IB Science: Atoms and Elements – Exam Essentials | IB 科学：原子与元素 考点精讲

📚 IB Science: Atoms and Elements – Exam Essentials | IB 科学：原子与元素 考点精讲

Every IB Science student must build a solid understanding of atoms and elements – the very building blocks of matter. This revision guide breaks down the essential concepts you need to master, from subatomic particles to the periodic table, isotopes, electron configurations, ions, chemical bonding, and practical separation techniques. Use this resource to consolidate your knowledge and prepare for assessment questions with confidence.

每一位 IB 科学学习者都必须打下关于原子和元素的坚实基础——它们是组成物质的基本单元。这份考点精讲涵盖了你必须掌握的核心概念，从亚原子粒子到元素周期表、同位素、电子排布、离子、化学键以及实用的分离技术。利用这份资料巩固知识，自信应对各类评估问题。

1. Atomic Structure and Subatomic Particles | 原子结构与亚原子粒子

Atoms are the smallest units of an element that retain its chemical properties. They consist of three main subatomic particles: protons (positive charge, located in the nucleus), neutrons (neutral, in the nucleus), and electrons (negative charge, orbiting the nucleus in shells). The nucleus is extremely dense and contains almost all the mass of the atom, whereas electrons contribute almost nothing to the mass but determine chemical behaviour.

原子是保留元素化学性质的最小单元。它们由三种主要的亚原子粒子组成：质子（带正电，位于原子核）、中子（不带电，位于原子核）和电子（带负电，在核外分层运动）。原子核极其致密，几乎包含了原子的全部质量；而电子虽对质量贡献极小，却决定了原子的化学行为。

• Relative masses and charges: Proton: mass ≈ 1, charge +1. Neutron: mass ≈ 1, charge 0. Electron: mass ≈ 1/1836, charge –1.
• 相对质量与电荷： 质子：质量≈1，电荷+1；中子：质量≈1，电荷0；电子：质量≈1/1836，电荷–1。

In a neutral atom, the number of protons equals the number of electrons. The identity of an element is determined by its proton number (atomic number).

在中性原子中，质子数等于电子数。决定元素身份的特征是其质子数（原子序数）。

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2. Atomic Number, Mass Number and Isotopes | 原子序数、质量数与同位素

The atomic number (Z) is the number of protons in the nucleus. The mass number (A) is the total number of protons and neutrons. For example, a carbon atom with 6 protons and 6 neutrons has Z = 6 and A = 12, often written as carbon‑12 (¹²C). Isotopes are atoms of the same element (same Z) that have different numbers of neutrons, hence different mass numbers.

原子序数（Z）是原子核中的质子数。质量数（A）是质子数和中子数之和。例如，一个碳原子有 6 个质子和 6 个中子，Z = 6，A = 12，通常写作碳‑12（¹²C）。同位素是同一元素（Z 相同）但中子数不同、因而质量数不同的原子。

• Examples: Carbon‑12 (¹²C), Carbon‑13 (¹³C), Carbon‑14 (¹⁴C). All have 6 protons, but 6, 7 and 8 neutrons respectively.
• 示例： 碳‑12（¹²C）、碳‑13（¹³C）、碳‑14（¹⁴C）。它们都有 6 个质子，但分别有 6、7、8 个中子。

Isotopes of an element exhibit identical chemical properties because they have the same electron configuration, but their physical properties (such as density and rate of diffusion) may differ slightly due to the mass difference.

同一元素的同位素化学性质完全相同，因为它们具有相同的电子排布；但由于质量差异，它们的物理性质（如密度和扩散速率）可能略有不同。

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3. Relative Atomic Mass (Aᵣ) | 相对原子质量

The relative atomic mass (Aᵣ) of an element is the weighted average mass of its isotopes, taking into account their natural abundances, compared to 1/12 of the mass of a carbon‑12 atom. It is a dimensionless number, typically shown on the periodic table under the element symbol.

元素的相对原子质量（Aᵣ）是其所有同位素按天然丰度计算出的加权平均质量，以碳‑12 原子质量的 1/12 为基准。它是一个无量纲数，通常标注在周期表中元素符号的下方。

Aᵣ = (abundance₁ × mass number₁ + abundance₂ × mass number₂ + …) / 100

Example: Chlorine consists of 75% ³⁵Cl and 25% ³⁷Cl. Aᵣ = (75 × 35 + 25 × 37) / 100 = 35.5. This explains why the atomic mass of chlorine is not a whole number.

举例： 氯元素由 75% 的 ³⁵Cl 和 25% 的 ³⁷Cl 组成。Aᵣ = (75 × 35 + 25 × 37) / 100 = 35.5。这就解释了为什么氯的相对原子质量不是整数。

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4. Electron Arrangement and Energy Levels | 电子排布与能级

Electrons are arranged in shells (energy levels) around the nucleus. The first shell can hold up to 2 electrons, the second up to 8, and the third up to 8 for the first 20 elements (the pattern becomes more complex for heavier elements). The electronic configuration determines an element’s reactivity and bonding pattern.

电子在原子核外分层排布（能级）。第一层最多容纳 2 个电子，第二层最多 8 个，前 20 号元素的第三层最多 8 个（更重的元素排布规则更复杂）。电子排布决定了元素的反应活性和成键方式。

Example: Sodium (Na, Z = 11) – electronic configuration: 2, 8, 1. The single outer‑shell electron makes sodium highly reactive. Example: Argon (Ar, Z = 18) – configuration: 2, 8, 8. Being a full outer shell, argon is chemically inert.

示例： 钠（Na，Z = 11）——电子排布为 2, 8, 1。最外层有一个电子，使得钠极为活泼。示例： 氩（Ar，Z = 18）——排布为 2, 8, 8。最外层全满，因此氩化学性质惰性。

When drawing diagrams, IB Science often asks for simple shell diagrams (dots and crosses) to represent electrons in each energy level.

在绘制示意图时，IB 科学常要求用简单的电子层图（点和叉）来表示各能级上的电子。

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5. Formation of Ions | 离子的形成

Atoms become ions when they gain or lose electrons to achieve a full outer shell (the stable octet rule). Metals tend to lose electrons to form positive ions (cations), while non‑metals tend to gain electrons to form negative ions (anions).

当原子通过失去或得到电子以达到最外层全满（稳定的八隅体规则）时，就会形成离子。金属倾向于失去电子形成阳离子，非金属倾向于得到电子形成阴离子。

• Cation example: Sodium atom (2,8,1) loses one electron → Na⁺ (2,8).
• 阳离子示例： 钠原子（2,8,1）失去一个电子 → Na⁺（2,8）。
• Anion example: Chlorine atom (2,8,7) gains one electron → Cl⁻ (2,8,8).
• 阴离子示例： 氯原子（2,8,7）得到一个电子 → Cl⁻（2,8,8）。

The charge on an ion is written as a superscript: for example, Ca²⁺ for calcium ion, O²⁻ for oxide ion. Polyatomic ions such as sulfate (SO₄²⁻) and nitrate (NO₃⁻) contain groups of atoms with an overall charge; these are also important to recognise.

离子的电荷以上标形式书写：如钙离子 Ca²⁺，氧离子 O²⁻。硫酸根（SO₄²⁻）和硝酸根（NO₃⁻）等多原子离子由原子团带整体电荷，也需要准确识别。

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6. Introduction to the Periodic Table | 元素周期表简介

The modern periodic table arranges elements in order of increasing atomic number (Z). Rows are called periods; columns are called groups. Elements in the same group have the same number of outer‑shell electrons and therefore similar chemical properties.

现代元素周期表按原子序数递增的顺序排列元素。横排称为周期，纵列称为族。同一族的元素具有相同的最外层电子数，因而表现出相似的化学性质。

• Group 1: Alkali metals – very reactive metals (Li, Na, K…), 1 outer electron.
• 第 1 族：碱金属——非常活泼的金属（Li, Na, K…），最外层 1 个电子。
• Group 7: Halogens – reactive non‑metals (F, Cl, Br, I…), 7 outer electrons.
• 第 7 族：卤素——活泼的非金属（F, Cl, Br, I…），最外层 7 个电子。
• Group 8/0: Noble gases – unreactive gases (He, Ne, Ar…), full outer shell.
• 第 8/0 族：稀有气体——不活泼气体（He, Ne, Ar…），最外层全满。

The table is divided into metals (left and centre), non‑metals (right) and

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