📚 S1.2 The Nuclear Atom | S1.2 核原子
The nuclear atom model describes the structure of an atom as a tiny, dense, positively charged nucleus containing protons and neutrons, surrounded by electrons that move through the rest of an almost empty volume. Almost all of the mass of an atom is packed into the nucleus, yet the nucleus itself is around one hundred thousand times smaller in diameter than the atom as a whole. Understanding exactly what the nucleus is made of is essential for explaining chemical identity, isotopic variation and atomic mass, because it is the number of protons that decides which element an atom is, while the number of neutrons decides which isotope of that element it is. This section covers the subatomic particles and their relative masses and charges, atomic number and mass number, nuclide notation, isotopes, relative isotopic mass, relative atomic mass calculations, and mass spectrometry as the key experimental technique used to measure these quantities.
核原子模型把原子描述为：一个微小、致密、带正电的原子核，内含质子和中子，核外电子在几乎空荡的空间中运动。原子几乎全部的质量都集中在原子核内，而原子核本身的直径却只有整个原子的大约十万分之一。准确理解原子核的组成，对解释化学身份、同位素差异和原子质量至关重要，因为正是质子数决定了一个原子属于哪种元素，而中子数决定了它属于该元素的哪一种同位素。本节涵盖亚原子粒子及其相对质量和相对电荷、原子序数与质量数、核素符号、同位素、相对同位素质量、相对原子质量的计算，以及作为关键实验技术的质谱法。
1. The Nuclear Atom Model | 核原子模型
The nuclear model replaced the older idea that electrons were scattered evenly through a positive "pudding". In the nuclear model, the positive charge and nearly all of the mass are concentrated in a single central nucleus, and the electrons occupy the surrounding space. The atom is therefore mostly empty space: if an atom were enlarged to the size of a football stadium, the nucleus would be roughly the size of a pea placed at the centre spot, while the electrons would be tiny specks moving around the stands.
核式模型取代了早期"电子均匀散布在正电布丁中"的旧观念。在核式模型中，正电荷和几乎全部质量都集中在一个中心原子核上，电子则占据核外的空间。因此原子大部分是空的：若把原子放大到一个足球场那么大，原子核大约只有放在中圈的一颗豌豆大小，电子则是绕着看台运动的微小颗粒。
eus is therefore about 10⁴ times smaller in radius and about 10¹² times smaller in volume than the whole atom, yet it contains more than 99.9% of the mass. This is why the mass of an atom is effectively the mass of its nucleus.
值得记住的典型数值：原子半径约为 1 × 10⁻¹⁰ m，而原子核半径约为 1 × 10⁻¹⁴ m。因此原子核的半径大约只有整个原子的 10⁻⁴，体积只有约 10⁻¹²，但它却包含超过 99.9% 的质量。这就是为什么原子的质量实际上就是其原子核的质量。
The experimental evidence for this model came from the deflection of alpha particles fired at thin metal foil: most passed straight through (the atom is mostly empty space), a few were deflected through large angles (the nucleus is positively charged), and very few bounced almost straight back (the nucleus is dense and carries most of the mass). All of these observations are direct consequences of the nuclear atom model.
支持该模型的实验证据来自用α粒子轰击薄金属箔时的偏转现象：大多数α粒子直接穿过（说明原子大部分是空的），少数发生大角度偏转（说明原子核带正电），极少数几乎被弹回（说明原子核致密且集中了大部分质量）。这些观测结果都是核原子模型的直接推论。
2. The Three Subatomic Particles | 三种亚原子粒子
Atoms are composed of three fundamental subatomic particles: protons, neutrons, and electrons. Protons and neutrons together are called nucleons because they are found inside the nucleus, whereas electrons occupy the shells outside the nucleus.
原子由三种基本亚原子粒子组成：质子、中子和电子。质子和中子合称核子，因为它们位于原子核内；电子则占据原子核外的电子层。
Protons are positively charged particles located in the nucleus. They have a relative mass of 1 and a relative charge of +1. The number of protons in the nucleus is unique to each element and is called the atomic number.
质子是位于原子核内带正电荷的粒子。其相对质量为 1，相对电荷为 +1。核内质子数对每种元素都是独一无二的，称为原子序数。
Neutrons are neutral particles also found in the nucleus. They have a relative mass of 1 and no electric charge (0). Neutrons contribute mass but no charge, so changing the number of neutrons changes the mass of an atom without changing its chemical identity.
中子是不带电的粒子，同样存在于原子核内。其相对质量为 1，没有电荷（0）。中子只贡献质量而不贡献电荷，因此改变中子数会改变原子的质量，却不改变它的化学身份。
Electrons are negatively charged particles that occupy regions of space called shells around the nucleus. Their relative mass is approximately 1/1836, which is often considered negligible, and they carry a relative charge of −1. Electrons are responsible for chemical bonding and for the chemical properties of an element.
电子是带负电荷的粒子，分布在原子核外的电子层中。其相对质量约为 1/1836，常被视为可忽略不计，相对电荷为 −1。电子决定化学键的形成以及元素的化学性质。
Because an atom is electrically neutral overall, the number of electrons in a neutral atom equals the number of protons. When an atom loses or gains electrons it becomes an ion, but the nucleus itself is unchanged in ordinary chemical processes.
由于原子整体呈电中性，中性原子中的电子数等于质子数。当原子失去或得到电子时它会变成离子，但在普通化学过程中原子核本身并不改变。
3. Relative Mass, Relative Charge and the Summary Table | 相对质量、相对电荷与汇总表
Comparing particles with the actual SI values of mass (around 10⁻²⁷ kg) and charge (around 10⁻¹⁹ C) is inconvenient, so chemists use relative mass and relative charge. Relative mass is measured on the carbon-12 scale, and relative charge is measured in units of the charge on one electron.
用实际的国际单位制数值（质量约 10⁻²⁷ kg、电荷约 10⁻¹⁹ C）来比较粒子很不方便，因此化学家使用相对质量和相对电荷。相对质量以碳-12 为标准，相对电荷则以一个电子的电荷为单位。
The properties of these particles are summarised as follows: proton mass ~1 u, charge +1; neutron mass ~1 u, charge 0; electron mass ~0.0005 u, charge −1. Note that 1/1836 is approximately 0.0005, so the original statement in older notes is consistent with the accepted value.
这些粒子的性质总结如下：质子质量约为 1 u，电荷 +1；中子质量约为 1 u，电荷 0；电子质量约为 0.0005 u，电荷 −1。注意 1/1836 约等于 0.0005，因此与常见教材给出的数值是一致的。
| Particle 粒子 | Relative mass 相对质量 | Relative charge 相对电荷 | Location 位置 |
| Proton 质子 | 1 | +1 | Nucleus 原子核 |
| Neutron 中子 | 1 | 0 | Nucleus 原子核 |
| Electron 电子 | 1/1836 ≈ 0.0005 | −1 | Shells 电子层 |
A quick sense check: since an electron is about 1836 times lighter than a proton, the mass of an atom is determined almost entirely by the number of protons plus the number of neutrons, and the mass of the electrons can usually be ignored in calculations of relative atomic mass.
一个快速的检验：由于电子比质子轻约 1836 倍，原子的质量几乎完全由质子数加中子数决定，在相对原子质量的计算中通常可以忽略电子的质量。
4. Atomic Number, Mass Number and Nuclide Notation | 原子序数、质量数与核素符号
The atomic number (symbol Z) is the number of protons in the nucleus of an atom. It defines the element: every atom with 6 protons is carbon, every atom with 17 protons is chlorine. In a neutral atom, Z also equals the number of electrons.
原子序数（符号 Z）是原子核内的质子数。它决定了元素种类：凡是含 6 个质子的原子都是碳，凡是含 17 个质子的原子都是氯。在中性原子中，Z 也等于电子数。
The mass number (symbol A) is the total number of protons and neutrons in the nucleus, also called the nucleon number. It is a count of particles, not a mass in grams, although it is numerically close to the relative isotopic mass.
质量数（符号 A）是原子核内质子数与中子数之和，也称为核子数。它是粒子计数，而不是以克为单位的质量，尽管其数值与相对同位素质量很接近。
A = Z + number of neutrons 中子数 = A − Z
a is a sodium atom with 11 protons and 23 − 11 = 12 neutrons, and ³⁵₁₇Cl is a chlorine atom with 17 protons and 18 neutrons.
核素符号的写法是：把质量数写在元素符号前的上标位置，把原子序数写在元素符号前的下标位置。例如 ²³₁₁Na 表示含 11 个质子、23 − 11 = 12 个中子的钠原子；³⁵₁₇Cl 表示含 17 个质子、18 个中子的氯原子。
For ions, the charge is written as a superscript after the symbol. For example, ²³₁₁Na⁺ is a sodium ion formed by losing one electron, so it still has 11 protons and 12 neutrons but only 10 electrons. The nuclear composition does not change when an ion forms.
对于离子，电荷写在元素符号后的上标位置。例如 ²³₁₁Na⁺ 是钠原子失去一个电子后形成的钠离子，它仍然有 11 个质子和 12 个中子，但只有 10 个电子。形成离子时原子核的组成并不改变。
5. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, and therefore different mass numbers. Because they have the same atomic number, isotopes occupy the same place in the periodic table.
同位素是同一元素的原子，它们具有相同的质子数但不同的中子数，因而质量数不同。由于原子序数相同，同位素在元素周期表中占据同一位置。
Chlorine provides a classic example: ³⁵₁₇Cl has 17 protons and 18 neutrons, while ³⁷₁₇Cl has 17 protons and 20 neutrons. Both are chlorine, both have 17 electrons in the neutral atom, and both form the same chloride ion, Cl⁻, but their mass numbers differ by two.
氯是经典例子：³⁵₁₇Cl 有 17 个质子和 18 个中子，而 ³⁷₁₇Cl 有 17 个质子和 20 个中子。两者都是氯，中性原子的电子数都是 17，都能形成相同的氯离子 Cl⁻，但质量数相差 2。
Because chemical properties depend on the number and arrangement of electrons, isotopes of the same element have almost identical chemical properties. Their physical properties differ slightly: heavier isotopes diffuse more slowly, have slightly higher boiling points in molecular form, and may be radioactive if the nucleus is unstable. Reaction rates can also differ slightly in kinetic isotope effects.
由于化学性质取决于电子的数量和排布，同一元素的同位素化学性质几乎完全相同。它们的物理性质略有差异：较重的同位素扩散更慢，以分子形式存在时沸点略高，若原子核不稳定还可能具有放射性。在动力学同位素效应中，反应速率也可能略有不同。
Isotopes can be separated by physical methods that depend on mass, such as fractional distillation, gas diffusion or mass spectrometry, but never by ordinary chemical reactions.
同位素可以依靠与质量有关的物理方法分离，例如分馏、气体扩散或质谱法，但绝不可能通过普通化学反应来分离。
6. Relative Isotopic Mass and Relative Atomic Mass | 相对同位素质量与相对原子质量
The carbon-12 scale is the reference for all relative masses. One atom of carbon-12 is defined as having a mass of exactly 12 units on this scale, so the standard is one-twelfth of the mass of a single carbon-12 atom.
碳-12 基准是所有相对质量的标准。一个碳-12 原子的质量被定义为该标度上正好 12 个单位，因此标准就是一个碳-12 原子质量的十二分之一。
ative isotopic mass = (mass of one atom of the isotope) ÷ (1/12 × mass of one atom of ¹²C)
The relative isotopic mass of a particular isotope is the mass of one atom of that isotope compared with one-twelfth of the mass of one atom of carbon-12. It is a pure number with no units, and for most isotopes it is very close to the mass number.
某一种同位素的相对同位素质量，是该同位素一个原子的质量与一个碳-12 原子质量十二分之一的比值。它是一个没有单位的纯数，多数情况下与质量数非常接近。
Relative atomic mass = (weighted mean mass of one atom) ÷ (1/12 × mass of one atom of ¹²C)
The relative atomic mass (symbol Ar) of an element is the weighted mean mass of one atom of that element compared with one-twelfth of the mass of one atom of carbon-12. The word "weighted" is essential: it takes account of the relative abundance of each isotope in the sample.
元素的相对原子质量（符号 Ar）是该元素一个原子的加权平均质量与一个碳-12 原子质量十二分之一的比值。"加权"一词至关重要：它考虑了样品中每种同位素的相对丰度。
Because it is a weighted mean over isotopes, relative atomic mass is often not a whole number. Chlorine has an Ar of 35.5, copper 63.6 and magnesium 24.3, even though no single atom of these elements has exactly that mass.
由于它是对各同位素取的加权平均，相对原子质量往往不是整数。氯的 Ar 为 35.5，铜为 63.6，镁为 24.3，尽管这些元素的任何一个单独原子都不具有恰好等于该值的质量。
7. Calculating Relative Atomic Mass from Isotopic Abundances | 由同位素丰度计算相对原子质量
If the percentage abundances of the isotopes are given, the relative atomic mass is calculated by multiplying each isotopic mass by its percentage abundance, adding the products, and dividing by 100.
若已知各同位素的百分比丰度，计算相对原子质量的步骤是：用每种同位素的质量乘以其百分比丰度，将各乘积相加，再除以 100。
Ar = (Σ (isotopic mass × percentage abundance)) ÷ 100
Worked example 1: chlorine. A sample contains 75.0% of ³⁵Cl and 25.0% 625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5. This matches the value quoted in the periodic table.
例 1：氯。某样品含 75.0% 的 ³⁵Cl 和 25.0% 的 ³⁷Cl。则 Ar = (75.0 × 35 + 25.0 × 37) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5，与元素周期表中给出的数值一致。
Worked example 2: copper. Copper consists of 69.2% ⁶³Cu and 30.8% × 65) ÷ 100 = (4359.6 + 2002.0) ÷ 100 = 6361.6 ÷ 100 = 63.6 to three significant figures.
例 2：铜。铜由 69.2% 的 ⁶³Cu 和 30.8% 的 ⁶⁵Cu 组成。则 Ar = (69.2 × 63 + 30.8 × 65) ÷ 100 = (4359.6 + 2002.0) ÷ 100 = 6361.6 ÷ 100 = 63.6（三位有效数字）。
Worked example 3: three isotopes. Magnesium consists of 78.99% ²⁴Mg, 10.00% ²⁵Mg and 11.01% + 286.26) ÷ 100 = 2432.02 ÷ 100 = 24.3. The same method extends to any number of isotopes.
例 3：三种同位素。镁由 78.99% 的 ²⁴Mg、10.00% 的 ²⁵Mg 和 11.01% 的 ²⁶Mg 组成。则 10.00 × 25 + 11.01 × 26) ÷ 100 = (1895.76 + 250.00 + 286.26) ÷ 100 = 2432.02 ÷ 100 = 24.3。同样的方法可以推广到任意多种同位素。
pic mass × fractional abundance), with the fractions summing to 1. The reverse calculation is also examinable: given Ar and one abundance, find the other abundance by setting up a single linear equation.
若丰度以分数而非百分数给出，只需把每种同位素质量乘以其分数丰度后相加：Ar = Σ（同位素质量 × 分数丰度），各分数之和为 1。反向计算也常考：已知 Ar 和其中一个丰度，通过列一个一元一次方程求出另一个丰度。
8. Mass Spectrometry: How It Works | 质谱法：工作原理
A mass spectrometer is the instrument used to measure the masses and relative abundances of isotopes and of molecular ions. It operates under a high vacuum so that ions do not collide with air molecules on their way through the instrument.
质谱仪是用来测量同位素及分子离子的质量和相对丰度的仪器。它在高真空下工作，以免离子在飞行途中与空气分子碰撞。
Stage 1 - Vaporisation: the sample is injected and vaporised, or for a solid it may be heated and ionised directly, so that particles are free to move individually.
第一步——汽化：样品被注入并汽化；若是固体，也可加热后直接电离，使粒子能够自由、单独地运动。
ng positive ions. For an atom X, the process is X(g) + e⁻ → X⁺(g) + 2e⁻. Most ions formed carry a single positive charge.
第二步——电离：电子枪发射的高能电子轰击已汽化的原子或分子，将其电子打出，生成正离子。对原子 X 而言，过程为 X(g) + e⁻ → X⁺(g) + 2e⁻。生成的大多数离子带一个单位正电荷。
Stage 3 - Acceleration: the positive ions are accelerated by an electric field so that they all enter the analysing region with the same kinetic energy.
第三步——加速：正离子被电场加速，使它们以相同的动能进入分析区。
Stage 4 - Deflection: a magnetic field bends the path of the moving ions into a circular arc. The extent of deflection depends on the mass-to-charge ratio, m/z. Ions with a smaller m/z value (lighter ions, or ions with a higher charge) are deflected more strongly, while heavier ions with a larger m/z value are deflected less.
第四步——偏转：磁场使运动离子的路径弯曲成圆弧。偏转程度取决于质荷比 m/z。m/z 较小的离子（较轻的离子，或电荷较高的离子）偏转得更厉害；m/z
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