📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
Understanding atomic structure is the foundation of all IGCSE Chemistry. Every element, compound, and reaction can be explained by the arrangement and behaviour of atoms. This guide covers the essential concepts, from subatomic particles to the patterns of the Periodic Table, with clear explanations in both English and Chinese.
理解原子结构是所有 IGCSE 化学的基础。每一种元素、化合物和反应都可以通过原子的排列和行为来加以解释。本指南涵盖关键概念，从亚原子粒子到元素周期表的规律，提供中英双语简明清晰的讲解。
1. The Components of an Atom | 原子的组成
Every atom is made up of three main subatomic particles: protons, neutrons, and electrons. Protons and neutrons are tightly packed inside the central nucleus, while electrons orbit the nucleus in shells (energy levels).
每个原子都由三种主要的亚原子粒子构成：质子、中子和电子。质子和中子紧密地存在于原子中心的原子核内，而电子则在原子核外的电子壳层（能级）中运动。
| Particle | Charge | Relative Mass | Location |
| Proton (质子) | +1 | 1 | Nucleus (原子核) |
| Neutron (中子) | 0 | ≈1 | Nucleus (原子核) |
| Electron (电子) | -1 | ≈1/1836 | Shells (电子壳层) |
An atom is electrically neutral because the number of protons equals the number of electrons. The positive charge of the protons exactly balances the negative charge of the electrons.
原子是电中性的，因为质子数等于电子数。质子的正电荷与电子的负电荷恰好相互抵消。
2. Atomic Number and Mass Number | 原子序数与质量数
The number of protons in an atom is called the atomic number (Z). It defines the identity of an element. For example, any atom with 6 protons is carbon, and any atom with 8 protons is oxygen.
原子中所含质子数称为原子序数（Z），它决定了元素的种类。例如，含有 6 个质子的原子一定是碳，含有 8 个质子的原子一定是氧。
The mass number (A) is the total number of protons and neutrons in the nucleus. Because electrons have negligible mass, they are not included in the mass number.
质量数（A）是原子核中质子数和中子数的总和。由于电子质量可忽略不计，因此它们不计入质量数。
mass number = number of protons + number of neutrons
质量数 = 质子数 + 中子数
When writing an element in nuclear notation, the mass number is placed as a superscript before the symbol, and the atomic number as a subscript. For example, carbon-14 is written as ¹⁴₆C. Here, 6 is the atomic number, and 14 is the mass number.
书写元素核素符号时，质量数以左上标形式放在元素符号前，原子数以下标形式放在左下方。例如，碳-14 写作 ¹⁴₆C。其中 6 是原子序数，14 是质量数。
3. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Consequently, they have the same atomic number but 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。因此，它们的原子序数相同而质量数不同。
A classic example is the element carbon. Carbon-12 (¹²₆C) has 6 protons and 6 neutrons. Carbon-13 (¹³₆C) has 6 protons and 7 neutrons. Carbon-14 (¹⁴₆C) has 6 protons and 8 neutrons. All are isotopes of carbon.
一个典型的例子是碳元素。碳-12（¹²₆C）有 6 个质子和 6 个中子；碳-13（¹³₆C）有 6 个质子和 7 个中子；碳-14（¹⁴₆C）有 6 个质子和 8 个中子。它们都是碳的同位素。
Isotopes have identical chemical properties because they share the same electron configuration. However, their physical properties may differ, such as mass and density. Some isotopes are unstable and emit radiation – these are radioactive isotopes.
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。然而，它们的物理性质可能不同，例如质量和密度。有些同位素不稳定并能放出辐射，这些称为放射性同位素。
- Carbon-14 is used in archaeology to date organic remains. | 碳-14 用于考古学中测定有机遗骸的年代。
- Cobalt-60 is used in cancer radiotherapy. | 钴-60 用于癌症放射治疗。
- Uranium-235 is used as fuel in nuclear reactors. | 铀-235 用作核反应堆的燃料。
4. Relative Atomic Mass | 相对原子质量
Most elements exist as a mixture of isotopes. Therefore, the mass quoted in the Periodic Table is a weighted average of the masses of all naturally occurring isotopes. This value is called the relative atomic mass (Aᵣ).
大多数元素以多种同位素的混合物形式存在。因此，元素周期表中所列的质量是所有天然同位素质量的加权平均值，该值称为相对原子质量（Aᵣ）。
To calculate Aᵣ, multiply each isotope’s relative mass by its relative abundance (as a fraction), then sum the results.
要计算相对原子质量，需将每种同位素的相对质量乘以其相对丰度（以小数表示），然后求和。
Aᵣ = Σ (isotope mass × relative abundance)
相对原子质量 = Σ（同位素质量 × 相对丰度）
Example: Chlorine contains 75% chlorine-35 (mass 35) and 25% chlorine-37 (mass 37). Its Aᵣ = (35 × 0.75) + (37 × 0.25) = 26.25 + 9.25 = 35.5.
示例：氯元素含 75% 的氯-35（质量 35）和 25% 的氯-37（质量 37）。其相对原子质量 =（35 × 0.75）+（37 × 0.25）= 26.25 + 9.25 = 35.5。
5. Electron Configuration | 电子排布
Electrons are arranged in shells around the nucleus. Each shell can hold a maximum number of electrons: the first shell holds 2, the second shell holds 8, and the third shell also holds 8 for the first 20 elements.
电子在原子核周围分层排列。每个壳层所能容纳的最大电子数为：第一层容纳 2 个，第二层容纳 8 个，对于前 20 号元素，第三层也容纳 8 个。
- Shell 1 (n=1): maximum 2 electrons | 第一层（n=1）：最多 2 个电子
- Shell 2 (n=2): maximum 8 electrons | 第二层（n=2）：最多 8 个电子
- Shell 3 (n=3): maximum 8 electrons (for elements 1–20) | 第三层（n=3）：最多 8 个电子（对前 20 号元素）
To determine the electron configuration of an atom, fill the shells from the inside outward. For example, sodium (Na) has 11 electrons: 2 in the first shell, 8 in the second, and 1 in the third. This can be written as 2,8,1.
要确定原子的电子排布，从内向外填充各壳层。例如，钠（Na）有 11 个电子：第一层 2 个，第二层 8 个，第三层 1 个，可写作 2,8,1。
The outermost shell is called the valence shell, and the electrons in it are called valence electrons. These determine the atom’s chemical reactivity.
最外层壳称为价电子层，其中的电子称为价电子。它们决定了原子的化学活性。
6. Periods and Groups in the Periodic Table | 元素周期表中的周期与族
The modern Periodic Table arranges elements by increasing atomic number. The horizontal rows are called periods; the vertical columns are called groups.
现代元素周期表按原子序数递增排列。横排称为周期，纵列称为族。
Each period corresponds to the number of electron shells used by the element. For example, elements in Period 2 have two shells of electrons. Each group contains elements with the same number of valence electrons, so they have similar chemical properties.
每个周期对应元素所用的电子层数。例如，第二周期元素具有两个电子壳层。每个族中的元素具有相同的价电子数，因此化学性质相似。
| Group Number | Number of Valence Electrons | Example Element |
| 1 (Alkali Metals) | 1 | Na (sodium) |
| 2 (Alkaline Earth Metals) | 2 | Mg (magnesium) |
| 7 (Halogens) | 7 | Cl (chlorine) |
| 0 (Noble Gases) | 8 (except He: 2) | Ar (argon) |
7. Group 1 – Alkali Metals | 第 1 族——碱金属
Group 1 elements include lithium, sodium, and potassium. They are soft, low-density metals that react with water to form alkaline solutions and hydrogen gas.
第 1 族元素包括锂、钠和钾。它们是质软、密度低的金属，能与水反应生成碱性溶液和氢气。
All alkali metals have one valence electron. They easily lose this electron to form a positive ion with a charge of +1. As you go down the group, the reactivity increases because the outer electron is further from the nucleus and more easily lost.
所有碱金属都有一个价电子。它们容易失去该电子，形成带 +1 电荷的阳离子。从上到下，金属性增强，因为最外层电子离原子核更远，更容易失去。
For example, the reaction of sodium with water is vigorous, while lithium reacts steadily, and potassium reacts even more violently.
例如，钠与水的反应较为剧烈，锂反应平稳，而钾反应更加猛烈。
8. Group 7 – The Halogens | 第 7 族——卤素
Group 7 elements are fluorine, chlorine, bromine, iodine, and astatine. They are non-metals and exist as diatomic molecules, such as Cl₂, Br₂, and I₂.
第 7 族元素包括氟、氯、溴、碘和砹。它们是非金属，以双原子分子形式存在，如 Cl₂、Br₂ 和 I₂。
Halogens have seven valence electrons. They gain one electron to complete their outer shell, forming a negatively charged ion with a charge of -1. As you go down the group, reactivity decreases because the outer shell is farther from the nucleus and attracts an extra electron less easily.
卤素原子有七个价电子。它们会获得一个电子从而填满最外层，形成带 -1 电荷的阴离子。从上到下，反应性减弱，因为最外层离原子核更远，对额外电子的吸引力变弱。
- Fluorine (F₂) is the most reactive halogen. | 氟（F₂）是最活泼的卤素。
- Chlorine (Cl₂) is a greenish-yellow gas used to disinfect water. | 氯（Cl₂）是黄绿色气体，用于水的消毒。
- Iodine (I₂) is a grey-black solid at room temperature. | 碘（I₂）在室温下是黑灰色固体。
A more reactive halogen can displace a less reactive halogen from its salt solution. For example, chlorine can displace bromine from potassium bromide solution.
较活泼的卤素能将较不活泼的卤素从其盐溶液中置换出来。例如，氯可将溴从溴化钾溶液中置换出来。
9. Group 0 – The Noble Gases | 第 0 族——稀有气体
Group 0 elements are helium, neon, argon, krypton, xenon, and radon. They are colorless, odourless gases with very low reactivity.
第 0 族元素包括氦、氖、氩、氪、氙和氡。它们是无色、无味、反应性极低的气体。
Noble gases have full outer shells (8 electrons, except helium with 2). This stable configuration means they have little tendency to gain, lose, or share electrons. Hence they are described as unreactive or inert.
稀有气体具有全满的最外层（8 个电子，氦除外为 2 个）。这种稳定结构意味着它们几乎没有得失或共享电子的趋势，因此被称为不活泼或惰性。
Because they are unreactive, noble gases are used in applications where reactions are unwanted. Argon is used in electric light bulbs; helium is used in balloons and airships.
由于稀有气体不活泼，它们被用于不希望发生反应的场合。氩用于电灯泡，氦用于气球和飞艇。
10. Summary and Exam Tips | 总结与考试提示
At IGCSE level, you must be comfortable with interpreting atomic diagrams, calculating Aᵣ from isotope abundances, and predicting chemical properties based on group position.
在 IGCSE 阶段，你必须熟练掌握原子结构图的解读、根据同位素丰度计算相对原子质量，以及根据元素周期表中的位置预测化学性质。
- Memorise the relative masses and charges of protons, neutrons and electrons. | 牢记质子、中子和电子的相对质量和电荷。
- Know how to write electron configurations for the first 20 elements. | 知道如何书写前 20 号元素的电子排布。
- Understand the trend in reactivity for Group 1 and Group 7. | 理解第 1 族和第 7 族元素的反应性变化趋势。
- Practice using the periodic table to determine atomic number, mass number and electron arrangement. | 练习使用元素周期表确定原子序数、质量数和电子排布。
Be careful with isotope notation: the superscript is always the mass number, and the subscript is always the atomic number. In any neutral atom, the atomic number also gives the number of electrons.
注意同位素符号：上标始终是质量数，下标始终是原子序数。在任意中性原子中，原子序数也等于电子数。
With a solid understanding of these core ideas, you will be ready to tackle many of the questions that appear on the Edexcel IGCSE Science exam.
牢固掌握这些核心概念后，你就能应对 Edexcel IGCSE 科学考试中的许多问题。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导