volume much larger than the nucleus.
中性原子由一个由质子和中子组成的微小致密原子核以及占据周围空间的电子构成。原子核几乎承载了全部质量，而电子决定化学性质，并占据远大于原子核的体积。
Protons and neutrons are collectively called nucleons because they reside in the nucleus. Electrons are not nucleons; they belong to the lepton family and are fundamental particles as far as current experiments can tell.
质子和中子统称为核子，因为它们位于原子核内。电子不是核子；电子属于轻子家族，就目前实验所知，电子是基本粒子。
2. Properties of Nucleons | 核子的性质
neutron is electrically neutral and has a slightly larger mass of about 1.675 × 10⁻²⁷ kg.
质子携带一个单位正基本电荷，+e = +1.60 × 10⁻¹⁹ C，质量约为 1.672 × 10⁻²⁷ kg。中子呈电中性，质量略大，约为 1.675 × 10⁻²⁷ kg。
Both protons and neutrons are baryons, meaning they are hadrons composed of three quarks. A proton is made of two up quarks and one down quark (uud), while a neutron is made of one up quark and two down quarks (udd).
质子和中子都是重子，也就是说它们是由三个夸克组成的强子。质子由两个上夸克和一个下夸克（uud）组成，而中子由一个上夸克和两个下夸克（udd）组成。
1 u = 1.66 × 10⁻²⁷ kg. On this scale, protons and neutrons each have a mass close to 1 u.
在核计算中，质量通常用原子质量单位表示，1 u = 1.66 × 10⁻²⁷ kg。以这个单位衡量，质子和中子的质量都接近 1 u。
3. Properties of Electrons | 电子的性质
-1.60 × 10⁻¹⁹ C, and has a rest mass of 9.11 × 10⁻³¹ kg, about 1/1836 of the proton mass.
电子携带一个单位负基本电荷，-e = -1.60 × 10⁻¹⁹ C，静止质量为 9.11 × 10⁻³¹ kg，约为质子质量的 1/1836。
Electrons are leptons and are not made of quarks. They are treated as point-like particles with no measurable internal structure in current experiments.
电子是轻子，不是由夸克组成的。在目前的实验中，电子被视为点状粒子，没有可测量的内部结构。
In an atom, electrons occupy discrete energy levels. Their arrangement explains atomic spectra and chemical bonding, but for nuclear physics the key point is that electrons can be emitted or absorbed in beta decay and electron capture.
在原子中，电子占据分立的能级。电子的排列解释了原子光谱和化学键，但在核物理中，关键点是电子可以在 β 衰变和电子俘获中被发射或吸收。
4. Nuclide Notation and Isotopes | 核素符号与同位素
A nuclide is represented by the notation AZX, where X is the chemical symbol, A is the mass number (number of nucleons), and Z is the atomic number (number of protons).
核素用符号 AZX 表示，其中 X 是化学符号，A 是质量数（核
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