📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
The periodic table is a central tool in chemistry, organising all known elements by their atomic structure. Understanding atoms — the tiny particles that make up matter — is the key to explaining why elements behave the way they do. This revision guide covers the essential points of atomic structure and periodicity for Edexcel IGCSE Science.
元素周期表是化学中的核心工具，它根据原子结构对所有已知元素进行排列。认识原子——构成物质的微小粒子——是解释元素行为方式的关键。本复习指南涵盖了 Edexcel IGCSE 科学中原子结构与周期性的重点内容。
1. The Atom | 原子
Every element is made of atoms. An atom has a tiny, dense central nucleus surrounded by electrons. The nucleus contains protons and neutrons, which are collectively called nucleons. Electrons move in energy levels (shells) around the nucleus at very high speed, occupying almost all of the atom’s volume.
每种元素都由原子组成。原子的中心是一个微小且致密的原子核，周围环绕着电子。原子核包含质子和中子，统称为核子。电子在原子核周围极高的速度运动，排列在不同的能级（电子壳层）上，占据了原子几乎全部的体积。
| 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 | 1/1836 (≈0) |
The number of protons in an atom is called the atomic number (Z). For a neutral atom, the number of electrons equals the number of protons. The mass number (A) is the total number of protons and neutrons.
原子中质子的数目称为原子序数（Z）。对于中性原子，电子数等于质子数。质量数（A）是质子数与中子数之和。
Number of neutrons = Mass number − Atomic number
中子数 = 质量数 − 原子序数
2. Isotopes | 同位素
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Therefore they have the same atomic number but different mass numbers. For example, carbon-12 (⁶¹²C) and carbon-14 (⁶¹⁴C) are both carbon atoms: each has 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons.
同位素是同一元素中质子数相同但中子数不同的原子。因此它们具有相同的原子序数，但质量数不同。例如，碳-12（⁶¹²C）和碳-14（⁶¹⁴C）都是碳原子：它们都有6个质子，但碳-12有6个中子，而碳-14有8个中子。
- Isotopes have identical chemical properties because chemical behaviour depends on electron arrangements, which are the same for isotopes of the same element. 同位素的化学性质完全相同，因为化学行为取决于电子排布，而同一元素不同同位素的电子排布是相同的。
- Physical properties, such as density and melting point, may differ slightly because of the difference in mass. 物理性质（如密度和熔点）因质量不同而略有差异。
- Isotopes of the same element are represented using mass number and atomic number: ⁱₙX where i = mass number, n = atomic number, X = element symbol. 同一元素的同位素用质量数和原子序数表示：ⁱₙX，其中 i = 质量数，n = 原子序数，X = 元素符号。
3. Relative Atomic Mass (Aᵣ) | 相对原子质量（Aᵣ）
Relative atomic mass is the weighted average mass of naturally occurring isotopes of an element compared with the mass of one atom of carbon-12 taken as exactly 12. Since isotopes exist in different abundances, we calculate Aᵣ using the formula:
相对原子质量是指元素中天然存在的各同位素的加权平均质量，与碳-12原子质量的1/12（即12作为基准）相比所得的值。由于各同位素的丰度不同，我们用以下公式计算Aᵣ：
Aᵣ = (mass of isotope₁ × % abundance₁ + mass of isotope₂ × % abundance₂ + …) / 100
Aᵣ =（同位素₁的质量 × 同位素₁的丰度% + 同位素₂的质量 × 同位素₂的丰度% + …）÷ 100
The periodic table shows the Aᵣ of each element. For example, chlorine has two common isotopes: chlorine-35 (75%) and chlorine-37 (25%).
元素周期表列出了每种元素的Aᵣ。例如，氯有两种常见同位素：氯-35（占75%）和氯-37（占25%）。
(35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 35.5
So the relative atomic mass of chlorine is 35.5. 因此氯的相对原子质量为35.5。
4. Electronic Configuration | 电子排布
Electrons are arranged in energy levels, often called shells, around the nucleus. Each shell has a maximum capacity: the first shell holds 2 electrons, the second holds 8, and the third also holds 8 (for elements up to calcium). These shells fill from the innermost to the outermost.
电子在原子核周围分层排列，这种层称为电子壳层。每个壳层有最大容量：第一层容纳2个电子，第二层容纳8个，第三层也容纳8个（对钙之前的元素而言）。电子从最内层开始向外填充。
- The group number of a main‑group element equals the number of electrons in its outer shell. 主族元素的族号等于其最外电子层中的电子数。
- The period number equals the number of occupied shells. 周期数等于被占据的电子壳层数。
- Elements in the same group have the same number of outer electrons, so they have similar chemical properties. 同一族中的元素具有相同的价电子数，因此化学性质相似。
Example: sodium (Na) has atomic number 11. Its electronic configuration is 2,8,1. It is in group 1, period 3. 示例：钠（Na）的原子序数为11，其电子排布为2,8,1，位于第1族、第3周期。
5. The Periodic Table | 周期表的结构
The periodic table arranges elements in order of increasing atomic number. A row is called a period, and a column is called a group. Most elements on the left and in the middle are metals, while non‑metals are found on the right side of the table.
元素周期表按原子序数递增的顺序排列元素。横行为周期，纵列称为族。表格左侧和中部的大多数元素是金属，非金属位于表格的右侧。
- Metals 金属: strong, malleable, good conductors of heat and electricity, shiny, high melting points (except mercury). 强度高、可延展、热和电的良导体、有光泽、熔点高（汞除外）。
- Non‑metals 非金属: brittle, dull, poor conductors, often gases or low‑melting solids. 脆、暗淡、导电导热性差，通常为气体或低熔点固体。
- Metalloids 类金属: have properties between metals and non‑metals (e.g. silicon). 具有介于金属与非金属之间的性质（如硅）。
Elements in a group have similar electron configurations, leading to trends in reactivity and physical properties. 同一族元素的电子排布相似，导致反应活性和物理性质呈现规律性变化。
6. Group 1 – Alkali Metals | 第1族——碱金属
All elements in Group 1, such as lithium (Li), sodium (Na) and potassium (K), have one outer electron. They are soft metals with low densities, and they are extremely reactive.
第1族的所有元素，如锂（Li）、钠（Na）和钾（K），都只有1个最外层电子。它们是软金属，密度低，并且反应性极强。
- They react with water to form a metal hydroxide and hydrogen gas. 它们与水反应生成金属氢氧化物和氢气。
- Reactivity increases going down the group because the outer electron is further from the nucleus and is shielded by inner electrons, making it easier to lose. 沿组向下反应性增强，因为最外层电子离核更远且受内层电子屏蔽，更容易失去。
- They have low melting points compared with most metals. 与大多数金属相比，它们的熔点较低。
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
2Na(固) + 2H₂O(液) → 2NaOH(溶液) + H₂(气)
7. Group 7 – Halogens | 第7族——卤素
Group 7 elements, including fluorine (F), chlorine (Cl), bromine (Br) and iodine (I), have seven outer electrons. They are non‑metals and exist as diatomic molecules (e.g. Cl₂, Br₂).
第7族元素包括氟（F）、氯（Cl）、溴（Br）和碘（I），它们有7个最外层电子。它们是非金属，以双原子分子形式存在（如Cl₂、Br₂）。
- They gain one electron in reactions to form negative ions (halides) with a charge of −1. 在反应中它们得到1个电子，形成带−1电荷的负离子（卤化物离子）。
- Reactivity decreases going down the group because the ability to attract an electron decreases as atomic radius increases. 沿组向下反应性减弱，因为随着原子半径增大，吸引电子的能力减弱。
- More reactive halogens displace less reactive halogens from their compounds. 更活泼的卤素会将较不活泼的卤素从其化合物中置换出来。
| Halogen 卤素 | Colour at room temp 室温颜色 | State 状态 |
|---|---|---|
| Chlorine | 氯气 | green‑yellow | 黄绿色 | gas | 气体 |
| Bromine | 溴 | red‑brown | 红棕色 | liquid | 液体 |
| Iodine | 碘 | dark grey / purple | 深灰色/紫色 | solid | 固体 |
8. Group 0 – Noble Gases | 第0族——稀有气体
Helium (He), neon (Ne), argon (Ar) and other group 0 elements have full outer shells of electrons. This makes them very stable and unreactive. They are colourless, odourless gases with very low boiling points.
氦（He）、氖（Ne）、氩（Ar）及其他第0族元素具有充满电子的最外层壳。这使得它们非常稳定、不活泼。它们都是无色、无味的低沸点气体。
- Their electron configurations are stable, so they do not usually form ions or molecules. 它们的电子排布稳定，因此通常不形成离子或分子。
- Argon is used in filament light bulbs to prevent the metal from reacting. 氩用于白炽灯泡，防止金属灯丝反应。
- Helium is used for floating balloons because it is less dense than air and is safe (non‑inflammable). 氦用于充气气球，因为它比空气轻且安全（不可燃）。
All noble gases have the maximum number of outer electrons possible: helium has 2, others have 8. 所有稀有气体都拥有最大外部电子数：氦有2个，其他稀有气体有8个。
9. Trends in Period 3 | 第3周期的变化趋势
Looking across a period, the atomic number increases, meaning more protons and more electrons. The electrons are added to the same outer shell, so the outer shell is gradually filled from 1 to 8 electrons.
横看一个周期，原子序数增加，意味着质子和电子都增多。电子被添加到同一个外壳层，因此外层电子数逐渐从1填充到8。
- Electronegativity (ability of an atom to attract bonding electrons) increases across a period. 电负性（原子吸引成键电子的能力）沿周期递增。
- Atomic radius generally decreases across a period because of the stronger attraction from the nucleus. 原子半径沿周期一般减小，因为原子核的吸引作用更强。
- Properties change from metallic (on the left) to non‑metallic (on the right). 性质从左侧的金属性变为右侧的非金属性。
These trends help predict how elements react and what compounds they form. 这些趋势有助于预测元素的反应方式以及它们形成的化合物。
10. Ions and Isotope Notation | 离子与同位素表示法
Atoms become ions by losing or gaining electrons. Metals lose electrons to form positive ions (cations); non‑metals gain electrons to form negative ions (anions). The charge is determined by the difference between the number of protons and the number of electrons.
原子通过失去或得到电子而成为离子。金属失去电子形成正离子（阳离子）；非金属得到电子形成负离子（阴离子）。电荷取决于质子数与电子数之差。
Magnesium atom: Mg (2,8,2) → Mg²⁺ (2,8) + 2e⁻
镁原子：Mg (2,8,2) → Mg²⁺ (2,8) + 2e⁻
Isotope notation also carries the charge if the atom is an ion. For example, ²³₁₁Na⁺ is a sodium ion with 11 protons, 12 neutrons, and 10 electrons. 同位素表示法在离子时还需标出电荷。例如，²³₁₁Na⁺ 是一个钠离子，含有11个质子、12个中子和10个电子。
Mastering these core ideas will allow you to interpret the periodic table and predict chemical behaviour across all areas of IGCSE Science. 掌握这些核心概念将帮助你理解元素周期表，并在 IGCSE 科学的所有领域中预测化学行为。
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