📚 Proton Number, Nucleon Number and Isotopes | 质子数、核子数与同位素
Atoms are often described as the smallest particles of matter, but they are not solid or indivisible. Inside every atom there are smaller particles called protons, neutrons and electrons. To identify an element, and to understand the different versions of the same element, chemists use two key numbers: the proton number and the nucleon number. This article explains what these numbers mean, how to work them out for any atom, and how they help us understand isotopes, ions, relative atomic mass and the patterns in the Periodic Table.
原子常被描述为物质的最小粒子，但原子并不是实心或不可分割的。每个原子内部都有更小的粒子：质子、中子和电子。为了识别一种元素，并理解同一元素的不同变体，化学家使用两个关键数字：质子数和核子数。本文将解释这两个数字的含义、如何对任意原子进行计算，以及它们如何帮助我们理解同位素、离子、相对原子质量和元素周期表中的规律。
1. Inside the Atom: Protons, Neutrons and Electrons | 原子内部：质子、中子与电子
Atoms contain three main subatomic particles. Protons carry a positive charge, neutrons carry no charge at all, and electrons carry a negative charge. Protons and neutrons are found in the dense central nucleus, while electrons move around the nucleus in shells, also called energy levels. Almost all of the mass of an atom is concentrated in the nucleus, but almost all of the volume is the empty space that the electrons occupy.
原子包含三种主要的亚原子粒子。质子带正电荷，中子完全不带电荷，电子带负电荷。质子和中子位于致密的中心原子核中，而电子在原子核外的电子层（也称能级）中运动。原子几乎全部的质量都集中在原子核中，但原子几乎全部的体积都是电子所占据的空旷空间。
If an atom is neutral, the number of electrons equals the number of protons, so the positive and negative charges cancel out exactly. If electrons are lost or gained, the particle becomes a charged ion instead. This simple charge balance is the reason why proton number controls so much of an atom's behaviour.
如果原子呈电中性，电子数就等于质子数，正负电荷恰好相互抵消。如果失去或得到电子，该粒子就会变成带电的离子。正是这种简单的电荷平衡，使得质子数在很大程度上决定了一个原子的行为。
2. Comparing the Three Subatomic Particles | 三种亚原子粒子的比较
The table below summarises the charge, relative mass and position of each particle. Notice that protons and neutrons have almost exactly the same mass, while an electron is roughly 1840 times lighter, which is why the mass of an atom is decided almost entirely by its nucleus.
下表总结了每种粒子的电荷、相对质量和位置。请注意，质子与中子的质量几乎完全相同，而电子大约比它们轻 1840 倍，这就是原子的质量几乎完全由原子核决定的原因。
| Particle | 粒子 | Relative charge | 相对电荷 | Relative mass | 相对质量 | Location | 位置 |
|---|---|---|---|
| Proton 质子 | +1 | 1 | Nucleus 原子核 |
| Neutron 中子 | 0 | 1 | Nucleus 原子核 |
| Electron 电子 | −1 | 1/1840 ≈ 0 | Shells 电子层 |
The word nucleon is used for any particle inside the nucleus, so both protons and neutrons are nucleons. This is why the total number of protons plus neutrons is called the nucleon number.
核子一词指原子核内的任何一种粒子，因此质子和中子都属于核子。这就是质子数与中子数之和被称为核子数的原因。
3. Proton Number (Atomic Number) | 质子数（原子序数）
The proton number is the number of protons in the nucleus of an atom. It is also called the atomic number and is given the symbol Z. The proton number is the identity card of an element: every atom with 6 protons is carbon, every atom with 17 protons is chlorine, and so on. No two different elements share the same proton number.
质子数是原子核内质子的数目，也称为原子序数，用符号 Z 表示。质子数是元素的"身份证明"：凡是含 6 个质子的原子就是碳，含 17 个质子的原子就是氯，依此类推。任何两种不同元素的质子数都不相同。
Proton number also decides the position of an element in the Periodic Table, because elements are arranged in order of increasing proton number. In a neutral atom, the number of electrons is also equal to the proton number, so the proton number controls the electron arrangement and therefore the chemical properties of the element.
质子数还决定元素在元素周期表中的位置，因为元素正是按质子数递增的顺序排列的。在中性原子中，电子数也等于质子数，因此质子数决定了电子排布，从而决定了元素的化学性质。
4. Nucleon Number (Mass Number) | 核子数（质量数）
The nucleon number is the total number of protons and neutrons in the nucleus of an atom. It is also called the mass number and is given the symbol A. Because protons and neutrons each have a relative mass of 1, the nucleon number tells us approximately how heavy the nucleus is compared with a single proton.
核子数是原子核内质子数与中子数的总和，也称为质量数，用符号 A 表示。由于质子和中子的相对质量都是 1，核子数可以告诉我们该原子核大约相当于多少个质子的质量。
Nucleon number A = protons + neutrons
核子数 A = 质子数 + 中子数
Be careful: the nucleon number is not the same as relative atomic mass. The nucleon number is always a whole number for a single atom, while relative atomic mass is usually a decimal because it is an average of all the isotopes of that element.
注意：核子数并不等于相对原子质量。对单个原子而言，核子数永远是整数；而相对原子质量通常是小数，因为它是该元素所有同位素的平均值。
5. Working Out Numbers of Particles | 计算各类粒子的数目
If you know the proton number and the nucleon number, you can work out every other particle count in a neutral atom. The number of neutrons is found by subtracting the proton number from the nucleon number, and the number of electrons equals the proton number.
如果已知质子数和核子数，你就能算出中性原子中其他所有粒子的数目。中子数由核子数减去质子数得到，而电子数等于质子数。
neutrons = A − Z electrons = Z (neutral atom)
中子数 = A − Z 电子数 = Z（中性原子）
, 20 neutrons and 17 electrons. For ions, only the electron number changes: a Mg²⁺ ion still has 12 protons and 12 neutrons, but only 10 electrons.
例如，钠-23 的 A = 23、Z = 11，因此它有 11 个质子、23 − 11 = 12 个中子和 11 个电子。氯-37 的 A = 37、Z = 17，因此有 17 个质子、20 个中子和 17 个电子。对于离子，只有电子数会改变：Mg²⁺ 离子仍含 12 个质子和 12 个中子，但只有 10 个电子。
| Species | 粒子 | Protons | 质子 | Neutrons | 中子 | Electrons | 电子 |
|---|---|---|---|
| ²³₁₁Na | 11 | 12 | 11 |
| ³⁷₁₇Cl | 17 | 20 | 17 |
| ²⁷₁₃Al³⁺ | 13 | 14 | 10 |
6. What Are Isotopes? | 什么是同位素
Isotopes are atoms of the same element that have the same proton number but different nucleon numbers. In other words, isotopes of an element have identical numbers of protons and electrons but different numbers of neutrons. Since the proton number is the same, they are the same element and they occupy the same place in the Periodic Table.
同位素是质子数相同但核子数不同的同一种元素的原子。换句话说，同一元素的同位素具有相同的质子数和电子数，但中子数不同。由于质子数相同，它们属于同一种元素，并在元素周期表中占据同一位置。
The different neutron numbers mean that isotopes have different masses, and some of them may be unstable and radioactive. However, they all react in the same way chemically, because chemical reactions depend on electrons, not on neutrons.
中子数不同意味着同位素的质量不同，其中一些可能不稳定并具有放射性。然而，它们的化学性质完全相同，因为化学反应取决于电子，而不是中子。
7. The Three Isotopes of Hydrogen | 氢的三种同位素
Hydrogen is the simplest example because all three of its isotopes have exactly one proton and one electron, but they contain zero, one and two neutrons respectively. Protium (¹₁H) is by far the most common, deuterium (²₁H) is used in heavy water, and tritium (³₁H) is radioactive and used in some nuclear applications.
氢是最简单的例子：它的三种同位素都恰好有 1 个质子和 1 个电子，但分别含有 0 个、1 个和 2 个中子。氕（¹₁H）最为常见，氘（²₁H）用于制造重水，氚（³₁H）具有放射性，用于某些核技术领域。
| Isotope | 同位素 | Protons | 质子 | Neutrons | 中子 | Nucleon number | 核子数 |
|---|---|---|---|
| Protium 氕 ¹₁H | 1 | 0 | 1 |
| Deuterium 氘 ²₁H | 1 | 1 | 2 |
| Tritium 氚 ³₁H | 1 | 2 | 3 |
Because deuterium is about twice as heavy as protium, heavy water (D₂O) has a slightly higher density and a slightly higher boiling point than ordinary water, even though both are chemically water.
由于氘的质量大约是氕的两倍，重水（D₂O）的密度和沸点都比普通水略高，尽管两者在化学上都是水。
8. Chlorine-35, Chlorine-37 and Carbon-14 | 氯-35、氯-37 与碳-14
Chlorine is a classic exam example. Naturally occurring chlorine is a mixture of two isotopes: about 75% chlorine-35 and about 25% chlorine-37. Both isotopes have 17 protons and 17 electrons, but chlorine-35 has 18 neutrons while chlorine-37 has 20 neutrons. This mixture is the reason the relative atomic mass of chlorine is 35.5 rather than a whole number.
氯是考试中的经典例子。天然存在的氯是两种同位素的混合物：约 75% 的氯-35 与约 25% 的氯-37。两种同位素的质子数和电子数都是 17，但氯-35 有 18 个中子，氯-37 有 20 个中子。正是这种混合物使氯的相对原子质量为 35.5，而不是整数。
| Isotope | 同位素 | Protons | 质子 | Neutrons | 中子 | Abundance | 丰度 |
|---|---|---|---|
| ³⁵₁₇Cl | 17 | 18 | 75% |
| ³⁷₁₇Cl | 17 | 20 | 25% |
Carbon also has several isotopes: carbon-12 and carbon-13 are stable, while carbon-14 is radioactive with a half-life of about 5730 years. Living things constantly take in carbon-14, but when they die the amount slowly decreases, which is the basis of radiocarbon dating.
碳也有多种同位素：碳-12 和碳-13 是稳定的，而碳-14 具有放射性，其半衰期约为 5730 年。生物在活着时不断吸收碳-14，但死亡后其含量会缓慢减少，这正是放射性碳定年法的原理。
9. Why Isotopes Share the Same Chemical Properties | 为什么同位素化学性质相同
Chemical behaviour is controlled by the electrons, especially the electrons in the outermost shell. Isotopes of an element have the same proton number, therefore the same number of electrons and the same electron arrangement. As a result, isotopes react in exactly the same way, form the same compounds and show the same valency.
化学行为由电子决定，尤其是最外层电子。同一元素的同位素质子数相同，因此电子数和电子排布也相同。因此，同位素的反应方式完全相同，生成相同的化合物，表现出相同的化合价。
Their physical properties, however, can differ slightly, because physical properties such as density, boiling point and rate of diffusion depend on mass. Heavier isotopes diffuse more slowly and have slightly higher densities, which is why heavy water and ordinary water are not completely identical in physical behaviour.
然而，它们的物理性质可能略有差异，因为密度、沸点和扩散速率等物理性质与质量有关。较重的同位素扩散更慢、密度略高，这就是重水与普通水在物理行为上并不完全相同的缘故。
10. Relative Atomic Mass from Isotope Abundance | 由同位素丰度计算相对原子质量
Because an element in nature is usually a mixture of isotopes, chemists calculate a weighted average called the relative atomic mass, written Aᵣ. Each isotopic mass is multiplied by its percentage abundance, the results are added together, and the total is divided by 100.
由于自然界中的元素通常是同位素的混合物，化学家会计算一个加权平均值，称为相对原子质量，记作 Aᵣ。将每种同位素的质量乘以其百分比丰度，把结果相加，再除以 100。
Aᵣ = (mass₁ × %₁ + mass₂ × %₂ + …) ÷ 100
× 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5. For copper, with about 69% copper-63 and 31% copper-65, the answer is (63 × 69 + 65 × 31) ÷ 100 = (4347 + 2015) ÷ 100 = 63.6.
对氯而言，计算为：Aᵣ =（35 × 75 + 37 × 25）÷ 100 =（2625 + 925）÷ 100 = 3550 ÷ 100 = 35.5。对铜而言，约 69% 为铜-63、31% 为铜-65，结果为（63 × 69 + 65 × 31）÷ 100 =（4347 + 2015）÷ 100 = 63.6。
This is why relative atomic masses on the Periodic Table are usually decimals: they are averages over all naturally occurring isotopes, weighted by how common each one is.
这就是元素周期表上的相对原子质量通常是小数的原因：它们是对所有天然同位素按丰度加权后的平均值。
11. Isotopes Compared with Ions | 同位素与离子的比较
Isotopes and ions are often confused because both involve atoms that are not "standard". The key difference is which particle changes. In isotopes the number of neutrons changes while the proton number stays the same; in ions the number of electrons changes while the proton number and neutron number stay the same.
同位素和离子常被混淆，因为两者都涉及"非标准"的原子。关键区别在于发生变化的是哪种粒子。在同位素中，中子数改变而质子数不变；在离子中，电子数改变而质子数与中子数都不变。
For example, ³⁵₁₇Cl and ³⁷₁₇Cl are isotopes of chlorine, while Cl⁻ and Mg²⁺ are ions. A chloride ion still has 17 protons and 18 or 20 neutrons, but it has 18 electrons instead of 17, which gives it a single negative charge.
例如，³⁵₁₇Cl 和 ³⁷₁₇Cl 是氯的同位素，而 Cl⁻ 和 Mg²⁺ 是离子。氯离子仍有 17 个质子以及 18 或 20 个中子，但它的电子数为 18 而不是 17，因此带一个负电荷。
Remember the two rules: isotopes differ in neutrons, ions differ in electrons. If both the proton number and the neutron number change, the atoms are simply different elements, not isotopes.
请记住两条规则：同位素差别在中子，离子差别在电子。如果质子数与中子数都改变了，这些原子就属于不同的元素，而不是同位素。
12. Nuclear Notation, Exam Tips and Common Mistakes | 核符号、考试技巧与常见错误
as a superscript on the left and the proton number as a subscript on the left, in the form ᴬ he element name plus the nucleon number is used instead, such as carbon-12 or uranium-235.
同位素的书写形式为 ᴬ_Z X：核子数写在左上角作为上标，质子数写在左下角作为下标。例如 ¹²₆C 表示核子数为 12、质子数为 6 的碳原子，因此中子数为 12 − 6 = 6。有时也用元素名加核子数的写法，如碳-12 或铀-235。
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