📚 Atomic Structure | 原子结构
Welcome to this TutorHao revision guide on atomic structure. This topic is a cornerstone of the Edexcel IGCSE Science (Double Award) specification, and a solid understanding of it will help you master chemical bonding, the periodic table, and radioactivity.
欢迎阅读 TutorHao 关于原子结构的复习指南。本主题是 Edexcel IGCSE 科学（双奖）考纲的基石，牢固掌握它有助于你理解化学键、周期表和放射性等内容。
1. The Atom | 原子
An atom is the smallest particle of an element that still shows the chemical properties of that element. Atoms are incredibly small: a typical atom has a diameter of about 1 × 10⁻¹⁰ m.
原子是保持元素化学性质的最小微粒。原子极小：典型原子的直径约为 1 × 10⁻¹⁰ m。
Every atom has a central nucleus, which contains protons and neutrons. The nucleus is surrounded by electrons moving in shells (energy levels). The nucleus carries almost all of the atom’s mass but occupies very little of its volume.
每个原子都有一个中心原子核，核内含质子和中子。电子在核外的壳层（能级）中运动。原子核几乎集中了原子的全部质量，但只占据原子体积的极小部分。
In a neutral atom, the number of protons equals the number of electrons, so the overall charge is zero. If the number of protons and electrons were different, the particle would be an ion, not a neutral atom.
在电中性原子中，质子数等于电子数，因此整体不带电。如果质子数与电子数不同，该微粒就是离子而非电中性原子。
2. Subatomic Particles | 亚原子粒子
The three key subatomic particles are the proton, the neutron, and the electron. You must know their relative charges and relative masses perfectly for the exam.
三种重要的亚原子粒子是质子、中子和电子。你必须准确牢记它们的相对电荷和相对质量。
| Particle 粒子 | Relative Charge 相对电荷 | Relative Mass 相对质量 |
| Proton 质子 | +1 | 1 |
| Neutron 中子 | 0 | 1 |
| Electron 电子 | -1 | 1/1840 (taken as 0) |
Protons and neutrons are found in the nucleus and are collectively called nucleons. Electrons move in shells around the nucleus and have almost no mass compared with protons and neutrons.
质子和中子位于原子核内，统称为核子。电子在核外壳层中运动，与质子、中子相比，其质量几乎可以忽略不计。
A useful memory aid: the mass number equals protons plus neutrons, and the electron has the same size of charge as the proton but with the opposite sign.
一个有用的记忆技巧：质量数等于质子数加中子数；电子所带电荷的大小与质子相同，但符号相反。
3. Atomic Number and Mass Number | 原子序数与质量数
The atomic number (Z) is the number of protons in the nucleus. For a neutral atom, the atomic number also equals the number of electrons.
原子序数（Z）是原子核中的质子数。对于电中性原子，原子序数也等于电子数。
The mass number (A) is the total number of protons and neutrons in the nucleus.
质量数（A）是原子核中质子数与中子数之和。
Mass number A = number of protons + number of neutrons
质量数 A = 质子数 + 中子数
Atomic number Z = number of protons = number of electrons
原子序数 Z = 质子数 = 电子数
In nuclear notation, the mass number A is written as a superscript and the atomic number Z as a subscript, both on the left of the element symbol. For example, carbon-12 is written as ¹²₆C.
在核素符号中，质量数 A 写在左上角上标位置，原子序数 Z 写在左下角下标位置。例如，碳-12 写作 ¹²₆C。
To find the number of neutrons, subtract the atomic number from the mass number:
要求中子数，用质量数减去原子序数即可：
Number of neutrons = A – Z
中子数 = A – Z
4. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. This means they share the same atomic number but have different mass numbers.
同位素是同一元素的不同原子，它们具有相同的质子数，但中子数不同。这意味着它们具有相同的原子序数，但质量数不同。
For example, chlorine has two naturally occurring isotopes: ³⁵Cl (17 protons, 18 neutrons) and ³⁷Cl (17 protons, 20 neutrons). Both behave identically in chemical reactions because chemical properties depend on the number and arrangement of electrons.
例如，氯有两种天然存在的同位素：³⁵Cl（17 个质子，18 个中子）和 ³⁷Cl（17 个质子，20 个中子）。两者在化学反应中表现完全相同，因为化学性质取决于电子的数量和排列方式。
Key facts about isotopes:
关于同位素的要点：
- Isotopes of an element have identical chemical properties.
- They may have slightly different physical properties, such as density and rate of diffusion.
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