Atomic Structure and the Periodic Table | 原子结构与元素周期表

📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表

In this revision guide, we will explore the fundamental building blocks of matter — atoms — and how the periodic table organises the elements based on their structure and properties. This topic is essential for your IGCSE Edexcel Science examinations, covering key concepts such as subatomic particles, electronic configuration, and trends in the periodic table.

在本复习指南中，我们将探索物质的基本组成单位——原子，以及元素周期表如何根据结构和性质对元素进行组织。本主题对于IGCSE Edexcel科学考试至关重要，涵盖亚原子粒子、电子排布和周期表规律等核心概念。

1. The Structure of an Atom | 原子的结构

Every atom consists of a tiny, dense nucleus containing protons and neutrons, surrounded by electrons that move in energy levels (shells). Protons are positively charged, neutrons are neutral, and electrons are negatively charged.

每个原子都由一个微小而致密的原子核构成，核内包含质子和中子，核外是分层（电子壳层）运动的电子。质子带正电荷，中子不带电，电子带负电荷。

Proton (p⁺): relative mass 1, relative charge +1, located in the nucleus.
Neutron (n⁰): relative mass 1, relative charge 0, located in the nucleus.
Electron (e⁻): relative mass 1/1840 (negligible), relative charge −1, located in the shells around the nucleus.

质子（p⁺）：相对质量为1，相对电荷为+1，位于原子核内。
中子（n⁰）：相对质量为1，相对电荷为0，位于原子核内。
电子（e⁻）：相对质量为1/1840（可忽略不计），相对电荷为−1，位于原子核外的电子壳层中。

Atomic number (Z) = number of protons = number of electrons (in a neutral atom)

质子数（Z）= 质子数 = 电子数（对于中性原子）

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

质量数（A）= 质子数 + 中子数

2. Isotopes and Relative Atomic Mass | 同位素与相对原子质量

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

同位素是同一元素的不同原子，它们具有相同的质子数但不同的中子数。因此，它们具有相同的原子序数，但质量数不同。

For example, carbon-12 (¹²C) has 6 protons and 6 neutrons, while carbon-14 (¹⁴C) has 6 protons and 8 neutrons. Because they have the same electron arrangement, isotopes show identical chemical properties.

例如，碳-12（¹²C）有6个质子和6个中子，而碳-14（¹⁴C）有6个质子和8个中子。由于它们具有相同的电子排布，同位素表现出相同的化学性质。

Relative atomic mass (Aᵣ) is the weighted mean mass of an atom compared to 1/12th of the mass of a carbon-12 atom. If the abundance of each isotope is known, Aᵣ can be calculated:

相对原子质量（Aᵣ）是该元素一个原子的平均质量与碳-12原子质量的1/12之比。如果知道每种同位素的丰度，就可以计算Aᵣ：

Aᵣ = (mass of isotope 1 × abundance₁ + mass of isotope 2 × abundance₂) ÷ total abundance

Aᵣ =（同位素1的质量×丰度₁ + 同位素2的质量×丰度₂）÷ 总丰度

3. Electronic Configuration | 电子排布

Electrons occupy energy levels (shells) around the nucleus. The arrangement of electrons in these shells is called the electronic configuration or electronic structure.

电子占据原子核周围的能级（壳层）。电子在这些壳层中的排列称为电子排布或电子结构。

First shell: maximum 2 electrons
Second shell: maximum 8 electrons
Third shell: maximum 8 electrons (for the first 20 elements)

第一壳层：最多2个电子
第二壳层：最多8个电子
第三壳层：最多8个电子（对前20号元素而言）

For example, sodium (Na, Z=11) has electronic configuration 2,8,1. Chlorine (Cl, Z=17) has 2,8,7.

例如，钠（Na，Z=11）的电子排布为2,8,1。氯（Cl，Z=17）的电子排布为2,8,7。

Electron shells are filled from the innermost shell outward, and the outermost shell determines chemical reactivity.

电子壳层从内向外填充，最外层电子决定了化学活性。

4. The Periodic Table: Basic Organisation | 元素周期表：基本组织

The periodic table arranges elements in order of increasing atomic number. The vertical columns are called groups, and the horizontal rows are called periods.

元素周期表按原子序数递增的顺序排列元素。垂直的列称为族，水平的行称为周期。

Group number: tells you the number of electrons in the outer shell (for Groups 1 to 7).
Period number: tells you the number of occupied electron shells.

族号：表示最外层电子数（对于第1至7族）。
周期号：表示被占据的电子壳层数。

Group / 族	Outer electrons / 最外层电子	Example / 示例
1	1	Li (2,1), Na (2,8,1)
2	2	Be (2,2), Mg (2,8,2)
7	7	F (2,7), Cl (2,8,7)
0 (noble gases)	8 (He has 2)	Ne (2,8), Ar (2,8,8)

5. Trends in Groups: Group 1 – Alkali Metals | 族内规律：第1族 – 碱金属

Group 1 elements (lithium, sodium, potassium, etc.) are soft, low-density metals that react vigorously with water. They all have one electron in their outer shell, which they easily lose to form a +1 ion.

第1族元素（锂、钠、钾等）是柔软、低密度的金属，能与水剧烈反应。它们最外层均有一个电子，容易失去电子形成+1价离子。

Going down the group, the reactivity increases. This is because the outer electron is further from the nucleus, more shielded by inner shells, and therefore less strongly attracted to the nucleus, making it easier to lose.

从上往下，反应活性递增。这是因为最外层电子离核更远，受到内层电子的屏蔽作用更大，因此原子核对其引力较弱，更容易失去电子。

Sodium reacting with water: forms sodium hydroxide and hydrogen gas.
Potassium reacts more violently than sodium.

钠与水反应：生成氢氧化钠和氢气。
钾与水的反应比钠更剧烈。

2Na + 2H₂O → 2NaOH + H₂

6. Group 7 – Halogens | 第7族 – 卤素

Group 7 elements (fluorine, chlorine, bromine, iodine) are non-metals with seven outer electrons. They gain one electron to form a −1 ion or share one electron in covalent bonds.

第7族元素（氟、氯、溴、碘）是最外层有7个电子的非金属。它们通过得到一个电子形成−1价离子，或通过共用一对电子形成共价键。

Going down the group, the reactivity decreases. The atomic radius increases, and the outer shell is further from the nucleus, so attracting an extra electron is harder.

从上往下，反应活性递减。原子半径增大，最外层离核更远，因此获得额外电子更加困难。

More reactive halogens can displace less reactive halogens from their compounds:

较活泼的卤素可以将较不活泼的卤素从其化合物中置换出来：

Cl₂ + 2KBr → 2KCl + Br₂

Chlorine is more reactive than bromine, so it displaces bromine from potassium bromide solution, turning the solution orange.

氯比溴活泼，因此氯能从溴化钾溶液中置换出溴，使溶液变为橙色。

7. Group 0 – Noble Gases | 第0族 – 稀有气体

Noble gases (helium, neon, argon, krypton, xenon) have full outer shells (helium has 2, others have 8). This makes them very stable and chemically unreactive — they exist as single atoms.

稀有气体（氦、氖、氩、氪、氙）具有饱满的最外层电子壳（氦为2个，其他为8个）。这使得它们非常稳定且化学性质不活泼——以单原子形式存在。

The boiling points of noble gases increase as you go down the group because the atoms become larger and the weak intermolecular forces become stronger, requiring more energy to overcome.

稀有气体的沸点随原子序数增加而升高，因为原子变大，分子间弱相互作用力加强，需要更多能量来克服。

Noble gas / 稀有气体	Outer shell / 最外层	Use / 用途
Helium (He)	2	Balloons, cooling
Neon (Ne)	8	Advertising signs
Argon (Ar)	8	Welding, light bulbs

8. Metals vs Non-Metals | 金属与非金属

The periodic table is divided by a stepped line separating metals (left) from non-metals (right). Metals typically have the following properties:

元素周期表由一条阶梯线分为金属（左侧）和非金属（右侧）。金属通常具有以下性质：

High melting and boiling points
Good conductors of heat and electricity
Malleable and ductile
Shiny when freshly cut
Form positive ions

高熔点和沸点
良好的热和电导体
具有延展性和可锻性
新切面有光泽
形成正离子

Non-metals are usually the opposite: poor conductors (except graphite), dull, brittle, and form negative ions or covalent compounds.

非金属通常相反：导电性差（石墨除外）、外观暗淡、易碎，形成负离子或共价化合物。

9. Ions and Ionic Bonding | 离子与离子键

When a metal reacts with a non-metal, electrons are transferred from the metal to the non-metal. This creates positively charged metal ions and negatively charged non-metal ions, which are held together by strong electrostatic forces called ionic bonds.

当金属与非金属反应时，电子从金属转移到非金属。这产生带正电的金属离子和带负电的非金属离子，它们通过强静电引力——即离子键——结合在一起。

For example, sodium (2,8,1) loses one electron to become Na⁺ (2,8). Chlorine (2,8,7) gains one electron to become Cl⁻ (2,8,8). The resulting compound is sodium chloride (NaCl).

例如，钠（2,8,1）失去一个电子变为Na⁺（2,8）。氯（2,8,7）得到一个电子变为Cl⁻（2,8,8）。生成的化合物是氯化钠（NaCl）。

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

10. Covalent Bonding | 共价键

Non-metal atoms can share pairs of electrons to achieve a full outer shell. Each shared pair of electrons constitutes one covalent bond.

非金属原子可以通过共用电子对来达到满的最外层电子壳。每对共享电子构成一个共价键。

Hydrogen molecule (H₂): each H shares its one electron, forming a single bond.
Water (H₂O): oxygen shares two pairs with two hydrogen atoms.
Carbon dioxide (CO₂): each double bond contains two shared pairs of electrons.

氢分子（H₂）：每个H共享其一个电子，形成一个单键。
水（H₂O）：氧与两个氢原子共享两对电子。
二氧化碳（CO₂）：每个双键包含两个共享电子对。

Covalent compounds typically have low melting and boiling points, are poor conductors, and can exist as simple molecules or giant structures such as diamond and graphite.

共价化合物通常具有较低的熔点和沸点，不导电，可以以简单分子或巨型结构（如金刚石和石墨）存在。

11. Group Trends in Electron Shell Loss/Gain | 电子得失的族内趋势

In Group 1, elements have one outer electron; losing that electron gives a stable noble-gas configuration. Reactivity increases down the group because the electron is lost more easily.

在第1族中，元素有一个最外层电子；失去该电子获得稳定的稀有气体构型。由于电子更容易失去，活性向下增大。

In Group 7, elements need one more electron to complete their outer shell. Reactivity decreases down the group because it becomes harder to attract an extra electron as the atomic radius increases.

在第7族中，元素需要再获得一个电子以完成最外层。由于原子半径增大，吸引额外电子变得更困难，活性向下减小。

These opposing trends are a classic exam question. Always compare the distance of the outer shell from the nucleus and the strength of the nuclear attraction.

这两种相反的趋势是经典考点。务必比较最外层离核的距离以及原子核引力的强弱。

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

Do not confuse atomic number and mass number: atomic number is the number of protons; mass number is protons + neutrons.
Use the correct notation: for example, Mg²⁺ has 12 protons and 10 electrons, not 12 electrons.
Remember that isotopes have identical chemical properties because they have the same electronic configuration.
When drawing electronic configurations, always write the number of electrons in each shell separated by commas (e.g., 2,8,1).
For group trends, always explain in terms of electron shielding and nuclear charge.

不要混淆原子序数与质量数：原子序数是质子数；质量数是质子数加中子数。
使用正确的符号：例如，Mg²⁺有12个质子和10个电子，而不是12个电子。
记住同位素具有相同的化学性质，因为它们具有相同的电子排布。
写出电子排布时，用逗号分隔每个壳层的电子数（例如2,8,1）。
对于族内规律，解释时务必涉及电子屏蔽和核电荷。

Practice drawing Bohr diagrams and writing electronic configurations for the first 20 elements. Then, compare their positions in the periodic table to predict their behaviour in chemical reactions.

练习绘制玻尔模型图并为前20号元素写出电子排布。然后，比较它们在周期表中的位置，以预测它们在化学反应中的行为。

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Atomic Structure and the Periodic Table | 原子结构与元素周期表