📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
Welcome to this revision guide on atomic structure and the periodic table. This topic forms the foundation of all chemistry and appears in every IGCSE Science paper. By the end of this article, you will be able to describe the structure of an atom, calculate the number of subatomic particles, and explain how the periodic table is organised.
欢迎阅读本篇关于原子结构和元素周期表的复习指南。这一主题是化学的基础，在每一份 IGCSE 科学试卷中都会出现。学完本文后，你将能够描述原子结构、计算亚原子粒子的数目，并解释周期表是如何排列的。
1. The Building Blocks of Matter | 物质的构成单元
All matter is made of atoms. An atom is the smallest particle of an element that keeps the chemical identity of that element. In IGCSE chemistry, you need to picture the atom as a tiny, dense nucleus surrounded by electrons moving in energy levels called shells.
一切物质都由原子构成。原子是保持元素化学性质的最小微粒。在 IGCSE 化学中，你需要把原子想象成一个微小而致密的原子核，周围是在称为电子壳层的能级中运动的电子。
The nucleus contains protons and neutrons, which are collectively called nucleons. Protons carry a positive charge, neutrons carry no charge, and electrons carry a negative charge. Since the atom is neutral overall, the number of protons must equal the number of electrons.
原子核中包含质子和中子，统称为核子。质子带正电，中子不带电，电子带负电。由于整个原子呈电中性，质子数必须等于电子数。
| Particle | Relative Charge | Relative Mass |
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | 1/1840 ≈ 0.0005 |
Notice that the relative mass of an electron is only about 1/1840 that of a proton, so almost all of the atom's mass is concentrated in the nucleus. This is why the nucleus is described as dense.
注意电子的相对质量仅为质子的约1/1840，因此原子几乎全部质量都集中在原子核内。这就是为什么原子核被描述为致密。
2. Atomic Number and Mass Number | 原子序数与质量数
Every element can be identified by its atomic number, sometimes called the proton number. The atomic number (Z) is the number of protons in the nucleus, and it is this value that decides which element the atom belongs to.
每种元素都可以通过其原子序数（有时也叫质子数）来识别。原子序数（Z）是原子核中的质子数，正是这个数值决定了原子属于哪种元素。
The mass number (A) is the total number of protons and neutrons in the nucleus. Therefore the number of neutrons can be calculated simply as A − Z.
质量数（A）是原子核中质子数与中子数的总和。因此中子数可以简单地用 A − Z 算出。
12 neutrons. In exam questions, always circle the two numbers and subtract them carefully.
例如，钠的原子序数为11，质量
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