Atomic Structure and Bonding | 原子结构与化学键

📚 Atomic Structure and Bonding | 原子结构与化学键

Understanding atomic structure and the three main types of chemical bonding is fundamental to all of IGCSE science. This article explains the key concepts in a clear, exam-focused way.

理解原子结构和三种主要化学键是IGCSE科学的基础。本文将以清晰、紧扣考点的方式解释关键概念。

1. Subatomic Particles | 亚原子粒子

An atom consists of three types of subatomic particles: protons, neutrons and electrons. Protons carry a positive charge, neutrons are neutral, and electrons carry a negative charge.

原子由三种亚原子粒子组成：质子、中子和电子。质子带正电荷，中子不带电，电子带负电荷。

Protons and neutrons are found in the nucleus, which is dense and positively charged.

质子和中子位于原子核内，原子核密度大且带正电。
Electrons orbit the nucleus in energy levels (shells) and have very little mass.

电子在核外的能级（电子层）中运动，质量非常小。
The relative masses and charges are: proton – mass 1, charge +1; neutron – mass 1, charge 0; electron – mass 1/1840, charge -1.

相对质量和电荷分别为：质子 – 质量1，电荷+1；中子 – 质量1，电荷0；电子 – 质量约1/1840，电荷-1。

Proton (p⁺): mass ≈ 1, charge +1
Neutron (n⁰): mass ≈ 1, charge 0
Electron (e⁻): mass ≈ 1/1840, charge -1

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. In a neutral atom, it also equals the number of electrons. The mass number (A) is the total number of protons and neutrons.

原子序数（Z）是原子核中的质子数。在中性原子中，它也等于电子数。质量数（A）是质子和中子的总数。

For example, a carbon atom has atomic number 6 and mass number 12, so it contains 6 protons, 6 electrons and 6 neutrons.

例如，碳原子的原子序数为6，质量数为12，因此含有6个质子、6个电子和6个中子。

Number of neutrons = Mass number – Atomic number
中子数 = 质量数 – 原子序数

3. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers.

同位素是同一元素中质子数相同但中子数不同的原子。它们具有相同的原子序数，但质量数不同。

Chlorine has two common isotopes: chlorine-35 and chlorine-37. Both have 17 protons and 17 electrons, but chlorine-35 has 18 neutrons while chlorine-37 has 20 neutrons.

氯有两种常见同位素：氯-35和氯-37。两者都有17个质子和17个电子，但氯-35有18个中子，氯-37有20个中子。

Isotopes of an element have identical chemical properties because chemical behaviour depends on the electron arrangement.

同一元素的同位素具有相同的化学性质，因为化学行为取决于电子排布。
Physical properties such as density or melting point may differ slightly.

物理性质如密度或熔点可能略有不同。

4. Electron Configuration | 电子排布

Electrons are arranged in energy levels (shells) around the nucleus. The filling order follows a simple rule: the first shell can hold up to 2 electrons, the second and third shells can hold up to 8 each.

电子围绕原子核排列在能级（电子层）中。填充顺序遵循简单规则：第一层最多容纳2个电子，第二层和第三层最多各容纳8个电子。

For example, sodium (Z = 11) has the electronic configuration 2, 8, 1. The outer shell (valence shell) contains 1 electron.

例如，钠（Z = 11）的电子排布为2, 8, 1。其最外层（价电子层）含有1个电子。

Writing electron configurations is essential for predicting how atoms bond. Elements in Group 1 have 1 outer electron, Group 2 have 2, Group 3 have 3, Group 4 have 4, Group 5 have 5, Group 6 have 6, Group 7 have 7, and Group 0 (noble gases) have 8 (except helium with 2).

写出电子排布对于预测原子如何成键至关重要。第1族元素有1个外层电子，第2族有2个，第3族有3个，第4族有4个，第5族有5个，第6族有6个，第7族有7个，第0族（稀有气体）有8个（氦除外，为2个）。

5. Chemical Bonding: Ionic Bonds | 化学键：离子键

Ionic bonding occurs between metals and non-metals. Electrons are transferred from the metal atom to the non-metal atom to form ions with full outer shells.

离子键发生在金属和非金属之间。电子从金属原子转移到非金属原子，形成具有全满外层电子结构的离子。

Sodium (2, 8, 1) loses 1 electron to become a sodium ion Na⁺ (2, 8). Chlorine (2, 8, 7) gains 1 electron to become a chloride ion Cl⁻ (2, 8, 8).

钠（2, 8, 1）失去1个电子变为钠离子Na⁺（2, 8）。氯（2, 8, 7）得到1个电子变为氯离子Cl⁻（2, 8, 8）。

Ionic compounds form giant lattices where oppositely charged ions are held together by strong electrostatic forces. This results in high melting points and electrical conductivity when molten or dissolved in water.

离子化合物形成巨大的晶格，正负离子通过强大的静电作用结合在一起。这导致离子化合物熔点高，并且在熔融或溶于水时能导电。

Na (2, 8, 1) → Na⁺ (2, 8) + e⁻
Cl (2, 8, 7) + e⁻ → Cl⁻ (2, 8, 8)

6. Chemical Bonding: Covalent Bonds | 化学键：共价键

Covalent bonding occurs between non-metal atoms. Atoms share pairs of electrons to achieve stable outer shells.

共价键发生在非金属原子之间。原子通过共享电子对来达到稳定的外层结构。

For example, in a hydrogen molecule H₂, the two hydrogen atoms share a single pair of electrons. This can be shown as H–H or H:H.

例如，在氢分子H₂中，两个氢原子共享一对电子。这可以表示为H–H或H:H。

Water (H₂O) has two O–H covalent bonds. Methane (CH₄) has four C–H bonds. Carbon dioxide has two double bonds: O=C=O.

水（H₂O）有两个O–H共价键。甲烷（CH₄）有四个C–H键。二氧化碳有两个双键：O=C=O。

Simple covalent molecules have low melting and boiling points because the intermolecular forces are weak.

简单共价分子具有低熔点和沸点，因为分子间作用力很弱。
They do not conduct electricity because there are no free mobile ions or electrons.

它们不导电，因为没有自由移动的离子或电子。
Giant covalent structures like diamond and silicon dioxide are hard and have very high melting points.

像金刚石和二氧化硅这样的巨大共价结构很硬，具有极高的熔点。

7. Metallic Bonding | 金属键

Metallic bonding occurs in metals. The atoms lose their outer electrons to form a sea of delocalised electrons, while the positive metal ions form a regular lattice.

金属键发生在金属中。金属原子失去外层电子，形成离域电子的“电子海”，而正金属离子则形成规则的晶格。

The electrostatic attraction between the positive ions and the delocalised electrons is called a metallic bond.

正离子与离域电子之间的静电吸引力称为金属键。

Metals are malleable and ductile because the layers of ions can slide over each other without breaking the metallic bond.

金属具有延展性和可锻性，因为离子层之间可以相互滑动而不会破坏金属键。
Metals are good conductors of electricity because delocalised electrons can move freely throughout the structure.

金属是良好的电导体，因为离域电子可以自由移动。
Metals have high melting points and high density due to the strong metallic bonds holding the lattice together.

金属具有高熔点和高密度，因为强金属键将晶格紧密地结合在一起。

8. Comparing Bonding Types | 键型对比

The table below summarises the key differences between ionic, covalent and metallic bonding.

下表总结了离子键、共价键和金属键的主要区别。

Property	Ionic	Covalent (simple molecular)	Metallic
Particles involved	Metal + non-metal	Non-metal + non-metal	Metal atoms
Bond form	Transfer of electrons	Sharing of electrons	Sea of electrons
Melting point	High	Low	High
Electrical conductivity	Only when molten/dissolved	Poor	Good

9. Writing Formulae for Ionic Compounds | 离子化合物化学式

To write the formula of an ionic compound, the total positive charge must equal the total negative charge. The formula shows the simplest whole-number ratio of ions.

写离子化合物化学式时，总正电荷必须等于总负电荷。化学式表示离子的最简整数比。

For example, magnesium (Mg²⁺) and chloride (Cl⁻): two chloride ions are needed for one magnesium ion, so the formula is MgCl₂.

例如，镁（Mg²⁺）和氯离子（Cl⁻）：一个镁离子需要两个氯离子，因此化学式为MgCl₂。

Aluminium oxide contains Al³⁺ and O²⁻. The lowest common multiple of charges is 6, so the formula is Al₂O₃.

氧化铝含有Al³⁺和O²⁻。电荷的最小公倍数为6，因此化学式为Al₂O₃。

Al³⁺ + O²⁻ → Al₂O₃
Mg²⁺ + Cl⁻ → MgCl₂

10. Exam Tips and Practice | 考点提示与练习

When answering questions on atomic structure, always state the number of protons, neutrons and electrons using the mass number and atomic number. For bonding, identify the type of elements involved first.

回答原子结构问题时，务必根据质量数和原子序数说明质子、中子和电子的数量。对于化学键，先判断涉及的元素类型。

Common mistakes include confusing atomic number with mass number, forgetting that electrons have negligible mass, and thinking that ionic compounds conduct electricity in solid form.

常见错误包括混淆原子序数与质量数，忘记电子质量可忽略，以及认为离子化合物在固态时能导电。

Practice drawing dot-and-cross diagrams for molecules like H₂O and CH₄.

练习绘制H₂O和CH₄等分子的电子点叉图。
Memorise the electron configurations of the first 20 elements.

记住前20号元素的电子排布。
Use the periodic table to determine ion charges for Group 1 (1+), Group 2 (2+), Group 6 (2-) and Group 7 (1-).

利用元素周期表确定离子电荷：第1族为1+，第2族为2+，第6族为2-，第7族为1-。

Try this quick question: An atom has atomic number 6 and mass number 14. How many neutrons does it have? (Answer: 8)

尝试快速测验：一个原子的原子序数为6，质量数为14。它有多少个中子？（答案：8个）

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Atomic Structure and Bonding | 原子结构与化学键