📚 509: Atomic Structure and the Periodic Table | 509：原子结构与周期表
Atoms are the fundamental building blocks of all matter, and understanding their structure is the key to unlocking the patterns behind chemical behaviour. In IGCSE Science, you will explore how protons, neutrons and electrons are arranged, how we represent atoms with symbols and numbers, and how the Periodic Table organises elements into groups and periods that reveal repeating trends.
原子是构成所有物质的基本单位，理解原子结构是打开化学行为规律之门的钥匙。在 IGCSE 科学课程中，你将探索质子、中子与电子如何排布，如何用符号和数字表示原子，以及周期表如何将元素按族和周期组织起来，展现出重复出现的趋势。
1. What is an Atom? | 什么是原子？
An atom is the smallest part of an element that can take part in a chemical reaction. It consists of a central nucleus surrounded by a cloud of electrons. The nucleus contains protons and neutrons, while electrons orbit in shells or energy levels around the nucleus. Most of the atom is empty space, and the overall charge is neutral because the number of protons equals the number of electrons.
原子是能参与化学反应的元素最小单位。它由一个中心原子核和包围它的电子云组成。原子核包含质子和中子，电子则在核外的电子层或能级上运动。原子内部绝大部分是空的，且整体呈电中性，因为质子数与电子数相等。
2. Subatomic Particles | 亚原子粒子
The three key subatomic particles you need to remember are protons, neutrons and electrons. Their relative masses and charges are summarised in the table below. Protons have a relative mass of 1 and a charge of +1. Neutrons also have a mass of 1 but no charge (0). Electrons have a negligible mass (often taken as 0) and a charge of −1.
你需要记住三种关键的亚原子粒子：质子、中子和电子。它们的相对质量与电荷总结如下表。质子的相对质量为 1，电荷为 +1。中子的质量也是 1，但不带电荷 (0)。电子的质量可忽略不计（常视为 0），电荷为 −1。
| Particle 粒子 | Relative mass 相对质量 | Relative charge 相对电荷 |
|---|---|---|
| Proton 质子 | 1 | +1 |
| Neutron 中子 | 1 | 0 |
| Electron 电子 | ∼0 (negligible) | −1 |
3. Atomic Number and Mass Number | 原子序数和质量数
Every element is defined by its atomic number (Z), which is the number of protons in the nucleus. The mass number (A) is the total number of protons plus neutrons. You will often see an element written with the mass number as a superscript and the atomic number as a subscript, for example ¹²₆C. To find the number of neutrons, subtract the atomic number from the mass number.
每种元素由其原子序数 (Z) 定义，即原子核内质子的数量。质量数 (A) 是质子数与中子数之和。你常会见到用上标标出质量数、下标标出原子序数的写法，如 ¹²₆C。要计算中子数，就用质量数减去原子序数。
Number of neutrons = Mass number − Atomic number
中子数 = 质量数 − 原子序数
4. Isotopes | 同位素
Isotopes are atoms of the same element that have the same atomic number but different mass numbers. This means they contain the same number of protons but different numbers of neutrons. For example, chlorine has two common isotopes: ³⁵Cl (17 protons, 18 neutrons) and ³⁷Cl (17 protons, 20 neutrons). Isotopes share identical chemical properties because they have the same electron arrangement, but their physical properties such as mass differ slightly.
同位素是同一元素中原子序数相同而质量数不同的原子。也就是说，它们的质子数相同但中子数不同。例如，氯有两种常见同位素：³⁵Cl（17个质子，18个中子）与 ³⁷Cl（17个质子，20个中子）。同位素由于电子排布相同而具有完全相同的化学性质，但它们的物理性质（如质量）略有差异。
5. Electronic Configuration | 电子排布
Electrons occupy shells (energy levels) around the nucleus. The first shell can hold up to 2 electrons, the second up to 8, and the third also holds up to 8 for the first 20 elements. The electronic configuration of an atom is written as a series of numbers showing how many electrons are in each shell, e.g. sodium (Na) with 11 electrons has the configuration 2,8,1. The arrangement determines an element’s chemical reactivity.
电子占据原子核外的电子层（能级）。第一层最多容纳 2 个电子，第二层最多容纳 8 个，前 20 号元素的第三层同样最多容纳 8 个。原子的电子排布用一串数字表示，标明每一层的电子数目，例如钠 (Na) 有 11 个电子，排布为 2,8,1。电子排布决定了元素的化学活泼性。
6. The Periodic Table Overview | 周期表概览
The modern Periodic Table arranges elements in order of increasing atomic number. It is organised into horizontal rows called periods and vertical columns called groups. Elements in the same group have the same number of electrons in their outer shell, which gives them similar chemical properties. This structure allows scientists to predict the behaviour of unfamiliar elements.
现代周期表按原子序数递增的顺序排列元素。它由水平的行（周期）和垂直的列（族）组成。同一族的元素具有相同的最外层电子数，因此化学性质相似。这种结构使科学家能够预测未接触过的元素的行为。
7. Groups and Periods | 族与周期
Going across a period, the atomic number increases, and the number of outer electrons increases, while the atoms get slightly smaller. Moving down a group, the number of electron shells increases, so atoms become larger. The group number (for Groups 1 to 8) tells you the number of electrons in the outermost shell. For example, Group 2 elements all have 2 outer electrons.
在同一个周期中从左到右，原子序数递增，最外层电子数增加，同时原子半径略微减小。沿同一族从上到下，电子层数增多，因此原子半径增大。族号（对于第 1 至第 8 族）代表最外层电子数。例如第 2 族元素的最外层都是 2 个电子。
8. Metals and Non-metals | 金属与非金属
Metals are found on the left and centre of the Periodic Table, while non-metals are located on the right. Metals typically have high melting points, conduct electricity and heat, and are malleable. Non-metals are usually poor conductors, have lower boiling points, and are brittle when solid. The dividing line starts around boron and steps down toward astatine, with elements near this staircase often showing intermediate properties as metalloids.
金属位于周期表的左侧和中部，非金属位于右侧。金属通常具有高熔点，能导电和导热，并具有延展性。非金属一般是不良导体，沸点较低，固体状态下较脆。分界线大致从硼开始，呈阶梯状延伸至砹；靠近这条阶梯的元素通常表现出类金属的中间性质。
9. Group 1 – The Alkali Metals | 第1族 – 碱金属
Group 1 elements (lithium, sodium, potassium, rubidium, caesium) are known as the alkali metals. They all have one electron in the outer shell, making them very reactive. Reactivity increases down the group because the outer electron is further from the nucleus and more easily lost. They react vigorously with water to form a metal hydroxide and hydrogen gas, for example: sodium + water → sodium hydroxide + hydrogen.
第 1 族元素（锂、钠、钾、铷、铯）被称为碱金属。它们最外层都有一个电子，因此非常活泼。从上到下活泼性增强，因为最外层电子离核越来越远，更容易失去。它们与水剧烈反应，生成金属氢氧化物和氢气，例如：钠 + 水 → 氢氧化钠 + 氢气。
10. Group 7 – The Halogens | 第7族 – 卤素
Group 7 elements (fluorine, chlorine, bromine, iodine, astatine) are the halogens. They have seven electrons in their outer shell, so they need to gain one electron to achieve a full outer shell. Reactivity decreases down the group because the incoming electron is further from the nucleus. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Halogens react with alkali metals to form ionic compounds called halides.
第 7 族元素（氟、氯、溴、碘、砹）被称为卤素。它们最外层有七个电子，因此需要获得一个电子来达到满层。活泼性从上到下递减，因为获取的电子离核越来越远。室温下，氟和氯是气体，溴为液体，碘为固体。卤素与碱金属反应生成叫做卤化物的离子化合物。
11. Group 0 – Noble Gases | 第0族 – 稀有气体
The Group 0 elements (helium, neon, argon, krypton, xenon, radon) are the noble gases. They have a full outer shell of electrons, which makes them extremely unreactive, or inert. They exist as single atoms (monatomic) and have very low boiling points. The boiling points increase down the group as the atoms become larger and the intermolecular forces strengthen.
第 0 族元素（氦、氖、氩、氪、氙、氡）是稀有气体。它们拥有全满的最外层电子，因此极其不活泼，即惰性。它们以单原子形式存在，沸点非常低。沸点从上到下逐渐升高，因为原子变大，分子间作用力增强。
12. Trends in the Periodic Table | 周期表中的趋势
Key periodic trends include decreasing atomic radius across a period, increasing atomic radius down a group, and the variation in metallic character. Metallic character decreases from left to right across a period, meaning elements become less likely to lose electrons and more likely to gain them. Electronegativity, the ability of an atom to attract bonding electrons, also increases across a period and decreases down a group. Recognising these patterns helps you compare and contrast the behaviour of different elements.
周期表中的主要趋势包括：同一周期内原子半径递减，同一族内原子半径递增，以及金属性的变化。金属性在一个周期中从左到右减弱，意味着元素失去电子的倾向变小，获得电子的倾向增大。电负性（原子吸引键合电子的能力）在同一周期内递增，在同一族内递减。识别这些规律有助于你比较和对照不同元素的行为。
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