📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
Understanding atomic structure and the periodic table is a fundamental part of the Edexcel IGCSE Science specification. This knowledge helps you explain how elements behave, why they react, and how their properties are organised in a predictable way.
理解原子结构和元素周期表是 Edexcel IGCSE 科学大纲的基础部分。这些知识帮助你解释元素如何表现、为什么发生反应，以及它们的性质如何以可预测的方式排列。
1. The Atom | 原子
An atom is the smallest particle of an element that can exist and still retain the properties of that element. It is made up of three types of sub‑atomic particles: protons, neutrons and electrons.
原子是元素存在且仍保持该元素性质的最小粒子。它由三种亚原子粒子组成：质子、中子和电子。
Protons and neutrons are located in the nucleus at the centre of the atom, while electrons move in energy levels (shells) around the nucleus. The nucleus carries a positive charge overall because protons are positively charged and neutrons are neutral.
质子和中子位于原子中心的原子核中，而电子在原子核周围的能层（壳层）中运动。由于质子带正电、中子不带电，原子核整体带正电荷。
Electrons have a very small mass compared to protons and neutrons, but they occupy most of the volume of the atom. Their negative charge balances the positive charge of the nucleus, making the atom neutral overall.
电子质量与质子、中子相比很小，但它们占据了原子大部分体积。其负电荷抵消了原子核的正电荷，使整个原子呈电中性。
2. Atomic Number and Mass Number | 原子序数与质量数
Every element is defined by its atomic number (Z), which is the number of protons in the nucleus. In a neutral atom, the number of electrons equals the number of protons.
每种元素由原子序数（Z）定义，即原子核中的质子数。在中性原子中，电子数等于质子数。
The mass number (A) is the total number of protons and neutrons in the nucleus. The number of neutrons can be found by subtracting the atomic number from the mass number.
质量数（A）是原子核中质子数与中子数之和。中子数可通过质量数减去原子序数得出。
Mass number = number of protons + number of neutrons
质量数 = 质子数 + 中子数
For example, sodium has a mass number of 23 and an atomic number of 11, so it contains 11 protons, 11 electrons and 12 neutrons. Different elements have different atomic numbers, so the atomic number is sometimes called the ‘fingerprint’ of an element.
例如，钠的质量数为23，原子序数为11，因此它含有11个质子、11个电子和12个中子。不同元素具有不同的原子序数，因此原子序数有时被称为元素的“指纹”。
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.
同位素是指同一种元素的原子，具有相同的质子数但不同的中子数。因此，它们具有相同的原子序数但不同的质量数。
For instance, carbon‑12 (¹²C) has 6 protons and 6 neutrons, while carbon‑14 (¹⁴C) has 6 protons and 8 neutrons. Both are carbon isotopes and behave identically in chemical reactions.
例如，碳‑12（¹²C）有6个质子和6个中子，而碳‑14（¹⁴C）有6个质子和8个中子。两者都是碳的同位素，在化学反应中表现相同。
Isotopes have the same electron configuration because the number of electrons is equal to the number of protons. This is why their chemical properties are identical, even though their physical properties such as density may differ slightly.
由于电子数等于质子数，同位素具有相同的电子排布。这就是为什么它们的化学性质相同，即使物理性质如密度略有不同。
4. Electron Configuration | 电子排布
Electrons occupy shells (energy levels) around the nucleus. The first shell can hold up to 2 electrons, the second shell can hold up to 8, and the third shell also holds 8 before the next shell begins to fill.
电子占据原子核周围的壳层（能层）。第一壳层最多可容纳2个电子，第二壳层最多可容纳8个，第三壳层也容纳8个，然后下一个壳层开始填充。
For example, the electron configuration of sodium (atomic number 11) is 2,8,1. This means there are 2 electrons in the first shell, 8 in the second and 1 in the third. The outer shell is called the valence shell, and the electrons in it are called valence electrons.
例如，钠（原子序数11）的电子排布是2,8,1。这意味着第一壳层有2个电子，第二壳层有8个，第三壳层有1个。最外层称为价电子层，其中的电子称为价电子。
The number of electrons in the outer shell determines the chemical properties of the element. Atoms are most stable when their outer shell is full, which explains why noble gases are unreactive and why atoms tend to gain, lose or share electrons to achieve a full outer shell.
最外层电子数决定元素的化学性质。当最外层排满时原子最稳定，这就解释了为什么稀有气体不活泼，也解释了原子为什么会获得、失去或共享电子以达到满壳层。
5. The Periodic Table: Groups and Periods | 元素周期表：族和周期
The periodic table arranges all known elements in order of increasing atomic number. Elements are placed in rows called periods and columns called groups.
元素周期表按原子序数递增的顺序排列所有已知元素。元素排列在称为“周期”的行和称为“族”的列中。
There are 18 groups in the modern periodic table. Elements in the same group have the same number of outer‑shell electrons, so they show similar chemical properties. For example, all elements in Group 1 have one electron in their outer shell.
现代元素周期表有18个族。同一族元素具有相同的最外层电子数，因此表现出相似的化学性质。例如，第1族所有元素的最外层都有1个电子。
Each period shows a gradual change from metallic to non‑metallic behaviour. Across a period from left to right, the number of outer‑shell electrons increases from 1 to 8, and the outer shell becomes more filled. Group numbers tell you how many electrons are in the outer shell, except for helium which has 2.
每个周期从左到右呈现从金属性到非金属性的逐渐变化。在同一周期中，从左到右最外层电子数从1增加到8，最外壳层逐渐填满。族号告诉你最外层电子数（氦除外，它有2个）。
6. Group 1: Alkali Metals | 第1族：碱金属
Group 1 elements, known as the alkali metals, include lithium, sodium and potassium. They are soft, have low densities, and are stored under oil because they react vigorously with air and water.
第1族元素称为碱金属，包括锂、钠和钾。它们柔软、密度低，由于与空气和水剧烈反应，因此保存在油中。
All Group 1 metals have one electron in their outer shell. In reactions, they lose this electron to form a positive ion with a charge of +1. For example, sodium loses its outer electron to become Na⁺.
所有第1族金属最外层都有1个电子。在反应中，它们失去这个电子形成带+1电荷的正离子。例如，钠失去其最外层电子变成Na⁺。
As you go down the group, the elements become more reactive. The outer electron is further from the nucleus and is more easily lost, so the atom is more likely to take part in reactions. Melting points and boiling points decrease down the group.
随着沿族向下移动，元素变得更活泼。最外层电子离原子核更远，更容易失去，因此原子更容易参与反应。熔点和沸点沿族向下降低。
7. Group 7: Halogens | 第7族：卤素
Group 7 elements, called the halogens, include fluorine, chlorine, bromine and iodine. They are non‑metals and exist as diatomic molecules, such as Cl₂, Br₂ and I₂.
第7族元素称为卤素，包括氟、氯、溴和碘。它们是非金属，以双原子分子形式存在，如Cl₂、Br₂和I₂。
Halogens have seven electrons in their outer shell. They tend to gain one electron to form a negative ion with a charge of −1, for example Cl⁻ and Br⁻. This makes them oxidising agents.
卤素最外层有7个电子。它们倾向于获得1个电子形成带−1电荷的负离子，例如Cl⁻和Br⁻。这使得它们成为氧化剂。
Reactivity decreases as you go down the group. Fluorine is the most reactive halogen and iodine is the least reactive. A more reactive halogen can displace a less reactive one from its salt solution. For example, chlorine displaces iodine from potassium iodide solution.
随着沿族向下移动，活泼性降低。氟是最活泼的卤素，碘最不活泼。更活泼的卤素可以将较不活泼的卤素从其盐溶液中置换出来。例如，氯可以从碘化钾溶液中置换出碘。
8. Group 0: Noble Gases | 第0族：稀有气体
Group 0 elements are the noble gases: helium, neon, argon, krypton, xenon and radon. They are all monatomic and have a full outer shell of electrons, so they are very unreactive.
第0族元素是稀有气体：氦、氖、氩、氪、氙和氡。它们都是单原子分子，具有满壳层电子，因此非常不活泼。
Helium has 2 electrons in its only shell, while the other noble gases have 8 electrons in their outer shell. This full outer‑shell arrangement is extremely stable, so noble gases do not easily form compounds.
氦在其唯一壳层中有2个电子，而其他稀有气体最外层有8个电子。这种满壳层排列非常稳定，因此稀有气体不易形成化合物。
Because they are unreactive, noble gases are used in situations where reactions must be avoided. For example, argon is used in filament lamps to prevent the filament from oxidising, and helium is used in balloons because it is less dense than air and non‑flammable.
由于不活泼，稀有气体用于需要避免反应的情况。例如，氩用于白炽灯中以防止灯丝氧化，氦用于气球中，因为它的密度比空气小且不可燃。
9. Metals and Non‑metals | 金属与非金属
The periodic table is divided roughly into metals on the left and non‑metals on the right, with metalloids along the zig‑zag line. Metals tend to lose electrons to form positive ions, while non‑metals tend to gain electrons to form negative ions.
元素周期表大致分为左侧的金属和右侧的非金属，准金属位于锯齿线附近。金属倾向于失去电子形成正离子，而非金属倾向于获得电子形成负离子。
Metal properties include high electrical conductivity, high thermal conductivity, malleability, ductility and a shiny appearance. They are usually solid at room temperature (except mercury) and have high melting points.
金属的性质包括高导电性、高导热性、延展性、展性和光泽外观。它们在室温下通常是固体（汞除外），熔点较高。
Non‑metals are generally poor conductors of electricity and heat, are brittle when solid, and have lower melting points. However, graphite is an exception because it conducts electricity, while diamond is extremely hard. Both are forms of carbon.
非金属通常是电和热的不良导体，固态时易碎，熔点较低。但石墨是个例外，因为它导电，而金刚石极硬。两者都是碳的同素异形体。
10. Trends in the Periodic Table | 元素周期表中的趋势
Across a period from left to right, the number of outer‑shell electrons increases, so elements change from metallic to non‑metallic behaviour. The atomic radius decreases because the increasing nuclear charge pulls electrons closer to the nucleus.
同一周期从左到右，最外层电子数增加，因此元素从金属性转变为非金属性。原子半径减小，因为核电荷增加将电子拉得更靠近原子核。
Down a group, elements gain new electron shells, so the atomic radius increases. For metals, ionisation energy and reactivity (in Group 1) increase down the group; for non‑metals (Group 7), reactivity decreases because the incoming electron is less attracted to the nucleus.
沿族向下，元素增加新的电子壳层，因此原子半径增大。对于金属，电离能和（第1族）活泼性沿族向下增大；对于非金属（第7族），活泼性降低，因为新进入的电子受原子核吸引较小。
Knowing these trends helps predict the properties of unfamiliar elements. For example, if you know that strontium is below calcium in Group 2, you can deduce that it will be more reactive than calcium and form Sr²⁺ ions.
了解这些趋势有助于预测不熟悉元素的性质。例如，如果你知道锶在第2族钙的下方，你可以推断它比钙更活泼，会形成Sr²⁺离子。
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