Elements and Atoms | 元素与原子

📚 Elements and Atoms | 元素与原子

Everything around you – the air you breathe, the water you drink, the screen you are staring at – is built from fewer than 100 naturally occurring building blocks. These building blocks are called elements, and each element is made of tiny particles called atoms. Understanding what an atom is and how atoms differ from one another is the first step towards mastering A-Level Chemistry. In this article we will explore atomic structure, isotopes, relative atomic mass, electron arrangement, and the link to the periodic table.

你周围的一切——呼吸的空气、饮用的水、正盯着的屏幕——都是由不到100种天然存在的“积木”搭建而成。这些积木叫做元素，而每种元素都由叫做原子的微小粒子组成。理解原子是什么以及不同元素的原子有何区别，是掌握A-Level化学的第一步。本文将探讨原子结构、同位素、相对原子质量、电子排布以及与周期表的关系。

1. What Are Elements and Atoms? | 元素与原子是什么？

An element is a pure substance that cannot be broken down into simpler substances by chemical means. Every element is composed of one type of atom. An atom is the smallest particle of an element that still retains the chemical properties of that element. For example, a piece of copper metal contains billions of copper atoms, each with the same chemical behaviour.

元素是一种不能用化学方法分解成更简单物质的纯净物。每种元素都由一种类型的原子构成。原子是保留该元素化学性质的最小粒子。例如，一片铜金属含有数十亿个铜原子，每个原子都有相同的化学行为。

Today we know of 118 elements, of which roughly 90 occur naturally. Each element is represented by a one- or two-letter chemical symbol, such as O for oxygen, Fe for iron, and Na for sodium (from the Latin natrium).

今天我们知道118种元素，其中大约90种是天然存在的。每种元素用一个或两个字母的化学符号表示，如O代表氧，Fe代表铁，Na代表钠（源自拉丁语natrium）。

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2. The Structure of the Atom | 原子的结构

Atoms are made up of three subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found in a tiny, dense nucleus at the centre of the atom, while electrons move around the nucleus in regions of space. The table below summarises their properties:

原子由三种亚原子粒子组成：质子、中子和电子。质子和中子位于原子中心微小且致密的原子核内，而电子在核外的空间区域运动。下表总结了它们的性质：

Particle 粒子	Relative charge 相对电荷	Relative mass 相对质量	Symbol 符号
Proton 质子	+1	1	p⁺
Neutron 中子	0	1	n⁰
Electron 电子	−1	1/1836 ≈ 0	e⁻

The number of protons in the nucleus determines the identity of the element. A neutral atom contains an equal number of protons and electrons, so their charges cancel out.

原子核中的质子数决定了元素的种类。中性原子的质子数与电子数相等，因此电荷相互抵消。

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3. 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. Every element has a unique atomic number. For instance, carbon always has Z = 6, oxygen Z = 8, and uranium Z = 92. In a neutral atom the atomic number also tells us the number of electrons.

原子序数（Z）是原子核中质子的数目。每种元素都有唯一的原子序数。例如，碳的Z = 6，氧的Z = 8，铀的Z = 92。在中性原子中，原子序数也告诉我们电子的数目。

The mass number (A) is the total number of protons and neutrons in the nucleus. It is always a whole number. We can write the symbol of an element with its A and Z values like this:

质量数（A）是原子核中质子和中子的总数。它总是整数。我们可以像这样表示带有A和Z的元素符号：

ᴬZX   e.g.   ¹²₆C,   ²³₁₁Na

From this notation, the number of neutrons can be found by subtracting Z from A: Number of neutrons = A – Z.

根据这种表示法，中子数可以通过质量数减去原子序数得到：中子数 = A – Z。

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4. Isotopes | 同位素

Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons. Because they have the same atomic number, they exhibit identical chemical behaviour, but their masses differ slightly. Chlorine, for example, has two important natural isotopes: chlorine‑35 (₁₇³⁵Cl) and chlorine‑37 (₁₇³⁷Cl).

同位素是同一元素中质子数相同但中子数不同的原子。由于原子序数相同，它们表现出相同的化学性质，但质量略有不同。例如，氯有两种重要的天然同位素：氯‑35 (₁₇³⁵Cl)和氯‑37 (₁₇³⁷Cl)。

Some isotopes are radioactive and decay over time. Carbon‑14, used in archaeological dating, is a radioactive isotope of carbon. All isotopes of an element share the same chemical symbol but are often identified by their mass number, e.g. carbon‑12 and carbon‑14.

有些同位素具有放射性，会随时间衰变。用于考古定年的碳‑14就是碳的一种放射性同位素。一种元素的所有同位素共享相同的化学符号，但通常用其质量数来区分，如碳‑12和碳‑14。

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5. Relative Atomic Mass | 相对原子质量

Because the masses of individual atoms are incredibly small, chemists use a scale based on the carbon‑12 isotope. One unified atomic mass unit (u) is defined as exactly 1/12 the mass of a carbon‑12 atom. On this scale, the relative isotopic mass of a ¹²C atom is exactly 12.

由于单个原子的质量极其微小，化学家使用基于碳‑12同位素的标度。一个统一的原子质量单位（u）被严格定义为碳‑12原子质量的1/12。按照这个标度，¹²C原子的相对同位素质量恰好是12。

The relative atomic mass (Aᵣ) of an element is the weighted average mass of all its naturally occurring isotopes relative to 1/12 of the mass of a carbon‑12 atom. It is calculated using the formula:

元素的相对原子质量（Aᵣ）是其所有天然同位素的质量相对于碳‑12原子质量1/12的加权平均值。它的计算公式为：

Aᵣ = Σ (isotopic mass × percentage abundance) / 100

For chlorine, with 75.77% ³⁵Cl (mass 34.97 u) and 24.23% ³⁷Cl (mass 36.97 u), Aᵣ ≈ (34.97×75.77 + 36.97×24.23)/100 = 35.45 (to 4 s.f.). This value is used in all quantitative chemistry.

对于氯，含有75.77%的³⁵Cl（质量34.97 u）和24.23%的³⁷Cl（质量36.97 u），Aᵣ ≈ (34.97×75.77 + 36.97×24.23)/100 = 35.45（保留四位有效数字）。所有定量化学计算都使用这个数值。

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6. Electron Energy Levels and Shells | 电子能级与电子层

Electrons are arranged around the nucleus in specific energy levels, sometimes called shells. The lowest energy level is closest to the nucleus. Each energy level can hold a maximum number of electrons given by 2n², where n is the principal quantum number (shell number).

电子围绕原子核排列在特定的能级上，有时被称为电子层。最低的能级离核最近。每个能级最多能容纳的电子数由2n²给出，其中n是主量子数（层数）。

For the first four shells: n=1 holds up to 2 electrons, n=2 holds up to 8, n=3 holds up to 18, and n=4 holds up to 32. However, the outermost shell of a stable atom never contains more than 8 electrons (the octet rule), which governs chemical reactivity.

对于前四个电子层：n=1最多容纳2个电子，n=2最多容纳8个，n=3最多容纳18个，n=4最多容纳32个。不过，稳定原子的最外层绝不会多于8个电子（八隅律），这支配着化学活泼性。

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7. Subshells and Orbitals | 亚层与轨道

Energy levels are further divided into subshells, labelled s, p, d, and f. Each subshell contains a specific number of orbitals. An orbital is a region of space where there is a high probability of finding an electron. Each orbital can hold a maximum of two electrons.

能级进一步分为亚层，记作s、p、d、f。每个亚层含有特定数量的轨道。轨道是电子出现概率很高的空间区域。每个轨道最多能容纳两个电子。

• s subshell: 1 orbital, holds 2 electrons. Spherical shape.
• p subshell: 3 orbitals, holds 6 electrons. Dumbbell‑shaped, oriented along x-, y-, z‑axes.
• d subshell: 5 orbitals, holds 10 electrons. Complex shapes.
• f subshell: 7 orbitals, holds 14 electrons.

• s亚层：1个轨道，容纳2个电子。球形。
• p亚层：3个轨道，容纳6个电子。哑铃形，沿x、y、z轴取向。
• d亚层：5个轨道，容纳10个电子。形状复杂。
• f亚层：7个轨道，容纳14个电子。

The type of subshell present in a shell depends on the shell number. The first shell (n=1) has only an s subshell. The second shell (n=2) has s and p subshells. The third shell (n=3) has s, p, and d subshells, and so on.

一个电子层中含有哪类亚层取决于层数。第一层（n=1）只有s亚层。第二层（n=2）有s和p亚层。第三层（n=3）有s、p和d亚层，以此类推。

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8. Filling Orbitals: Aufbau, Hund, Pauli | 轨道填充规则

Electrons occupy orbitals in a way that minimises the energy of the atom. Three key principles govern this process:

电子以尽量降低原子能量的方式占据轨道。三个关键原则支配这个过程：

Aufbau principle: Electrons fill the lowest available energy level and subshell first before moving to higher ones. The order of filling for the first elements is: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p. Note that the 4s orbital is filled before 3d because it has slightly lower energy.

构造原理：电子首先填充最低的可用能级和亚层，然后再进入较高的能级。前几个元素的填充顺序为：1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p。注意4s轨道在3d之前填充，因为它的能量略低。

Pauli exclusion principle: Each orbital can hold a maximum of two electrons, and these two electrons must have opposite spins. (Often indicated by arrows pointing up ↑ and down ↓.)

泡利不相容原理：每个轨道最多容纳两个电子，且这两个电子必须自旋相反。（常用向上的箭头↑和向下的箭头↓表示。）

Hund’s rule: When filling a set of degenerate orbitals (orbitals of equal energy, like the three p orbitals), electrons occupy separate orbitals singly with parallel spins before any orbital receives a second electron. This arrangement reduces electron‑electron repulsion.

洪特规则：在填充一组简并轨道（能量相等的轨道，如三个p轨道）时，电子会先以平行自旋独自占据不同轨道，之后才会让某个轨道接受第二个电子。这种排布降低了电子间的排斥。

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9. Electron Configuration Notation | 电子排布式

The electron configuration of an atom describes how its electrons are distributed among the various shells and subshells. It is written using the subshell notation: the shell number, the subshell letter, and a superscript for the number of electrons in that subshell. For example, the configuration of a carbon atom (Z=6) is 1s² 2s² 2p²; for oxygen (Z=8) it is 1s² 2s² 2p⁴.

原子的电子排布描述了电子在各个电子层和亚层中的分布。书写时使用亚层符号：先写层数，再写亚层字母，右上方用上标表示该亚层中的电子数。例如，碳原子（Z=6）的排布为1s² 2s² 2p²；氧原子（Z=8）是1s² 2s² 2p⁴。

For transition metals or heavier elements, the configuration becomes longer. Iron (Z=26) is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. The 4s electrons are written before 3d following energy order, even though the 4s subshell is the outermost in terms of principal quantum number. When forming positive ions, the 4s electrons are lost first.

对于过渡金属或更重的元素，排布会变长。铁（Z=26）的排布为1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶。按照能量顺序，4s电子写在3d之前，尽管4s亚层在主量子数上是最外层。形成阳离子时，4s电子会先失去。

Abbreviated configurations use the nearest noble gas in square brackets, e.g., Fe: [Ar] 4s² 3d⁶.

简写排布用方括号中的最近惰性气体，例如Fe：[Ar] 4s² 3d⁶。

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10. The Periodic Table and Electronic Structure | 周期表与电子结构

A-Level Chemistry relies heavily on the link between electron configuration and the layout of the periodic table. The table is arranged in order of increasing atomic number, and elements with similar outer‑shell electron configurations fall into the same group, giving them similar chemical properties.

A-Level化学非常依赖电子排布与周期表布局之间的联系。周期表按原子序数递增排列，外层电子排布相似的元素归入同一族，赋予它们相似的化学性质。

• s‑block: Groups 1 and 2, plus helium. Outermost electrons occupy an s subshell.
• p‑block: Groups 13 to 18. Outermost electrons occupy p subshell.
• d‑block: Transition metals. Outermost d subshell is being filled.
• f‑block: Lanthanides and actinides. Outermost f subshell is being filled.

• s区：第1族和第2族，加上氦。最外层电子占据s亚层。
• p区：第13族至第18族。最外层电子占据p亚层。
• d区：过渡金属。外层的d亚层正在填充。
• f区：镧系和锕系元素。外层的f亚层正在填充。

The period number equals the highest principal quantum number of the element’s electrons. Group number for s‑ and p‑block elements often indicates the total number of valence electrons.

周期号等于该元素电子所具有的最高主量子数。s区和p区元素的族号通常指示价电子的总数。

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11. Ions and Valence Electrons | 离子与价电子

When atoms lose or gain electrons, they form ions. Metals tend to lose electrons to achieve a stable noble‑gas configuration, forming cations (positive ions). Non‑metals tend to gain electrons, forming anions (negative ions). The chemical reactivity of an element is largely determined by the number of valence electrons – the electrons in the outermost shell.

原子失去或得到电子时，会形成离子。金属倾向于失去电子以达到稳定的稀有气体结构，形成阳离子（正离子）。非金属倾向于获得电子，形成阴离子（负离子）。元素的化学活泼性在很大程度上由其价电子数目决定——即最外层的电子。

For example, sodium (Na, 1s² 2s² 2p⁶ 3s¹) has one valence electron. Losing it gives Na⁺ with the electron configuration of neon. Chlorine (Cl, 1s² 2s² 2p⁶ 3s² 3p⁵) has seven valence electrons and gains one to become Cl⁻, acquiring the argon configuration.

例如，钠（Na, 1s² 2s² 2p⁶ 3s¹）有一个价电子。失去它形成Na⁺，电子排布与氖相同。氯（Cl, 1s² 2s² 2p⁶ 3s² 3p⁵）有七个价电子，获得一个电子成为Cl⁻，获得氩的电子排布。

Isoelectronic species – atoms or ions with the same number of electrons – can have very different nuclear charges and sizes. O²⁻, F⁻, Ne, Na⁺, and Mg²⁺ all have 10 electrons (1s² 2s² 2p⁶) but differ in ionic radius and chemical behaviour.

等电子物种——具有相同电子数的原子或离子——其核电荷和大小可能截然不同。O²⁻、F⁻、Ne、Na⁺和Mg²⁺都有10个电子（1s² 2s² 2p⁶），但离子半径和化学行为各异。

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12. Summary | 总结

Starting from the simple idea that matter is made of atoms, we have seen how the structure of the atom – protons, neutrons, and electrons – defines an element. The concepts of atomic number, mass number, and isotopes allow us to account for the masses we measure in the laboratory. Electron configuration, governed by the Aufbau principle, Pauli exclusion principle, and Hund’s rule, explains the periodicity of the elements and the basis of chemical bonding. A firm grasp of elements and atoms is an essential foundation for every topic you will encounter in A‑Level Chemistry.

从物质由原子构成的简单思想出发，我们了解了原子的结构——质子、中子和电子——如何定义一种元素。原子序数、质量数和同位素的概念使我们能够解释实验室内测得的原子质量。由构造原理、泡利不相容原理和洪特规则支配的电子排布，解释了元素的周期性及化学键合的基础。扎实掌握元素与原子的知识，是你在A-Level化学中遇到的每一个课题的必需基础。

Published by TutorHao | Chemistry Revision Series | aleveler.com

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