📚 Understanding Atomic Structure: Protons, Neutrons, and Electrons | 理解原子结构：质子、中子与电子
In the Edexcel IGCSE Science specification, atomic structure is the foundation of both chemistry and physics. You need to describe the atom using a simple model, recall the properties of subatomic particles, define atomic number and mass number, interpret nuclear symbols, calculate relative atomic mass, and deduce electronic configurations. This article covers all these skills step by step and focuses on the types of questions you are likely to meet in the Edexcel IGCSE examination.
在 Edexcel IGCSE 科学考纲中，原子结构是化学和物理的共同基础。你需要用简单模型描述原子，记住亚原子粒子的性质，定义原子序数和质量数，解读核素符号，计算相对原子质量，并推导电子排布。本文将一步步全面覆盖这些考点，并聚焦于 Edexcel IGCSE 考试中常见的题型。
1. What Is an Atom? | 什么是原子？
An atom is the smallest particle of an element that can take part in a chemical reaction. Every element is made up of atoms, and each atom consists of a central nucleus surrounded by electrons. Atoms are normally neutral because the number of protons in the nucleus is equal to the number of electrons moving around the nucleus. For example, a carbon atom has six protons and six electrons, so its overall charge is zero.
原子是元素参与化学反应的最小粒子。每种元素都由原子组成，每个原子由一个中心原子核以及核外电子构成。原子通常呈电中性，因为原子核中的质子数等于核外运动的电子数。例如，碳原子有六个质子和六个电子，因此总电荷为零。
Although atoms are extremely small, they have a structure that can be modelled and predicted. At IGCSE level, you are expected to use the nuclear model of the atom: a tiny, dense, positively charged nucleus containing protons and neutrons, surrounded by negatively charged electrons in energy levels.
尽管原子极其微小，但它们的结构可以被模拟和预测。在 IGCSE 阶段，你需要使用原子核模型：一个微小、致密、带正电的原子核，内含质子和中子，周围是分层排列的带负电的电子。
2. The Nuclear Model of the Atom | 原子的核模型
The modern nuclear model was developed from Ernest Rutherford’s gold-foil experiment in 1911. In this experiment, positively charged alpha particles were fired at a very thin gold foil. Most passed straight through, which showed that atoms are mostly empty space. A small number were deflected at large angles, and a very few bounced back. This could only be explained if there was a tiny, dense, positively charged nucleus at the centre of the atom.
现代核模型源于 1911 年欧内斯特·卢瑟福的金箔实验。在这个实验中，带正电的 α 粒子被射向极薄的金箔。大多数粒子直接穿过，这表明原子内部大部分是空的。少数粒子以大角度偏转，极少数甚至被反弹回来。这只能用原子中心存在一个微小、致密、带正电的原子核来解释。
Later experiments identified protons and neutrons inside the nucleus. Protons are positively charged, neutrons have no charge, and both have a relative mass of about 1. Electrons were found to occupy distinct energy levels, or shells, around the nucleus. This simple nuclear model is the one needed for the Edexcel IGCSE course.
后来的实验在原子核中发现了质子和中子。质子带正电，中子不带电，两者的相对质量都约为 1。电子分布在原子核周围不同的能级，也就是电子壳层中。这个简单的核模型正是 Edexcel IGCSE 课程所需要的。
3. The Three Subatomic Particles | 三种亚原子粒子
You must be able to recall the relative charge and relative mass of the proton, neutron and electron. The table below summarises the key data.
你需要能够记住质子、中子和电子的相对电荷与相对质量。下表总结了关键数据。
| Particle | Symbol | Relative Charge | Relative Mass | Location |
| Proton | p⁺ | +1 | 1 | Nucleus |
| Neutron | n⁰ | 0 | 1 | Nucleus |
| Electron | e⁻ | -1 | 1/1836 ≈ 0.0005 | Electron shells |
Protons and neutrons are found in the nucleus and therefore are called nucleons. The electron has a negligible mass compared with a proton or neutron, but its negative charge exactly balances the positive charge of a proton in a neutral atom.
质子和中子位于原子核中，因此被称为核子。电子的质量与质子或中子相比可以忽略不计，但它的负电荷在电中性原子中恰好抵消一个质子的正电荷。
A common exam point is that the atomic number is equal to the number of protons, not the number of electrons or neutrons. In a neutral atom, the number of electrons is equal to the number of protons, but this can change when an ion forms.
一个常见考点是：原子序数等于质子数，而不是电子数或中子数。在电中性原子中，电子数等于质子数，但在形成离子时会发生改变。
4. Atomic Number and Mass Number | 原子序数与质量数
The atomic number, sometimes called the proton number, is the number of protons in the nucleus of an atom. The mass number is the total number of protons and neutrons in the nucleus. The relationship can be written as:
原子序数，也叫质子数，是指原子核中的质子数。质量数是指原子核中质子数和中子数的总和。它们的关系可以写成：
mass number = protons + neutrons
atomic number = protons
number of neutrons = mass number − atomic number
For example, a sodium atom has 11 protons and 12 neutrons. Its atomic number is 11, and its mass number is 23. It can be written using the standard nuclear notation:
例如，一个钠原子有 11 个质子和 12 个中子。它的原子序数是 11，质量数是 23。它可以写成标准的核素符号：
²³₁₁Na
In this symbol, the top number is the mass number and the bottom number is the atomic number. For a neutral sodium atom, the number of electrons is also 11. Using this notation, carbon-12 is written as ¹²₆C, and carbon-14 is written as ¹⁴₆C.
在这个符号中，上面的数字是质量数，下面的数字是原子序数。对于电中性钠原子，电子数也等于 11。使用这种符号，碳-12 写作 ¹²₆C，碳-14 写作 ¹⁴₆C。
5. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because they have the same number of protons, they also have the same atomic number, but their mass numbers differ.
同位素是指同一种元素中具有相同质子数但不同中子数的原子。由于它们质子数相同，原子序数也相同，但质量数不同。
For example, chlorine has two common isotopes: chlorine-35 and chlorine-37. Both have 17 protons, but chlorine-35 has 18 neutrons, while chlorine-37 has 20 neutrons. Isotopes of the same element have the same chemical properties because they have the same electron arrangement.
例如，氯元素有两种常见同位素：氯-35 和氯-37。它们都有 17 个质子，但氯-35 有 18 个中子，而氯-37 有 20 个中子。同种元素的同位素具有相同的化学性质，因为它们具有相同的电子排布。
You should also be able to represent isotopes using nuclear notation. Hydrogen has three isotopes: ¹H, ²H and ³H. All have one proton, but they contain zero, one and two neutrons respectively.
你还应该能用核素符号表示同位素。氢有三种同位素：¹H、²H 和 ³H。它们都含有一个质子，但中子数分别为零、一和二。
6. Electronic Configuration | 电子排布
Electrons are arranged in shells, also called energy levels. The lowest energy level is filled first. At IGCSE level, you should be able to use the simple rule 2, 8, 8 for the first three shells. The number of electrons in each shell can be written as a sequence separated by commas.
电子排列在壳层中，也就是能级中。能量最低的壳层首先被填满。在 IGCSE 阶段，你需要掌握简单的 2、8、8 排布规则。每个壳层中的电子数可以用逗号分隔的数列表示。
For example:
例如：
- Sodium (Na): 11 electrons, configuration 2,8,1
- 钠（Na）： 11 个电子，排布为 2,8,1
- Chlorine (Cl): 17 electrons, configuration 2,8,7
- 氯（Cl）： 17 个电子，排布为 2,8,7
- Argon (Ar): 18 electrons, configuration 2,8,8
- 氩（Ar）： 18 个电子，排布为 2,8,8
The outermost shell is called the valence shell, and the electrons in it are called valence electrons. These electrons are responsible for chemical bonding. Elements in the same group have the same number of outer-shell electrons.
最外层电子壳层称为价电子壳层，其中的电子称为价电子。价电子决定元素的化学成键方式。同一族的元素具有相同的最外层电子数。
2, 8, 8… rule
7. Ions and the Octet Rule | 离子与八隅体规则
Atoms form ions by losing or gaining electrons. They do this to obtain a full outer shell, usually eight electrons, which is the stable electronic configuration of a noble gas. This idea is called the octet rule.
原子通过失去或获得电子来形成离子。它们这样做是为了获得全满的最外层，通常是八个电子，也就是稀有气体元素所具有的稳定电子排布，这一观点称为八隅体规则。
Metal atoms, such as sodium, lose electrons to form positive ions called cations. Sodium has the electronic configuration 2,8,1. When it loses its one outer electron, it becomes a sodium ion with the configuration 2,8:
金属原子，如钠，失去电子形成正离子，称为阳离子。钠的电子排布是 2,8,1。当它失去一个外层电子后，形成电子排布为 2,8 的钠离子：
Na → Na⁺ + e⁻
Non-metal atoms, such as chlorine, gain electrons to form negative ions called anions. Chlorine has the configuration 2,8,7. It gains one electron to fill its outer shell, becoming a chloride ion with the configuration 2,8,8:
非金属原子，如氯，获得电子形成负离子，称为阴离子。氯的电子排布是 2,8,7。它获得一个电子来填满最外层，形成电子排布为 2,8,8 的氯离子：
Cl + e⁻ → Cl⁻
The charge on an ion is equal to the number of electrons lost or gained. Group 1 metals form 1+ ions, group 2 metals form 2+ ions, group 6 non-metals form 2− ions, and group 7 non-met
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