Constituents of the Atom | 原子的组成

📚 Constituents of the Atom | 原子的组成

The atom is the fundamental building block of ordinary matter, and understanding its constituents is the first essential step in A-Level nuclear and particle physics. In this topic, you will learn about the protons, neutrons and electrons that make up atoms, the meaning of proton number and nucleon number, the concept of isotopes, and the fundamental particles — quarks and leptons — that lie beneath the structure of the nucleus.

原子是普通物质的基本组成单元，理解其组成是A-Level核物理与粒子物理的第一步。在本主题中，你将学习构成原子的质子、中子和电子，质子数与核子数的含义，同位素的概念，以及更深层的基本粒子——夸克与轻子。

1. The Atom: A Historical Overview | 原子：历史概览

In the early 20th century, Ernest Rutherford’s gold-foil experiment demonstrated that the atom consists of a tiny, dense, positively charged nucleus surrounded by a cloud of negatively charged electrons. This nuclear model replaced the earlier plum-pudding model proposed by J.J. Thomson, which had suggested that positive charge was spread uniformly throughout the atom with electrons embedded within it.

20世纪初，欧内斯特·卢瑟福的金箔实验证明，原子由一个微小、致密、带正电荷的原子核和周围带负电荷的电子云组成。这一核模型取代了J.J.汤姆孙提出的葡萄干布丁模型，后者认为正电荷均匀分布在整个原子中，电子嵌在其中。

The nucleus itself is composed of two types of nucleons: protons and neutrons. The electrons occupy quantised energy levels around the nucleus and are bound to it by the electrostatic attraction between opposite charges. Although the nucleus occupies only about 10⁻¹⁵ of the atom’s volume, it contains more than 99.9% of its mass.

原子核本身由两种核子组成：质子和中子。电子占据核外量子化的能级，并通过异种电荷之间的静电引力与核结合。尽管原子核的体积只占原子总体积的约10⁻¹⁵，却包含了超过99.9%的质量。

2. Protons, Neutrons and Electrons | 质子、中子和电子

The three constituents of the atom are the proton, the neutron and the electron. Each has distinct properties of charge and mass that must be memorised precisely for examinations.

原子的三种组成粒子是质子、中子和电子。每种粒子都有独特的电荷和质量性质，考试中需要精确记忆。

• Proton: charge +1.60 × 10⁻¹⁹ C, mass 1.673 × 10⁻²⁷ kg. It is written in nuclide notation as ¹₁p or ¹₁H.

  质子：电荷 +1.60 × 10⁻¹⁹ C，质量 1.673 × 10⁻²⁷ kg。核素记法写作 ¹₁p 或 ¹₁H。

• Neutron: charge 0 C, mass 1.675 × 10⁻²⁷ kg. It is written as ¹₀n.

  中子：电荷 0 C，质量 1.675 × 10⁻²⁷ kg。写作 ¹₀n。

• Electron: charge −1.60 × 10⁻¹⁹ C, mass 9.11 × 10⁻³¹ kg. It is written as ⁰₋₁e.

  电子：电荷 −1.60 × 10⁻¹⁹ C，质量 9.11 × 10⁻³¹ kg。写作 ⁰₋₁e。

Notice that the proton and neutron have very similar masses, whereas the electron is approximately 1836 times lighter than the proton. For this reason, nearly all of the atom’s mass is concentrated in the nucleus. The charges of the proton and electron are equal in magnitude but opposite in sign, which is why a neutral atom contains equal numbers of each.

注意，质子与中子的质量非常接近，而电子质量约为质子质量的1/1836。因此，原子质量的绝大部分集中在原子核内。质子与电子的电荷量值相等而符号相反，这就是为什么中性原子中两者的数目相等。

3. Proton Number and Nucleon Number | 质子数与核子数

Every element is defined by its proton number Z, also called the atomic number, which is the number of protons in the nucleus. The nucleon number A, also called the mass number, is the total number of protons and neutrons combined. The neutron number N is therefore given by:

每个元素由其质子数Z（也称为原子序数）定义，即原子核中质子的数目。核子数A（也称为质量数）是质子和中子的总数。因此中子数N由下式给出：

N = A − Z

A neutral atom has an equal number of protons and electrons, so the proton number also tells us the number of electrons in the atom. This is why the proton number dictates the element’s chemical behaviour — it determines the electron configuration.

中性原子中质子数与电子数相等，因此质子数也告诉我们原子中电子的数目。这就是为什么质子数决定元素的化学行为——它决定了电子排布。

In standard nuclide notation, the nucleon number is written as a superscript and the proton number as a subscript, both to the left of the element symbol. For example, carbon-12 is written as ¹²₆C, indicating A = 12 and Z = 6. The number of neutrons is then N = 12 − 6 = 6.

在标准核素记法中，核子数写在元素符号左上角作为上标，质子数写在左下角作为下标。例如，碳-12写作¹²₆C，表示A = 12，Z = 6。中子数即为N = 12 − 6 = 6。

4. Isotopes | 同位素

Isotopes are nuclei with the same number of protons (same Z) but different numbers of neutrons (different N), and therefore different nucleon numbers A. Isotopes of the same element have identical chemical properties because chemistry is governed by the electron configuration, which depends only on Z.

同位素是具有相同质子数（相同的Z）但不同中子数（不同的N），因而核子数A也不同的原子核。同一元素的同位素具有完全相同的化学性质，因为化学性质由电子排布决定，而电子排布仅取决于Z。

For example, hydrogen exists in three naturally occurring isotopic forms:

例如，氢存在三种天然的同位素形式：

• Protium (¹₁H): one proton, zero neutrons.

  氕（¹₁H）：一个质子，零个中子。

• Deuterium (²₁H): one proton, one neutron.

  氘（²₁H）：一个质子，一个中子。

• Tritium (³₁H): one proton, two neutrons.

  氚（³₁H）：一个质子

  Published by TutorHao | A-Level Physics Revision Series | aleveler.com

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