A-Level Chemistry: Definition and Calculation of Nucleon Number | A-Level化学：核子数的定义与计算

📚 A-Level Chemistry: Definition and Calculation of Nucleon Number | A-Level化学：核子数的定义与计算

The nucleon number, also known as the mass number, is a fundamental concept in A-Level chemistry. It represents the total number of protons and neutrons in the nucleus of an atom. Understanding its definition and calculation is essential for interpreting atomic structure, writing nuclide notation, and solving problems involving isotopes and relative atomic mass.

核子数，又称质量数，是 A-Level 化学中的一个基本概念。它表示原子核中质子与中子的总数。理解其定义与计算方法，对于解读原子结构、书写核素符号，以及解决涉及同位素和相对原子质量的问题至关重要。

1. Atomic Structure and Subatomic Particles | 原子结构与亚原子粒子

Before defining nucleon number, it is necessary to recall the basic particles that make up an atom. A proton is a positively charged particle found in the nucleus. A neutron is an electrically neutral particle also found in the nucleus. An electron is a negatively charged particle that moves around the nucleus.

在定义核子数之前，有必要回顾构成原子的基本粒子。质子是位于原子核中带正电的粒子；中子是不带电、也位于原子核中的粒子；电子是带负电、在原子核周围运动的粒子。

Because protons and neutrons have almost identical masses and are both located in the nucleus, they are grouped together under the name “nucleons”. The electron is much lighter and contributes essentially nothing to the total mass of an atom.

由于质子与中子质量几乎相同，并且都位于原子核内，因此它们被统称为“核子”。电子的质量小得多，对原子总质量的贡献几乎可以忽略不计。

2. Defining Nucleon Number (Mass Number) | 核子数（质量数）的定义

The nucleon number, usually given the symbol A, is the total number of protons and neutrons in the nucleus of an atom. The name “mass number” arises because almost all of an atom’s mass is concentrated in its protons and neutrons. In nuclear notation, A is written as a superscript to the left of the element symbol.

核子数通常用符号 A 表示，指原子核中质子与中子的总数。之所以称为“质量数”，是因为几乎全部原子质量都集中在质子和中子中。在核素符号中，A 写在元素符号的左上方。

For an atom or ion, the nucleon number can be expressed by the equation:

A = Z + N

Here, Z is the number of protons, and N is the number of neutrons. Since both Z and N are whole numbers, A is always an integer.

其中，Z 是质子数，N 是中子数。由于 Z 和 N 都是整数，A 也总是整数。

3. Nucleons: Protons and Neutrons | 核子：质子和中子

The term nucleon is a collective name for protons and neutrons. It is important to remember that electrons are not nucleons. In all neutral atoms, the number of electrons equals the number of protons, but electrons are never counted when calculating nucleon number.

“核子”是质子和中子的统称。需要特别注意，电子不是核子。在所有中性原子中，电子数等于质子数，但计算核子数时绝不把电子计算在内。

The number of protons in a nucleus is called the atomic number, Z, and it defines the element. The number of neutrons, N, can vary among atoms of the same element, giving rise to isotopes. The sum of Z and N gives the nucleon number, A.

原子核中的质子数称为原子序数 Z，它决定了元素种类。同一种元素的原子之间中子数 N 可以不同，从而产生同位素。Z 与 N 之和就是核子数 A。

For example, every carbon atom has Z = 6. If a carbon nucleus contains 6 neutrons, its nucleon number is 12; if it contains 7 neutrons, the nucleon number is 13.

例如，每个碳原子的 Z = 6。若某个碳原子核含 6 个中子，则其核子数为 12；若含 7 个中子，则核子数为 13。

4. Calculating Nucleon Number from Subatomic Particles | 从亚原子粒子计算核子数

To find the nucleon number of an atom or ion, use the relationship A = Z + N. In a neutral atom, the atomic number is equal to the number of electrons, so if you are given the electron count for a neutral species, you can identify Z directly. However, for ions, the electron count is different from the proton count because electrons are gained or lost; you must use the nuclear charge, not the electron count, to find Z.

求原子或离子的核子数时，使用关系式 A = Z + N。在中性原子中，原子序数等于电子数，因此若已知中性原子的电子数，可直接确定 Z。但对于离子，电子数因得失电子而不等于质子数；必须利用核电荷（即质子数）来确定 Z，而不能用电子数。

The neutron number is not normally shown on the periodic table. It can be calculated by subtracting the atomic number from the nucleon number:

N = A − Z

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