Numbers of Nucleons | 核子数

📚 Numbers of Nucleons | 核子数

In A-level chemistry, the term nucleon refers collectively to the two types of particle found in the atomic nucleus: the proton and the neutron. Both nucleons have a relative mass of approximately 1 atomic mass unit (u), whereas electrons are much lighter and are not nucleons. Since almost all of the mass of an atom is concentrated in the nucleus, the total number of nucleons largely determines the atom’s mass.

在 A-level 化学中，核子（nucleon）是原子核内两类粒子的统称：质子和中子。两类核子的相对质量都约为 1 原子质量单位（u），而电子要轻得多，不属于核子。由于原子几乎全部质量都集中在原子核，核子总数在很大程度上决定了原子的质量。

1. What Are Nucleons? | 什么是核子？

Nucleons are subatomic particles found in the nucleus of an atom. There are two types of nucleon: protons, which carry a positive charge, and neutrons, which are electrically neutral. Both protons and neutrons have a relative mass very close to 1 u, while electrons are about 1/1836 of this mass and are not counted as nucleons.

核子是存在于原子核内的亚原子粒子。核子有两种：带正电的质子和不带电的中子。质子和中子的相对质量都非常接近 1 u，而电子约为该质量的 1/1836，并且不属于核子。

The total number of nucleons in a nucleus is called the nucleon number or mass number. It is represented by the symbol A. Because protons and neutrons account for almost all the atomic mass, A gives a good indication of the relative mass of a nuclide.

原子核中核子的总数称为核子数或质量数，用符号 A 表示。由于质子和中子几乎贡献了全部原子质量，A 可以很好地反映一个核素的相对质量。

2. Proton Number, Neutron Number and Nucleon Number | 质子数、中子数与核子数

The proton number is given the symbol Z, and it defines the element. The neutron number is given the symbol N. The nucleon number, also called the mass number, is given the symbol A. These three quantities are related by the simple equation below.

质子数用符号 Z 表示，它决定元素的种类。中子数用符号 N 表示。核子数，又称质量数，用符号 A 表示。这三个量之间的关系如下面的简单公式所示。

A = Z + N

For example, a carbon-12 atom has 6 protons and 6 neutrons, so its nucleon number is A = 6 + 6 = 12. A uranium-235 atom has 92 protons and 143 neutrons, giving A = 92 + 143 = 235. Note that A, Z and N are always whole numbers for a given nuclide.

例如，碳-12 原子有 6 个质子和 6 个中子，因此其核子数 A = 6 + 6 = 12。铀-235 原子有 92 个质子和 143 个中子，因此 A = 92 + 143 = 235。注意，对于特定核素，A、Z 和 N 始终为整数。

3. Nuclide Notation | 核素符号

Nuclide notation shows the nucleon number and proton number alongside the element symbol. The nucleon number is written as a superscript to the left of the symbol, and the proton number as a subscript to the left. A neutral atom of carbon-12 is therefore written as ¹²₆C.

核素符号在元素符号左侧同时标明核子数和质子数。核子数写在元素符号左上角，质子数写在左下角。因此，碳-12 中性原子写作 ¹²₆C。

From the notation you can find the neutron number at once: N = A − Z. For example, ³⁵₁₇Cl has A = 35 and Z = 17, so it has 18 neutrons; ³⁷₁₇Cl has A = 37 and Z = 17, so it has 20 neutrons.

从核素符号可以直接求出中子数：N = A − Z。例如，³⁵₁₇Cl 的 A = 35、Z = 17，因此中子数为 18；³⁷₁₇Cl 的 A = 37、Z = 17，因此中子数为 20。

4. Isotopes: Same Proton Number, Different Nucleon Number | 同位素：质子数相同，核子数不同

Isotopes are atoms of the same element that have the same proton number but different nucleon numbers. In other words, they have the same Z but different A because their neutron numbers differ. For example, chlorine has two stable isotopes: ³⁵₁₇Cl and ³⁷₁₇Cl.

同位素是同一元素的不同原子，它们具有相同的质子数但不同的核子数。换句话说，它们的 Z 相同而 A 不同，因为中子数不同。例如，氯有两种稳定同位素：³⁵₁₇Cl 和 ³⁷₁₇Cl。

Isotope \| 同位素	Proton number Z \| 质子数 Z	Nucleon number A \| 核子数 A	Neutron number N \| 中子数 N
³⁵₁₇Cl	17	35	18
³⁷₁₇Cl	17	37	20

Because isotopes have the same electron arrangement, their chemical properties are essentially identical. However, their physical properties that depend on mass, such as density or rate of diffusion, can differ slightly.

由于同位素具有相同的电子排布，它们的化学性质基本相同。但取决于质量的物理性质（如密度或扩散速率）可能略有不同。

5. Why Nucleon Number Is Also Called Mass Number | 为什么核子数又称质量数

The nucleon number is often called the mass number because protons and neutrons account for almost all of an atom’s mass. Each proton and neutron has a relative mass close to 1 on the atomic mass scale, while an electron has a relative mass of only about 1/1836.

核子数常被称为质量数，因为质子和中子几乎构成了原子的全部质量。在原子质量标度上，每个质子和中子的相对质量都接近 1，而电子的相对质量仅为约 1/1836。

The isotope carbon-12 is used as the standard for the relative atomic mass scale: one atom of ¹²₆C is assigned a mass of exactly 12 u. This makes the link between nucleon number and atomic mass particularly clear.

碳-12 同位素被用作相对原子质量标度的标准：一个 ¹²₆C 原子的质量被规定为恰好 12 u。这使核子数与原子质量之间的联系尤为清晰。

6. Relative Atomic Mass and Isotopic Abundance | 相对原子质量与同位素丰度

The relative atomic mass Aᵣ of an element is the weighted average mass of its atoms compared with 1/12 of the mass of a carbon-12 atom. Because natural samples contain a mixture of isotopes, Aᵣ is often not a whole number.

元素的相对原子质量 Aᵣ 是其原子质量与碳-12 原子质量的 1/12 相比所得的加权平均值。由于天然样品含有多种同位素的混合物，Aᵣ 通常不是整数。

Aᵣ = Σ (isotopic mass × abundance) / 100

For example, chlorine has about 75.0% ³⁵Cl and 25.0% ³⁷Cl, so Aᵣ ≈ (35 × 75.0 + 37 × 25.0) / 100 = 35.5. This explains why chlorine has a relative atomic mass of 35.5 even though no single chlorine atom has a nucleon number of 35.5.

例如，氯约有 75.0% 的 ³⁵Cl 和 25.0% 的 ³⁷Cl，因此 Aᵣ ≈ (35 × 75.0 + 37 × 25.0) / 100 = 35.5。这就解释了为什么氯的相对原子质量为 35.5，尽管没有任何一个氯原子的核子数是 35.5。

7. Mass Spectrometry and Nucleon Numbers | 质谱与核子数

Mass spectrometry provides direct evidence for isotopes and their nucleon numbers. In a mass spectrometer, atoms are ionised and separated according to their mass-to-charge ratio, producing peaks at m/z values that correspond to the nucleon numbers of the ions.

质谱法为同位素及其核子数提供了直接证据。在质谱仪中，原子被电离并按质荷比进行分离，在 m/z 值处产生峰，这些值对应离子的核

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Numbers of Nucleons | 核子数