Atomic Structure | 原子结构

📚 Atomic Structure | 原子结构

The atom is the basic building block of all matter. In the Edexcel IGCSE Science course, you need to understand how the atom is organised, how to represent different atoms and ions, and how this structure explains chemical behaviour.

原子是所有物质的基本构成单位。在Edexcel IGCSE科学课程中，你需要理解原子如何组织、如何表示不同的原子和离子，以及这种结构如何解释化学行为。

1. Subatomic Particles | 亚原子粒子

Every atom contains three kinds of subatomic particles. Protons are positively charged, neutrons have no charge, and electrons are negatively charged. Protons and neutrons are found in the tiny nucleus at the centre of the atom, while electrons move around the nucleus in shells.

每个原子都包含三种亚原子粒子。质子带正电荷，中子不带电，电子带负电荷。质子和中子位于原子中心的微小原子核内，而电子则在原子核外的电子壳层中运动。

Proton: relative charge +1, relative mass 1, located in the nucleus.
质子：相对电荷 +1，相对质量 1，位于原子核中。
Neutron: relative charge 0, relative mass 1, located in the nucleus.
中子：相对电荷 0，相对质量 1，位于原子核中。
Electron: relative charge -1, relative mass about 1/1836, located in shells outside the nucleus.
电子：相对电荷 -1，相对质量约为 1/1836，位于原子核外的电子壳层中。

The number of protons determines which element the atom belongs to. In a neutral atom, the number of electrons equals the number of protons, so the overall charge is zero.

质子数决定了该原子属于哪种元素。在中性原子中，电子数等于质子数，因此总电荷为零。

2. Atomic Number and Mass Number | 原子序数与质量数

Each element is defined by its atomic number, Z, which is the number of protons in the nucleus. The mass number, A, is the total number of protons plus neutrons in the nucleus.

每种元素由其原子序数 Z 定义，也就是原子核中的质子数。质量数 A 是原子核中质子数与中子数的总和。

Atomic number (Z) = number of protons
Mass number (A) = number of protons + number of neutrons

原子序数（Z）= 质子数
质量数（A）= 质子数 + 中子数

So the number of neutrons can be found by the equation: neutrons = A – Z.

因此中子数可通过公式计算：中子数 = A – Z。

For example, carbon-12 has a mass number of 12 and an atomic number of 6, so it contains 6 protons and 6 neutrons. This is written as ¹²₆C. The small number on the bottom left is the atomic number, and the number on the top left is the mass number.

例如，碳-12 的质量数为12，原子序数为6，因此它含有6个质子和6个中子。可写成 ¹²₆C。左下角的小数字是原子序数，左上角的数字是质量数。

3. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Since the number of protons is the same, isotopes of an element 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。由于质子数相同，同一元素的同位素具有相同的原子序数，但质量数不同。

Hydrogen-1 (¹₁H): 1 proton, 0 neutrons.
氢-1 (¹₁H)：1个质子，0个中子。
Hydrogen-2 (²₁H), also called deuterium: 1 proton, 1 neutron.
氢-2 (²₁H)，也叫氘：1个质子，1个中子。
Hydrogen-3 (³₁H), also called tritium: 1 proton, 2 neutrons.
氢-3 (³₁H)，也叫氚：1个质子，2个中子。

Isotopes have the same chemical properties because they have the same number of outer electrons. However, their physical properties can differ, for example their density or rate of diffusion. Some isotopes are radioactive and are used in medicine, archaeology and industry.

同位素具有相同的化学性质，因为它们具有相同的最外层电子数。然而，它们的物理性质可能不同，例如密度或扩散速率。一些同位素具有放射性，可用于医学、考古和工业。

4. Electron Arrangement | 电子排布

Electrons are arranged in shells (also called energy levels) around the nucleus. Each shell can hold a certain maximum number of electrons. The first shell can hold up to 2 electrons, the second shell can hold up to 8 electrons, and the third shell can also hold up to 8 electrons for the first 20 elements.

电子在原子核周围的壳层（也称能级）中排列。每个壳层可容纳的最大电子数有限。第一层最多容纳2个电子，第二层最多容纳8个电子，对于前20号元素，第三层最多也容纳8个电子。

For example, a sodium atom has 11 electrons. They are arranged in three shells: 2 in the first shell, 8 in the second shell, and 1 in the third shell. This is written as 2,8,1.

例如，钠原子有11个电子。它们排列在三个壳层中：第一层2个，第二层8个，第三层1个。写作 2,8,1。

Electrons in the outermost shell are called valence electrons. They are responsible for the chemical reactivity of the atom. Atoms with a full outer shell are stable, like the noble gases.

最外层电子称为价电子，它们决定原子的化学反应活性。具有全满最外层的原子是稳定的，例如稀有气体。

5. The Periodic Table | 元素周期表

The periodic table arranges elements in order of increasing atomic number. Elements are organised into rows called periods and columns called groups. The group number of an element tells you the number of electrons in its outer shell (for groups 1 to 7).

周期表按原子序数递增的顺序排列元素。元素被组织成行（称为周期）和列（称为族）。元素的族号告诉你它最外层电子的数目（对第1至第7族而言）。

Elements in the same group have similar chemical properties because they have the same number of outer electrons. For example, Group 1 metals are all reactive, while Group 0 elements are unreactive.

同一族中的元素具有相似的化学性质，因为它们具有相同的最外层电子数。例如，第1族金属都很活泼，而第0族元素则不活泼。

Group \| 族	Outer electrons \| 最外层电子数
1	1
2	2
6	6
7	7
0	8 (except helium, which has 2) \| 8（氦除外，氦为2）

6. Ions and Ionic Bonding | 离子与离子键

Atoms can lose or gain electrons to achieve a full outer shell. When an atom loses electrons, it becomes a positively charged ion called a cation. When an atom gains electrons, it becomes a negatively charged ion called an anion.

原子可以通过失去或获得电子来达到全满的最外层。当原子失去电子时，变成带正电荷的离子，称为阳离子。当原子获得电子时，变成带负电荷的离子，称为阴离子。

For example, sodium (Na, electron arrangement 2,8,1) loses one electron to form a Na⁺ ion. Chlorine (Cl, 2,8,7) gains one electron to form a Cl⁻ ion. The strong electrostatic force of attraction between oppositely charged ions is called an ionic bond.

例如，钠（Na，电子排布2,8,1）失去一个电子形成 Na⁺ 离子。氯（Cl，2,8,7）获得一个电子形成 Cl⁻ 离子。带相反电荷离子之间的强烈静电吸引力称为离子键。

Ionic compounds form giant ionic lattices. They have high melting points, are usually soluble in water, and conduct electricity when molten or dissolved in water because the ions are free to move.

离子化合物形成巨大的离子晶格。它们熔点高，通常可溶于水，在熔融或溶于水时能导电，因为离子可以自由移动。

7. Covalent Bonding | 共价键

A covalent bond is formed when two atoms share a pair of electrons. This usually happens between non-metal atoms. By sharing electrons, both atoms can achieve a full outer shell.

共价键是两个原子共享一对电子时形成的。这通常发生在非金属原子之间。通过共享电子，两个原子都能达到全满的最外层。

For example, in a water molecule (H₂O), each hydrogen atom shares one electron with the oxygen atom. The oxygen atom shares two of its electrons, one with each hydrogen, so each hydrogen has a full shell of 2 electrons and oxygen has a full shell of 8 electrons.

例如，在水分子（H₂O）中，每个氢原子与氧原子共享一个电子。氧原子共享其两个电子，每个氢原子各一个，因此每个氢原子都有全满的2电子壳层，氧原子有全满的8电子壳层。

Covalent compounds can be simple molecules, such as CO₂ and CH₄, or giant structures, such as diamond and graphite. Simple molecules have low melting points because the intermolecular forces are weak.

共价化合物可以是简单分子，如 CO₂ 和 CH₄，也可以是巨大结构，如金刚石和石墨。简单分子因分子间作用力弱而熔点低。

8. Metallic Bonding | 金属键

In metals, atoms lose their outer electrons to become positive ions. The lost electrons form a sea of delocalised electrons around the ions. The strong electrostatic attraction between the positive metal ions and the negative electron sea is called metallic bonding.

在金属中，原子失去其最外层电子成为正离子。失去的电子形成在离子周围自由移动的电子海。正金属离子与负电子海之间的强烈静电吸引称为金属键。

This structure explains why metals are good conductors of electricity and heat: the delocalised electrons can move freely. Metals are also malleable and ductile because layers of positive ions can slide over each other without breaking the bond.

这种结构解释了为什么金属是电和热的良导体：自由电子可以移动。金属还具有延展性和可锻性，因为正离子层可以在不破坏键的情况下相互滑动。

9. Empirical Formulas and Equations | 实验式与化学方程式

The empirical formula of a compound shows the simplest whole-number ratio of atoms of each element in the compound. For example, the empirical formula of hydrogen peroxide is HO, although its molecular formula is H₂O₂.

化合物的实验式表示化合物中各元素原子的最简整数比。例如，过氧化氢的实验式是 HO，而它的分子式是 H₂O₂。

In chemical equations, the number of atoms of each element must be balanced on both sides. You can change the coefficients in front of formulas but never change the small numbers (subscripts) inside formulas.

在化学方程式中，各元素的原子数在等号两侧必须平衡。你可以改变化学式前面的系数，但绝不能改变化学式中的小数字（下标）。

2H₂ + O₂ → 2H₂O

This equation shows that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water.

该方程表示两个氢分子与一个氧分子反应生成两个水分子。

10. Radioactivity and Atomic Structure | 放射性与原子结构

Some atoms have unstable nuclei, meaning the balance between protons and neutrons is not stable. These atoms are radioactive and emit radiation as they decay to become more stable.

一些原子的原子核不稳定，即质子和中子之间的平衡不稳定。这些原子具有放射性，并通过衰变放出辐射以变得更稳定。

Alpha (α): a nucleus of helium-4 (²⁴He), with a charge of +2 and mass number 4.
α粒子：氦-4核（²⁴He），电荷 +2，质量数 4。
Beta (β): a fast-moving electron emitted from the nucleus when a neutron changes into a proton.
β粒子：当中子转变为质子时从核中放出的高速电子。
Gamma (γ): high-energy electromagnetic radiation, no mass and no charge.
γ射线：高能电磁辐射，无质量、无电荷。

During alpha decay, the mass number decreases by 4 and the atomic number decreases by 2. During beta decay, the mass number stays the same but the atomic number increases by 1.

在α衰变中，质量数减少4，原子序数减少2。在β衰变中，质量数不变，但原子序数增加1。

Radioactive isotopes are used in many applications, such as carbon-14 dating, medical imaging and sterilising surgical equipment.

放射性同位素在许多应用中使用，例如碳-14测年、医学成像和手术器械消毒。

Conclusion | 总结

The structure of the atom is central to the whole IGCSE Science course. By understanding subatomic particles, electron arrangements, and the different types of bonding, you can explain a vast range of chemical and physical phenomena. Remember: atomic number tells you the protons, mass number tells you protons plus neutrons, and the electron arrangement controls reactivity.

原子结构是整个IGCSE科学课程的核心。通过理解亚原子粒子、电子排布和不同类型的键，你可以解释大量化学和物理现象。请记住：原子序数告诉你质子数，质量数告诉你质子数加中子数，而电子排布决定了反应活性。

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Atomic Structure | 原子结构