📚 Atoms and Elements: The Ultimate IB & OCR Science Revision Guide | 原子与元素：IB 与 OCR 科学终极复习指南
Understanding atoms and elements is the foundation of all chemistry. This guide breaks down the key concepts you need to master for IB and OCR science courses, from subatomic particles to periodic trends. Let’s dive into the building blocks of matter.
理解原子与元素是整个化学学科的基础。本指南将为你拆解 IB 和 OCR 科学课程中必须掌握的核心概念，从亚原子粒子到元素周期律。让我们一起探索物质的基本组成单元。
1. The Structure of the Atom | 原子的结构
An atom consists of a tiny, dense nucleus surrounded by a cloud of electrons. The nucleus contains positively charged protons and neutral neutrons, while negatively charged electrons occupy regions of space called orbitals or energy levels. Almost all the mass of the atom is concentrated in the nucleus, yet the electrons determine the atom’s chemical behaviour.
原子由一个微小、致密的原子核及围绕其周围的电子云构成。原子核内包含带正电的质子和不带电的中子，而带负电的电子则占据称为轨道或能级的空间区域。原子几乎全部质量都集中在原子核上，但决定原子化学行为的是核外电子。
The relative masses and charges of these particles are crucial to remember. A proton and a neutron each have a relative mass of approximately 1 atomic mass unit (u), while an electron has a negligible mass of about 1/1836 u. The relative charge of a proton is +1, an electron is –1, and a neutron is 0.
记住这些粒子的相对质量和电荷至关重要。一个质子和一个中子各自的相对质量大约为1原子质量单位（u），而电子的质量极小，约为1/1836 u。质子的相对电荷为 +1，电子为 –1，中子为 0。
| 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 |
2. Atomic Number (Z) and Mass Number (A) | 原子序数（Z）与质量数（A）
The atomic number (Z) is the number of protons in the nucleus of an atom. It defines the element: all atoms of the same element have the same number of protons. In a neutral atom, Z also equals the number of electrons.
原子序数（Z）是原子核内质子的数目。它定义了元素的种类：同一种元素的所有原子都具有相同的质子数。在电中性原子中，Z 也等于电子数。
The mass number (A) is the total number of protons and neutrons in the nucleus. It is always a whole number. For example, carbon-12 has Z = 6 and A = 12, meaning 6 protons and 6 neutrons (12 – 6).
质量数（A）是原子核内质子数与中子数的总和，它始终是一个整数。例如，碳-12 的 Z = 6，A = 12，即含有6个质子和6个中子（12 – 6）。
The standard notation is shown as follows, where X is the chemical symbol:
标准表示法如下所示，其中 X 代表元素符号：
A
X
Z
From this you can calculate the number of neutrons = A – Z.
由此你可以计算中子数 = A – Z。
3. Isotopes: Same Element, Different Neutrons | 同位素：同种元素，中子数不同
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. This means they have the same atomic number but different mass numbers. For example, carbon-12 (⁶₁₂C) and carbon-14 (⁶₁₄C) are isotopes of carbon.
同位素是指质子数相同而中子数不同的同一种元素的原子。这意味着它们的原子序数相同，但质量数不同。例如，碳-12（⁶₁₂C）和碳-14（⁶₁₄C）是碳的同位素。
Chemical properties of isotopes are almost identical because chemical behaviour is governed by the electron configuration, which depends on the number of protons. Physical properties, such as mass and density, differ slightly due to the varying neutron count.
同位素的化学性质几乎完全相同，因为化学行为由电子排布决定，而电子排布取决于质子数。物理性质，如质量和密度，则因中子数不同而有细微差异。
Uses of isotopes include carbon-14 dating (radiocarbon dating) and medical tracers like iodine-131.
同位素的应用包括碳-14年代测定法（放射性碳定年）以及像碘-131这样的医学示踪剂。
4. Relative Atomic Mass (Ar) | 相对原子质量（Ar）
The relative atomic mass (Ar) is the weighted average mass of an atom of an element compared to 1/12 of the mass of a carbon-12 atom. It accounts for the abundance of each isotope. The formula is: Ar = Σ(isotopic mass × percentage abundance) / 100.
相对原子质量（Ar）是指某元素一个原子的平均质量与一个碳-12原子质量的1/12相比较所得的数值。它考虑了每种同位素的丰度。计算公式为：Ar = Σ(同位素质量 × 丰度百分比) / 100。
For IB and OCR exams, you may be asked to calculate Ar from given isotopic abundances. For example, naturally occurring chlorine consists of 75% ³⁵Cl and 25% ³⁷Cl. Its Ar ≈ (35 × 75 + 37 × 25) / 100 = 35.5.
在 IB 和 OCR 考试中，你可能需要根据给出的同位素丰度计算 Ar。例如，天然存在的氯由 75% 的 ³⁵Cl 和 25% 的 ³⁷Cl 组成。其 Ar 约等于 (35 × 75 + 37 × 25) / 100 = 35.5。
Note that relative atomic mass has no units because it is a ratio. This is a very common exam trap.
请注意，相对原子质量没有单位，因为它是一个比值。这是考试中非常常见的易错点。
5. Electron Configuration: Shells and Energy Levels | 电子排布：电子层与能级
Electrons occupy specific energy levels (shells) around the nucleus. The first shell can hold up to 2 electrons, the second up to 8, and the third up to 8 (or 18 at higher levels, but for the first 20 elements we use 2, 8, 8). The electronic configuration is written as a series of numbers, e.g., sodium (Na, Z=11) is 2,8,1.
电子占据原子核外特定的能级（电子层）。第一层最多容纳2个电子，第二层最多8个，第三层最多8个（更高层可达18个，但对前20号元素我们采用2, 8, 8的规则）。电子排布写成一系列数字，例如钠（Na，Z=11）为 2,8,1。
The electrons in the outermost shell are called valence electrons, and they determine the element’s reactivity and bonding type. Elements with the same number of valence electrons are in the same group of the periodic table and show similar chemical properties.
最外层电子称为价电子，它们决定了元素的反应活性和成键类型。具有相同价电子数的元素位于周期表的同一族，并表现出相似的化学性质。
For IB, you also need to understand the sub-level model: s, p, d ordering (1s, 2s, 2p, 3s, 3p, 4s, 3d…). For example, iron (Fe) has an electron configuration of [Ar] 4s² 3d⁶.
对于 IB 课程，你还需要理解亚层模型：s、p、d 的填充顺序（1s, 2s, 2p, 3s, 3p, 4s, 3d…）。例如，铁（Fe）的电子排布为 [Ar] 4s² 3d⁶。
6. The Periodic Table: Groups and Periods | 元素周期表：族与周期
The modern periodic table arranges elements in order of increasing atomic number, not atomic mass. The horizontal rows are called periods, and the vertical columns are groups. Elements in the same group have the same number of valence electrons, leading to similar chemical behaviour.
现代周期表按原子序数递增的顺序排列元素，而不是按原子质量。横行称为周期，纵列称为族。同一族元素的价电子数相同，因此化学行为相似。
Group numbers for the main groups: Group 1 (alkali metals, 1 valence electron), Group 2 (alkaline earth metals, 2 valence electrons), Group 17 (halogens, 7 valence electrons), Group 18 (noble gases, full outer shell, 8 except helium with 2).
主族的族号：第1族（碱金属，1个价电子），第2族（碱土金属，2个价电子），第17族（卤素，7个价电子），第18族（稀有气体，最外层满电子，除氦为2个外其余为8个）。
OCR may use the older IUPAC numbering 1–8 plus a letter, while IB uses the 1–18 system. Be familiar with both: for example, halogens are Group 17 (or Group VII).
OCR 可能使用较旧的 IUPAC 编号 1–8 加字母的系统，而 IB 使用 1–18 系统。请熟悉这两种体系：例如，卤素是第17族（或第VII族）。
7. Trends Across a Period: Atomic Radius and Electronegativity | 同周期变化趋势：原子半径与电负性
As you move from left to right across a period, the atomic number increases, adding more protons to the nucleus. This increases the nuclear charge, pulling the electrons closer. The atomic radius decreases across a period because the increased nuclear charge attracts the electrons more strongly, and the added electrons go into the same energy level (same shielding).
沿周期从左到右，原子序数递增，原子核内质子数增多。这增加了核电荷，将电子拉得更紧。原子半径沿周期减小，因为增大的核电荷更强地吸引电子，并且新增的电子进入同一能级（屏蔽效应不变）。
Electronegativity also increases across a period. Electronegativity is the ability of an atom to attract a bonding pair of electrons. As the atomic radius shrinks and nuclear charge grows, the attraction for bonding electrons becomes stronger.
电负性沿周期从左到右递增。电负性是原子吸引成键电子对的能力。随着原子半径缩小和核电荷增加，对成键电子的吸引力增强。
For example, sodium (Na) has a low electronegativity, while chlorine (Cl) has a very high electronegativity. This trend explains why metals (left) tend to lose electrons and non-metals (right) tend to gain electrons.
例如，钠（Na）电负性低，而氯（Cl）电负性很高。这一趋势解释了为什么金属（左侧）倾向于失去电子，而非金属（右侧）倾向于得到电子。
8. Trends Down a Group: Atomic Radius and Reactivity | 同族变化趋势：原子半径与反应活性
Going down a group, the number of electron shells increases, so the atomic radius gets larger. The outermost electrons are further from the nucleus and are more shielded by inner electron shells. As a result, the attractive force from the nucleus on the valence electrons decreases.
沿族向下，电子层数增加，原子半径变大。最外层电子离核更远，并受到更多内层电子的屏蔽。因此，原子核对价电子的吸引力减弱。
For Group 1 metals, reactivity increases down the group because the lone valence electron is more easily lost. For Group 17 halogens, reactivity decreases down the group because it becomes harder to attract an extra electron when the atom is larger and the nucleus is more shielded.
对于第1族金属，反应活性沿族向下增强，因为单个价电子更容易失去。对于第17族卤素，反应活性沿族向下减弱，因为原子变大且核屏蔽增大，更难吸引额外的电子。
This opposite trend in reactivity for metals and non-metals is a classic exam question.
金属和非金属的反应活性沿族呈现相反的规律，这是经典考题。
9. Ions and Ionic Bonding Preview | 离子与离子键预览
An ion is formed when an atom gains or loses electrons to achieve a stable noble gas electron configuration (full outer shell). Metals typically lose electrons to form positive ions (cations), while non-metals gain electrons to form negative ions (anions).
当原子获得或失去电子以达到稳定的稀有气体电子构型（满壳层）时，便形成离子。金属通常失去电子形成阳离子，而非金属获得电子形成阴离子。
For example, a sodium atom (2,8,1) loses one electron to become Na⁺ with a 2,8 configuration like neon. A chlorine atom (2,8,7) gains one electron to become Cl⁻ with a 2,8,8 configuration like argon.
例如，钠原子（2,8,1）失去一个电子变为 Na⁺，具有2,8的氖型构型。氯原子（2,8,7）获得一个电子变为 Cl⁻，具有2,8,8的氩型构型。
The electrostatic attraction between oppositely charged ions forms an ionic bond. Ionic compounds have high melting points and conduct electricity when molten or dissolved.
带相反电荷的离子之间的静电吸引力形成离子键。离子化合物具有高熔点，在熔融态或溶液中能导电。
10. Common Exam Mistakes and Tips | 常见考试错误与提示
- Mixing up atomic number and mass number. Always check which one defines the element. Z = protons; A = protons + neutrons.
- 混淆原子序数和质量数。务必核对哪个定义了元素。Z = 质子数；A = 质子数 + 中子数。
- Forgetting that relative atomic mass is weighted average. Do not simply average isotopic masses. Use abundance percentages.
- 忘记相对原子质量是加权平均值。不要简单地将同位素质量平均。要使用丰度百分比。
- Misunderstanding electron configuration for transition metals. For IB, remember that 4s fills before 3d, but when writing ions, 4s electrons are lost first.
- 误解过渡金属的电子排布。IB 考生请记住，4s 在 3d 之前填入，但形成离子时，4s 电子优先失去。
- Using the wrong group numbering system. Clarify with your exam board whether you need to use 1–18 or older symbols.
- 使用错误的族编号体系。事先弄清你的考试局要求使用的是 1–18 系统还是旧符号。
11. Summary and Key Equations | 总结与关键公式
Here is a quick recap of the essential equations and concepts for your revision:
以下是复习必备的基本方程和概念速览：
Number of neutrons = Mass number (A) – Atomic number (Z)
中子数 = 质量数 (A) – 原子序数 (Z)
Relative atomic mass (Ar) = Σ (isotopic mass × % abundance) / 100
相对原子质量 (Ar) = Σ (同位素质量 × 丰度百分比) / 100
In a neutral atom, number of electrons = Z. In an ion, electrons = Z – (charge). For example, Mg²⁺ has 12 – 2 = 10 electrons.
电中性原子中，电子数 = Z。对于离子，电子数 = Z – (电荷)。例如，Mg²⁺ 的电子数为 12 – 2 = 10。
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