📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
This article explores the internal structure of atoms and the systematic organisation of elements in the periodic table. These concepts are essential for Edexcel IGCSE Science and appear frequently in exam questions on bonding, reactivity and trends.
本文探讨原子的内部结构以及元素在周期表中的系统排列。这些概念是 Edexcel IGCSE 科学的核心内容，并频繁出现在关于化学键、反应活性和变化规律的考题中。
1. The Atomic Model | 原子模型
The modern atom is composed of a tiny, dense nucleus surrounded by electrons. The nucleus contains protons and neutrons, while electrons occupy specific energy levels called shells.
现代原子由一个微小而致密的原子核以及环绕其外的电子组成。原子核包含质子和中子，而电子占据称为电子壳层的特定能级。
In a neutral atom, the number of protons equals the number of electrons. This means the positive charge of the nucleus exactly balances the negative charge of the surrounding electrons.
在中性原子中，质子数等于电子数。这意味着原子核的正电荷与周围电子的负电荷完全平衡。
The diameter of an atom is about 0.1 nm, while the nucleus is roughly 10,000 times smaller. Despite this, nearly all the mass of an atom is concentrated in the nucleus.
原子的直径约为 0.1 nm，而原子核大约还要小一万倍。尽管如此，原子几乎全部的质量都集中在原子核中。
2. Subatomic Particles | 亚原子粒子
The three subatomic particles you must know are the proton, neutron and electron. Their relative masses and charges are summarised below.
你必须掌握的三种亚原子粒子是质子、中子和电子。它们的相对质量和相对电荷总结如下。
| Particle | Relative mass | Relative charge | Location |
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 (negligible) | -1 | Shells around nucleus |
Protons and neutrons are collectively called nucleons because they are found in the nucleus. The electron is very light and its mass is often taken as zero in IGCSE calculations.
质子和中子统称为核子，因为它们都位于原子核内。电子非常轻，在 IGCSE 计算中其质量通常忽略不计。
3. Atomic Number and Mass Number | 原子序数和质量数
The atomic number (Z) is the number of protons in the nucleus of an atom. For a neutral atom, the atomic number also equals the number of electrons.
原子序数（Z）是原子核中的质子数。对于中性原子，原子序数也等于电子数。
The mass number (A) is the total number of protons and neutrons in the nucleus. Neutrons can be calculated using the rule:
质量数（A）是原子核中质子数和中子数的总和。中子数可用如下规则计算：
Number of neutrons = A - Z
中子数 = 质量数 A - 原子序数 Z
For example, a sodium atom has 11 protons and 12 neutrons, so its mass number is 23. In exam notation this is written as sodium-23 or 23Na.
例如，钠原子有 11 个质子和 12 个中子，因此其质量数为 23。在考试书写中可记作钠-23 或 23Na。
4. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because they have the same proton number, their chemical properties are identical.
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，它们的化学性质相同。
Common examples include carbon-12 and carbon-14. Both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons.
常见例子包括碳-12 和碳-14。两者都有 6 个质子，但碳-12 有 6 个中子，而碳-14 有 8 个中子。
Isotopes may have different physical properties, such as density, melting point and rate of diffusion. They are used in medicine, archaeology and industry, for example carbon-14 for dating ancient materials.
同位素可能具有不同的物理性质，如密度、熔点和扩散速率。它们可用于医学、考古和工业，例如用碳-14 测定古代材料的年代。
5. Electron Configuration | 电子排布
Electrons occupy shells around the nucleus. For the first 20 elements, the shell capacities are: 2 in the first shell, 8 in the second shell and 8 in the third shell (for IGCSE purposes).
电子占据原子核周围的壳层。对于前 20 号元素，壳层容量为：第一层 2 个、第二层 8 个、第三层 8 个（在 IGCSE 范围内）。
The electron configuration lists the number of electrons in each shell. For example, sodium has 11 electrons arranged as 2, 8, 1. Its configuration is written as 2,8,1.
电子排布列出每个壳层中的电子数。例如，钠有 11 个电子，排列为 2, 8, 1。其电子排布写作 2,8,1。
Electron configuration determines how an atom reacts. Atoms with a full outermost shell (2 or 8 electrons) are stable and unreactive, while atoms with one or seven outer electrons are very reactive.
电子排布决定了原子如何反应。最外层满壳层（2 或 8 个电子）的原子稳定且不活泼，而最外层有 1 个或 7 个电子的原子则非常活泼。
6. The Periodic Table Layout | 周期表布局
The periodic table arranges elements in order of increasing atomic number. Horizontal rows are called periods, and vertical columns are called groups.
周期表按照原子序数递增的顺序排列元素。横的行称为周期，纵的列称为族。
Elements in the same group have the same number of outer electrons and therefore show similar chemical properties. For example, group 1 elements all have one electron in their outer shell.
同一族的元素具有相同的最外层电子数，因此表现出相似的化学性质。例如，第 1 族元素的最外层都有一个电子。
The period number tells you how many electron shells are occupied. For instance, elements in period 3 have electrons in three shells.
周期数告诉你原子的电子层数。例如，第 3 周期的元素拥有三个电子壳层。
7. Group 1: Alkali Metals | 第1族：碱金属
Group 1 elements include lithium, sodium and potassium. They are soft, have low densities and have relatively low melting points compared with typical metals.
第 1 族元素包括锂、钠和钾。它们质地柔软、密度较小，相对于典型金属而言熔点较低。
Alkali metals have one outer electron, which they lose easily to form a +1 ion. This makes them highly reactive, and their reactivity increases down the group.
碱金属只有一个最外层电子，容易失去形成 +1 离子。这使得它们非常活泼，且活泼性自上而下增强。
For example, lithium reacts slowly with water, sodium reacts vigorously, and potassium reacts explosively. The general equation is:
例如，锂与水反应缓慢，钠与水反应剧烈，而钾与水反应十分剧烈甚至爆炸。通式如下：
metal + water → metal hydroxide + hydrogen
金属 + 水 → 金属氢氧化物 + 氢气
8. Group 7: Halogens | 第7族：卤素
Group 7 elements are the halogens: fluorine, chlorine, bromine and iodine. At room temperature, their states change from gas (F₂, Cl₂) to liquid (Br₂) to solid (I₂), and their colours become darker down the group.
第 7 族元素是卤素：氟、氯、溴和碘。在室温下，它们的状态从气体（F₂、Cl₂）变为液体（Br₂）再变为固体（I₂），颜色也随原子序数增大而加深。
Halogens have seven outer electrons and gain one electron to form a -1 ion. Their reactivity decreases down the group, which is the opposite of group 1.
卤素最外层有 7 个电子，得到一个电子形成 -1 离子。它们的活泼性自上而下减弱，与第 1 族恰好相反。
A more reactive halogen will displace a less reactive halogen from its salt solution. For example, chlorine displaces bromine from potassium bromide solution.
较活泼的卤素能将较不活泼的卤素从其盐溶液中置换出来。例如，氯可以从溴化钾溶液中置换出溴。
9. Group 0: Noble Gases | 第0族：稀有气体
The noble gases are helium, neon, argon, krypton and xenon. They all have a full outer shell of electrons, which makes them very stable and chemically unreactive.
稀有气体包括氦、氖、氩、氪和氙。它们的最外层电子均已填满，因此非常稳定，化学性质极不活泼。
Noble gases exist as single atoms rather than molecules. For example, helium is used in balloons and argon is used in welding to provide an inert atmosphere.
稀有气体以单原子形式存在，而不是分子。例如，氦用于气球，氩用于焊接中提供惰性气氛。
Because they are unreactive, noble gases do not form compounds under ordinary conditions. This property is directly linked to their electron configuration.
由于稀有气体不活泼，在通常条件下不会形成化合物。这一特性与其电子排布直接相关。
10. Trends Across a Period | 周期内的变化趋势
Moving from left to right across a period, the atomic number increases while the number of electron shells remains the same. The number of outer electrons increases from 1 to 8.
从左到右横跨一个周期时，原子序数增加，但电子层数保持不变。最外层电子数从 1 增加到 8。
As a result, metallic character decreases and non-metallic character increases across a period. For example, period 3 starts with sodium (a reactive metal) and ends with argon (a noble gas).
因此，在一个周期内，金属性减弱，非金属性增强。例如，第 3 周期从钠（活泼金属）开始，到氩（稀有气体）结束。
Within the same period, the atomic radius tends to decrease because the increased nuclear charge pulls the electrons closer. This trend helps explain patterns in reactivity and ionisation energy.
在同一周期内，原子半径往往减小，因为核电荷增加会把电子拉得更近。这一趋势有助于解释反应活性和电离能的变化规律。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图