📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
The atom is the basic building block of all matter. Understanding its internal structure and how electrons are arranged is essential for explaining chemical reactions, physical properties, and the organisation of the periodic table. This article covers the key ideas you need for your Edexcel IGCSE Science examination.
原子是构成所有物质的基本单元。理解原子的内部结构以及电子的排布方式，是解释化学反应、物理性质和元素周期表排列规律的关键。本文涵盖 Edexcel IGCSE 科学考试中你需要掌握的核心概念。
1. The Structure of an Atom | 原子的结构
Every atom consists of a tiny, dense central nucleus surrounded by electrons. The nucleus contains protons and neutrons, which are collectively called nucleons.
每个原子都是由一个极小且致密的中心原子核以及围绕它运动的电子所组成。原子核中包含质子和中子，它们统称为核子。
Protons are positively charged particles. Their relative mass is 1.
质子是带正电荷的粒子，其相对质量为 1。
Neutrons have no charge (neutral) and a relative mass of 1.
中子不带电荷（呈电中性），相对质量也为 1。
Electrons are negatively charged, with a relative mass of about 1/1836, so their mass is usually ignored.
电子带负电荷，相对质量约为 1/1836，因此其质量通常忽略不计。
Proton : +1 charge, mass 1 | Neutron : 0 charge, mass 1 | Electron : −1 charge, mass ≈ 0
质子：电荷 +1，质量 1 | 中子：电荷 0，质量 1 | 电子：电荷 −1，质量 ≈ 0
2. Atomic Number and Mass Number | 原子序数与质量数
The atomic number (Z) is the number of protons in the nucleus of an atom. It defines the element. The mass number (A) is the total number of protons and neutrons in the nucleus.
原子序数（Z）是原子核中的质子数，它决定了该元素是哪一种元素。质量数（A）是原子核中质子数和中子数的总和。
Number of neutrons = Mass number − Atomic number
中子数 = 质量数 − 原子序数
For an atom, the number of electrons equals the number of protons because atoms are electrically neutral. For example, a sodium atom has 11 protons, so it also has 11 electrons.
对于原子而言，电子数等于质子数，因为原子是电中性的。例如，钠原子有 11 个质子，因此它也有 11 个电子。
3. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Therefore 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不同的中子数，因此原子序数相同而质量数不同。
For example, carbon has three natural isotopes: ¹²₆C, ¹³₆C and ¹⁴₆C. They all have 6 protons, but they contain 6, 7 and 8 neutrons respectively.
例如，碳有三种天然同位素：¹²₆C、¹³₆C 和 ¹⁴₆C。它们都有 6 个质子，但分别含有 6、7 和 8 个中子。
Isotopes of the same element show identical chemical properties because chemical behaviour is determined by the number and arrangement of electrons.
同一元素的同位素具有相同的化学性质，因为化学行为由电子的数目和排列决定。
Physical properties such as density or rate of diffusion may differ slightly due to the difference in mass.
物理性质如密度或扩散速率可能因质量不同而有微小差异。
4. Electron Configuration | 电子排布
Electrons are arranged in shells (energy levels) around the nucleus. Each shell can hold a limited number of electrons: the first shell holds up to 2, the second shell holds up to 8, and the third shell also holds up to 8 (in the first 20 elements).
电子在原子核外的壳层（能层）中排列。每个壳层能容纳的电子数有限：第一层最多 2 个，第二层最多 8 个，第三层在前 20 号元素中也最多 8 个。
t energy shell first. For example, the electronic configuration of sodium (Z=11) is 2,8,1.
电子首先填满能量最低的壳层。例如，钠（Z=11）的电子排布为 2,8,1。
Na: 2.8.1 | C: 2.4 | Cl: 2.8.7
钠：2.8.1 | 碳：2.4 | 氯：2.8.7
5. The Periodic Table: Groups and Periods | 元素周期表：族与周期
The periodic table arranges elements in order of increasing atomic number. A row is called a period, and a column is called a group.
元素周期表按原子序数递增的顺序排列元素。横向的一行称为周期，纵向的一列称为族。
Elements in the same group have the same number of outer-shell electrons, which gives them similar chemical properties.
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。
The group number for groups 1 and 2 equals the number of outer electrons. For groups 3 to 0, the group number can be related to the outer electron count as well (group 7 elements have 7 outer electrons).
第 1 族和第 2 族的族序数等于最外层电子数。对于第 3 族至第 0 族，族序数也与最外层电子数相关（如第 7 族元素有 7 个最外层电子）。
The period number tells us how many occupied electron shells an atom has.
周期数表示该元素原子中共有几个被占据的电子壳层。
6. Group 1: Alkali Metals | 第 1 族：碱金属
Group 1 metals, such as lithium, sodium and potassium, are highly reactive metals. They all have one electron in their outer shell.
第 1 族金属，如锂、钠和钾，是高反应活性的金属。它们的最外层都只有 1 个电子。
React with water to produce hydrogen gas and an alkaline solution of the metal hydroxide.
与水反应生成氢气和金属氢氧化物的碱性溶液。
Reactivity increases down the group because the outer electron becomes farther from the nucleus and is more easily lost.
反应活性沿族向下递增，因为最外层电子离原子核更远，更容易失去。
They are soft, have low densities, and melt at low temperatures.
它们质地软、密度低，熔点较低。
7. Group 7: Halogens | 第 7 族：卤素
Group 7 elements, the halogens, include fluorine, chlorine, bromine and iodine. They all have seven outer-shell electrons and form diatomic molecules.
第 7 族元素称为卤素，包括氟、氯、溴和碘。它们最外层都有 7 个电子，能形成双原子分子。
They are non-metals with colours ranging from pale yellow (fluorine) to dark grey (iodine).
它们是非金属，颜色从淡黄色（氟）到深灰色（碘）不等。
Reactivity decreases down the group because the atoms get larger and it is harder to gain an extra electron.
反应活性沿族向下递减，因为原子变大，越难获得额外的电子。
A more reactive halogen can displace a less reactive halogen from a solution of its salt. 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，氯可以从溴化钾溶液中置换出溴。
8. Group 0: Noble Gases | 第 0 族：稀有气体
Group 0 elements are helium, neon, argon, krypton and xenon. They have a full outer shell of electrons, so they are very stable and unreactive.
第 0 族元素包括氦、氖、氩、氪和氙。它们的最外层电子已经填满，因此非常稳定，几乎不反应。
They exist as monatomic gases at room temperature.
它们在室温下以单原子气体形式存在。
They have very low boiling points, which increase slightly down the group.
它们的沸点很低，且沿族向下略微升高。
Their full outer shell means they do not gain, lose or share electrons under normal conditions.
满的最外层意味着在通常条件下它们不获得、不失去也不共享电子。
9. Metals and Non-metals | 金属与非金属
The periodic table can be divided into metals and non-metals. A stepped line separates them: metals are on the left and bottom, non-metals are on the right and top.
元素周期表可以分为金属和非金属。一条阶梯线将它们分开：金属位于左侧和下方，非金属位于右侧和上方。
Metals conduct heat and electricity, are shiny, malleable and ductile.
金属能导电导热、有光泽、可延展、可拉成丝。
Non-metals are poor conductors (except graphite), are dull, brittle and may be gases or solids at room temperature.
非金属是差的导体（石墨除外），表面暗淡、脆，室温下可能是气体或固体。
Metals tend to lose electrons to form positive ions; non-metals tend to gain electrons to form negative ions.
金属倾向于失去电子形成正离子；非金属倾向于获得电子形成负离子。
10. Trends Across a Period | 周期内的变化趋势
Across a period from left to right, the number of protons increases while the number of electron shells stays the same. This results in a stronger nuclear attraction on the outer electrons.
在同一周期内从左到右，质子数增加而电子壳层数不变，这使得原子核对外层电子的吸引力更强。
Therefore, atomic radius decreases across a period, and ionisation energy generally increases. Elements become less metallic and more non-metallic.
因此，原子半径在同一周期内逐渐减小，电离能大致增大。元素从金属性逐渐过渡到非金属性。
Period 3: Na → Mg → Al → Si → P → S → Cl → Ar
第三周期：Na → Mg → Al → Si → P → S → Cl → Ar
Sodium and magnesium are metals, aluminium is a metal with some non-metal behaviour, and silicon is a metalloid, while P, S, Cl are non-metals, and Ar is a noble gas.
钠和镁是金属，铝是有一些非金属性的金属，硅是准金属，而磷、硫、氯是非金属，氩是稀有气体。
11. The Order in the Periodic Table | 元素周期表的排序规则
The modern periodic table is arranged by increasing atomic number, not by relative atomic mass. This corrects the problems in Mendeleev's early table where some elements were out of order if mass was used.
现代元素周期表按原子序数递增排列，而不是按相对原子质量排列。这修正了早期门捷列夫周期表中仅按质量排列时会出现的次序颠倒问题。
Mendeleev left gaps for undiscovered elements and predicted their properties.
门捷列夫为尚未发现的元素留出空位，并预言了它们的性质。
Isotopes of the same element occupy the same position because they share the same atomic number.
同一元素的同位素占据同一个位置，因为它们具有相同的原子序数。
12. Key Formulas and Summary | 关键公式与总结
When writing the symbol of an atom, the mass number is written at the top left and the atomic number at the bottom left.
书写原子符号时，质量数写在左上角，原子序数写在左下角。
General symbol: ᴬ bsp; e.g. ³⁵₁₇Cl represents a chlorine atom with 17 protons, 18 neutrons and 17 electrons.
通用符号：ᴬ_ZX，例如 ³⁵₁₇Cl 表示一个氯原子，含有 17 个质子、18 个中子和 17 个电子。
Atomic number = number of protons = number of electrons in a neutral atom.
原子序数 = 质子数 = 中性原子中的电子数。
Mass number = protons + neutrons.
质量数 = 质子数 + 中子数。
Same group → same outer shell electron number, similar chemical properties.
同族 → 最外层电子数相同，化学性质相似。
Mastering these concepts will help you predict the behaviour of elements and understand how the periodic table is built. Practice drawing electron diagrams and writing configurations for the first 20 elements.
掌握这些概念将帮助你预测元素的行为，并理解元素周期表是如何构成的。请练习绘制电子结构图并书写前 20 号元素的电子排布。
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