📚 Atoms and Elements: IB & WJEC Science Revision Guide | 原子与元素：IB WJEC 科学考点精讲
Atoms form the basis of all matter, and understanding their structure is a cornerstone of both IB and WJEC science courses. This guide breaks down every essential concept from subatomic particles to ion formation, equipping you with precise knowledge for exam success.
原子是所有物质的基础，理解其结构是 IB 和 WJEC 科学课程的核心。本指南逐一剖析从亚原子粒子到离子形成的每个重要概念，助你掌握精准知识，从容应对考试。
1. Basic Structure of the Atom | 原子的基本结构
An atom is the smallest unit of an element that retains its chemical properties. It consists of a dense central nucleus containing protons and neutrons, surrounded by electrons that move in regions called energy levels or shells.
原子是保持元素化学性质的最小单元。它由一个致密的中心原子核和核外电子组成，原子核含有质子和中子，电子在称为能级或电子层的区域内运动。
Protons carry a relative charge of +1 and a relative mass of 1. Neutrons have no charge (neutral) and also a relative mass of 1. Electrons possess a charge of −1 and a negligible relative mass (1/1836).
质子带一个单位的相对正电荷，相对质量为 1。中子不带电，相对质量也为 1。电子带一个单位的负电荷，相对质量极小（1/1836），可忽略不计。
Most of the atom’s volume is empty space, but the nucleus contains almost all of its mass. In a neutral atom, the number of protons equals the number of electrons, making the overall charge zero.
原子的大部分体积是空的，但几乎全部质量都集中在原子核中。在中性原子中，质子数等于电子数，因此净电荷为零。
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 identity of the element: all atoms of carbon have Z = 6, while oxygen atoms have Z = 8.
原子序数（Z）是原子核内的质子数。它决定了元素的种类：所有碳原子的 Z = 6，而氧原子的 Z = 8。
The mass number (A) is the total number of protons and neutrons in the nucleus. It is always a whole number and is used to distinguish between different isotopes of the same element.
质量数（A）是原子核中质子数与中子数的总和。它总是一个整数，用于区分同一元素的不同同位素。
You can calculate the number of neutrons by subtracting the atomic number from the mass number: neutrons = A − Z. For example, an atom with A = 23 and Z = 11 (sodium) has 12 neutrons.
中子数可以通过质量数减去原子序数计算：中子数 = A − Z。例如，一个 A = 23、Z = 11 的原子（钠）有 12 个中子。
3. Nuclear Symbol Notation | 核符号表示法
Nuclear symbols summarise an atom’s composition in a compact form. The element symbol (X) is written with the mass number as a superscript and the atomic number as a subscript on the left side: AZX. For example, the most common isotope of carbon is ¹²₆C.
核符号以简洁形式概括原子组成。元素符号（X）左侧上标是质量数，下标是原子序数：AZX。例如最常见的碳同位素表示为 ¹²₆C。
Using Unicode characters, we can write ¹²₆C, ²³₁₁Na, or ¹⁶₈O. When writing charges, the charge follows the symbol as a superscript: Cl⁻, Ca²⁺, or O²⁻. The negative or positive sign comes after the number for ions.
用 Unicode 字符可写作 ¹²₆C、²³₁₁Na 或 ¹⁶₈O。表示电荷时，将电荷以右上标形式跟在符号后，如 Cl⁻、Ca²⁺ 或 O²⁻。离子电荷中数字在前，正负号在后。
This notation allows chemists to quickly determine the numbers of protons, neutrons, and electrons in any atom or ion, which is essential for balancing nuclear equations and understanding isotopes.
这种表示法让化学家能迅速确定任何原子或离子中的质子、中子和电子数，这对于平衡核方程和理解同位素至关重要。
4. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they share the same atomic number but have different mass numbers.
同位素是同一元素中质子数相同而中子数不同的原子。它们具有相同的原子序数，但质量数不同。
Because the number of protons (and electrons) remains unchanged, isotopes exhibit almost identical chemical properties. Differences lie mainly in physical properties such as mass, density, and sometimes radioactivity.
由于质子数（和电子数）不变，同位素的化学性质几乎完全相同。差异主要体现在物理性质上，如质量、密度，有时还包括放射性。
Common examples include the three isotopes of hydrogen: protium (¹₁H), deuterium (²₁H), and tritium (³₁H). Carbon-12, carbon-13, and carbon-14 are another classic set. Carbon-14 is radioactive and used in radiocarbon dating.
常见的例子包括氢的三种同位素：氕（¹₁H）、氘（²₁H）和氚（³₁H）。碳-12、碳-13 和碳-14 是另一组典型同位素。碳-14 具有放射性，用于放射性碳定年。
5. Relative Atomic Mass (Aᵣ) | 相对原子质量
The relative atomic mass (Aᵣ) is the weighted average mass of all the naturally occurring isotopes of an element, compared to 1/12 of the mass of a carbon-12 atom. It has no units because it is a relative measure.
相对原子质量（Aᵣ）是某元素所有天然同位素的加权平均质量，与碳-12 原子质量的 1/12 相比。它是相对值，因此没有单位。
The calculation formula is:
Aᵣ = Σ (isotope mass × % abundance) / 100
计算公式为：
Aᵣ = Σ (同位素质量 × 丰度百分比) / 100
For example, chlorine has two main isotopes: ³⁵Cl (75% abundance) and ³⁷Cl (25% abundance). Its Aᵣ is calculated as (35 × 75 + 37 × 25) / 100 = 35.5. This explains why Aᵣ values on the periodic table are often not whole numbers.
例如，氯有两种主要同位素：³⁵Cl（丰度 75%）和 ³⁷Cl（丰度 25%）。其 Aᵣ 计算为 (35 × 75 + 37 × 25) / 100 = 35.5。这解释了为什么周期表上的 Aᵣ 值通常不是整数。
6. Electron Configuration | 电子排布
Electrons occupy discrete energy levels (shells) around the nucleus. The shells fill in order: the first shell holds up to 2 electrons, the second up to 8, the third up to 8 (for the first 20 elements), and the fourth begins to fill after that.
电子占据核外分立的能级（电子层）。电子层按顺序填充：第一层最多容纳 2 个电子，第二层最多 8 个，第三层（在前 20 号元素中）最多 8 个，此后第四层开始填充。
The electron configuration for an element can be written as a series of numbers representing the number of electrons in each shell, e.g., sodium (Z = 11) is 2,8,1. Using subshell notation, it is 1s² 2s² 2p⁶ 3s¹.
元素的电子排布可用一串数字表示每层电子数，例如钠（Z = 11）为 2,8,1。若用亚层符号表示，则为 1s² 2s² 2p⁶ 3s¹。
The valence electrons in the outermost shell determine the chemical reactivity and bonding behaviour of the element. Atoms tend to gain, lose, or share electrons to achieve a full outer shell (often 8 electrons, known as the octet rule).
最外层的价电子决定了元素的化学活性和成键行为。原子倾向于获得、失去或共用电子，以使最外层达到满层（通常为 8 个电子，称为八隅体规则）。
7. 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. Horizontal rows are called periods; the period number indicates the number of electron shells an atom possesses.
元素周期表按原子序数递增排列。横行称为周期；周期数表示该原子具有的电子层数。
Vertical columns are called groups. Elements in the same group have the same number of electrons in their outermost shell, giving them similar chemical properties. For instance, Group 1 (alkali metals) all have one outer electron, while Group 7 (halogens) have seven.
纵列称为族。同一族元素的最外层电子数相同，因此具有相似的化学性质。例如，第 1 族（碱金属）最外层均有 1 个电子，而第 7 族（卤素）有 7 个电子。
Metals are found on the left and centre, non-metals on the right. The noble gases (Group 0) have full outer shells and are exceptionally unreactive. The position of an element helps predict its ion charge and typical reactions.
金属位于周期表左侧和中部，非金属在右侧。稀有气体（第 0 族）具有满层电子，极不活泼。元素的位置有助于预测其离子电荷和典型反应。
8. Ions: Formation and Charge | 离子：形成与电荷
Ions form when atoms gain or lose electrons to achieve a stable electron arrangement, usually that of the nearest noble gas. Metals tend to lose electrons and form positively charged cations, while non-metals tend to gain electrons and form negatively charged anions.
当原子获得或失去电子以达到稳定的电子排布（通常是邻近稀有气体的结构）时，就会形成离子。金属易失去电子形成带正电的阳离子，非金属易获得电子形成带负电的阴离子。
The charge on a simple ion can be deduced from the group number: Group 1 elements form 1⁺ ions (Na⁺), Group 2 form 2⁺ (Mg²⁺), Group 6 form 2⁻ (O²⁻), and Group 7 form 1⁻ (Cl⁻). For elements in Groups 3 and 5, common charges are 3⁺ and 3⁻ respectively.
简单离子的电荷可根据族序数推断：第 1 族形成 1⁺ 离子（Na⁺），第 2 族形成 2⁺（Mg²⁺），第 6 族形成 2⁻（O²⁻），第 7 族形成 1⁻（Cl⁻）。第 3 族和第 5 族元素常见离子电荷分别为 3⁺ 和 3⁻。
In an ion, the number of protons remains unchanged from the neutral atom, but the number of electrons has changed. For example, a Ca²⁺ ion has 20 protons but only 18 electrons. The nuclear symbol for an ion includes the charge, e.g., ⁴⁰₂₀Ca²⁺.
离子中质子数与中性原子相同，但电子数已改变。例如，Ca²⁺ 离子有 20 个质子，却只有 18 个电子。离子的核符号包含电荷，如 ⁴⁰₂₀Ca²⁺。
9. Isotopes and Radioactivity | 同位素与放射性
Many isotopes are stable, but some are unstable because their nuclei contain an imbalance of protons and neutrons. Unstable isotopes undergo radioactive decay, emitting radiation to become more stable.
许多同位素是稳定的，但有些因原子核内质子数与中子数不平衡而不稳定。不稳定同位素会发生放射性衰变，释放辐射以变得更稳定。
Radioisotopes have important applications: carbon-14 is used to date archaeological specimens, iodine-131 is used in thyroid cancer treatment, and uranium-235 is used as fuel in nuclear reactors. These contexts often appear in IB and WJEC exam questions.
放射性同位素有重要应用：碳-14 用于考古样本定年，碘-131 用于治疗甲状腺癌，铀-235 用作核反应堆燃料。这些背景常出现在 IB 和 WJEC 的考题中。
Radiation can be in the form of alpha particles (⁴₂He²⁺), beta particles (electrons, ⁰₋₁e), or gamma rays. Their penetrating powers and ionising abilities differ, and safety precautions must be taken when handling radioactive materials.
辐射可以是 α 粒子（⁴₂He²⁺）、β 粒子（电子，⁰₋₁e）或 γ 射线。它们的穿透能力和电离能力不同，处理放射性物质时必须采取安全措施。
10. Common Exam Questions and Tips | 常见考题与技巧
Exam questions frequently ask you to state the numbers of subatomic particles from nuclear symbols, calculate relative atomic mass from isotopic abundance data, or draw electron configurations for the first 20 elements. Be prepared to write nuclear symbols for ions and atoms.
考题经常要求你根据核符号说明亚原子粒子数，根据同位素丰度数据计算相对原子质量，或画出前 20 号元素的电子排布图。要准备好写出离子和原子的核符号。
When calculating Aᵣ, always check that the percentage abundances add up to 100. If using decimal fractions, divide each fractional abundance by 1, not 100. Examiner tips: show all workings clearly and pay attention to units (Aᵣ has no units).
计算 Aᵣ 时，始终检查丰度百分比之和是否为 100。如果使用小数值，应将每个丰度小数乘以同位素质量后直接求和。阅卷人提示：清晰展示所有步骤，注意相对原子质量不带单位。
A common pitfall is confusing mass number with atomic mass. Remember, mass number is always a whole number (total protons + neutrons), while atomic mass is the weighted average shown on the periodic table. Use the correct terminology in written answers.
常见误区是将质量数与原子质量混淆。记住，质量数总是整数（质子数 + 中子数），而原子质量是周期表上显示的加权平均值。在书面答案中要使用正确的术语。
For IB students, data-based questions often require you to interpret information about isotopes or ionisation energies. For WJEC, questions frequently link atomic structure to the properties of elements in the periodic table. Practice past papers to master these common themes.
对 IB 学生而言，数据题常要求解读有关同位素或电离能的信息。对 WJEC 考生，题目常将原子结构与周期表中元素的性质联系起来。通过练习历年真题来掌握这些常见主题。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply