📚 Protons, Electrons, and the Periodic Table | 质子、电子与元素周期表
Atoms are the building blocks of all matter. To understand the periodic table, you first need to know about three subatomic particles: protons, neutrons and electrons. Protons give an element its identity, electrons control its chemical reactions, and the periodic table organises all known elements by the number of protons. This revision guide walks through the atom from the inside out, then shows how that structure explains the layout and behaviour of every element on the periodic table.
原子是所有物质的基本构成单元。要理解元素周期表，首先需要认识三种亚原子粒子：质子、中子和电子。质子决定元素的身份，电子控制元素的化学反应，而元素周期表则按照质子数将所有已知元素系统地排列起来。本复习指南将从原子内部结构讲起，再说明这一结构如何解释元素周期表的排列方式以及每一种元素的行为。
1. The Atom at a Glance | 原子速览
An atom is the smallest neutral particle of an element that can take part in chemical reactions. It contains a tiny, dense central nucleus made of protons and neutrons. Around the nucleus, electrons move in regions called shells or energy levels. Protons carry a positive charge, electrons carry a negative charge, and neutrons have no charge.
原子是元素能够参与化学反应的最小中性粒子。它含有一个微小而致密的中心原子核，核内由质子和中子组成。在原子核周围，电子在被称为电子层或能级的区域内运动。质子带正电荷，电子带负电荷，中子不带电荷。
Because the charges on protons and electrons are equal but opposite, a neutral atom has the same number of protons and electrons. The overall charge is therefore zero. This balance is central to understanding how atoms become ions later.
由于质子和电子所带电荷大小相等、符号相反，中性原子的质子数和电子数相同，因此总电荷为零。这种电荷平衡对以后理解原子如何变成离子至关重要。
The nucleus is extremely small compared with the whole atom. If an atom were the size of a sports stadium, the nucleus would be roughly the size of a pea at the centre, while the electrons would be moving somewhere out in the stands. This means most of an atom is empty space, yet almost all of its mass sits in the nucleus.
原子核与整个原子相比极其微小。如果把一个原子比作一座体育场，原子核大约只有场地中央的一颗豌豆那么大，而电子则在看台一带运动。这意味着原子内部大部分是空的，但几乎全部质量都集中在原子核里。
2. Relative Mass and Charge of Subatomic Particles | 亚原子粒子的相对质量与电荷
Scientists use relative mass and relative charge to compare subatomic particles easily. A proton is given a relative mass of 1 and a relative charge of +1. A neutron also has a relative mass of 1 but a relative charge of 0. An electron has a relative charge of -1 and a very small relative mass, often treated as 0 at KS3 level.
科学家使用相对质量和相对电荷来方便地比较亚原子粒子。质子的相对质量定为1，相对电荷为+1。中子的相对质量也是1，但相对电荷为0。电子的相对电荷为-1，相对质量非常小，在KS3阶段通常视为0。
| Particle / 粒子 | Relative mass / 相对质量 | Relative charge / 相对电荷 | Location / 位置 |
|---|---|---|---|
| Proton 质子 | 1 | +1 | Nucleus 原子核 |
| Neutron 中子 | 1 | 0 | Nucleus 原子核 |
| Electron 电子 | 1/1840 (treated as 0) 视为0 | -1 | Shells 电子层 |
An important consequence of these numbers is that the mass of an atom is almost entirely decided by the protons and neutrons in its nucleus. The electrons contribute so little mass that they are ignored in most KS3 calculations, even though they are the particles that actually do the chemistry.
这些数据的一个重要结果是：原子的质量几乎完全由原子核中的质子和中子决定。电子贡献的质量极小，因此在KS3阶段的大多数计算中都被忽略，尽管真正参与化学反应的恰恰是电子。
3. Atomic Number, Proton Number and Element Identity | 原子序数、质子数与元素身份
The number of protons in an atom is called the atomic number, or proton number. It is the single most important number for an element, because it defines which element the atom is. Change the number of protons and you no longer have the same element.
原子中质子的数目称为原子序数，也叫质子数。这是元素最重要的一个数值，因为它决定了这个原子究竟是哪种元素。一旦质子数改变，元素身份也随之改变。
| Element / 元素 | Proton number / 质子数 | Neutron number / 中子数 | Electron number (neutral) / 电子数（中性） |
|---|---|---|---|
| Hydrogen 氢 | 1 | 0 | 1 |
| Helium 氦 | 2 | 2 | 2 |
| Carbon 碳 | 6 | 6 | 6 |
| Sodium 钠 | 11 | 12 | 11 |
| Chlorine 氯 | 17 | 18 | 18 |
In a neutral atom, proton number equals electron number. This is why you can work out the electron arrangement of any element simply by knowing its position in the periodic table, or its proton number from the data sheet.
在中性原子中，质子数等于电子数。因此，只要知道元素在周期表中的位置，或者从数据表中查到它的质子数，就能推导出它的电子排布。
4. Electron Shells and Energy Levels | 电子层与能级
Electrons do not sit in fixed orbits like planets. Instead, they occupy shells (also called energy levels) around the nucleus. The first shell is closest to the nucleus and holds a maximum of 2 electrons. The second shell holds up to 8, and the third shell also holds up to 8 for the first 20 elements.
电子并不像行星那样沿固定轨道运动，而是分布在原子核周围的电子层（也叫能级）上。第一层离核最近，最多容纳2个电子；第二层最多8个；对前20号元素来说，第三层最多也是8个。
Shell pattern rule: 2, 8, 8, ... / 电子层规律：2、8、8……
Electrons fill the inner shells first. Sodium, with 11 electrons, fills two in the first shell, eight in the second shell, and one in the third shell. We write this as 2, 8, 1. The electrons in the outermost shell are called outer electrons, and they are the ones that matter for chemistry.
电子先填满内层。钠有11个电子，第一层2个、第二层8个、第三层1个，写作2, 8, 1。最外层上的电子称为外层电子，正是它们决定了元素的化学性质。
- Hydrogen 氢: 1 electron → arrangement 1
- Carbon 碳: 6 electrons → arrangement 2, 4
- Oxygen 氧: 8 electrons → arrangement 2, 6
- Sodium 钠: 11 electrons → arrangement 2, 8, 1
- Chlorine 氯: 17 electrons → arrangement 2, 8, 7
- Calcium 钙: 20 electrons → arrangement 2, 8, 8, 2
5. How the Periodic Table Is Organised | 元素周期表如何编排
The periodic table arranges all known elements in order of increasing atomic number (proton number). Elements are placed in rows called periods and columns called groups. Because the order is based on protons, elements with similar electron arrangements end up in the same group, and that is why they behave similarly.
元素周期表按原子序数（质子数）递增的顺序排列所有已知元素。元素被放入称为周期的横行和称为族的纵列中。由于排列依据是质子数，电子排布相似的元素最终落在同一族中，这正是它们性质相似的原因。
| Position / 位置 | Outer electrons / 外层电子数 | Typical behaviour / 典型行为 |
|---|---|---|
| Group 1 第1族 | 1 | Lose 1 electron, form 1+ ions 失去1个电子，形成1+离子 |
| Group 2 第2族 | 2 | Lose 2 electrons, form 2+ ions 失去2个电子，形成2+离子 |
| Group 7 第7族 | 7 | Gain 1 electron, form 1- ions 得到1个电子，形成1-离子 |
| Group 0 第0族 | 8 (full) 满层 | Unreactive 不活泼 |
For the first 20 elements, the group number is directly linked to the number of outer electrons. This is why the table is such a powerful predictive tool: you can often guess how an element will react just from where it sits.
对前20号元素而言，族号与外层电子数直接相关。这正是周期表如此有用的原因：往往只看元素的位置，就能推测它会有怎样的反应。
6. From Electron Arrangement to Chemical Behaviour | 从电子排布到化学性质
Protons give an element its identity, but electrons control its chemical reactions. Atoms react in order to achieve a full outer shell, usually eight electrons, which is the same arrangement as a noble gas. Metals tend to lose electrons and non-metals tend to gain or share them.
质子决定元素的身份，电子控制元素的化学反应。原子发生反应，是为了达到外层电子满层结构，通常是8个电子，与稀有气体相同。金属倾向于失去电子，非金属则倾向于得到或共用电子。
Sodium has the arrangement 2, 8, 1. Losing its single outer electron leaves it with 2, 8, which is the arrangement of neon. It therefore forms a Na⁺ ion. Chlorine has 2, 8, 7 and gains one electron to reach 2, 8, 8, the arrangement of argon, forming a Cl⁻ ion. The two ions then attract each other to make sodium chloride.
钠的排布是2, 8, 1。失去唯一的1个外层电子后剩下2, 8，正是氖的排布，因此它形成Na⁺离子。氯的排布是2, 8, 7，得到1个电子后达到2, 8, 8，即氩的排布，形成Cl⁻离子。两种离子相互吸引，就生成了氯化钠。
Na → Na⁺ + e⁻ and Cl + e⁻ → Cl⁻
Electron transfer like this produces ions. Electron sharing, on the other hand, produces covalent bonds, which you will meet in later topics. Either way, the outer electrons are the deciding factor.
这样的电子转移会产生离子。而电子共用则形成共价键，这会在后续内容中学习。无论哪种情况，起决定作用的都是外层电子。
7. Isotopes and Relative Atomic Mass | 同位素与相对原子质量
Atoms of the same element always have the same proton number, but they do not always have the same number of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes.
同一种元素的原子质子数总是相同的，但中子数不一定相同。同种元素中中子数不同的原子被称为同位素。
- Carbon-12 碳-12: 6 protons, 6 neutrons, mass number 12
- Carbon-13 碳-13: 6 protons, 7 neutrons, mass number 13
- Carbon-14 碳-14: 6 protons, 8 neutrons, mass number 14
- Chlorine-35 氯-35: 17 protons, 18 neutrons, mass number 35
- Chlorine-37 氯-37: 17 protons, 20 neutrons, mass number 37
Isotopes react in the same way chemically, because they have the same electron arrangement. What differs is their mass. The relative atomic mass shown on the periodic table is an average that takes the abundance of each isotope into account.
同位素的化学性质相同，因为它们的电子排布相同，不同的只是质量。周期表上标示的相对原子质量，就是考虑了各同位素丰度后的平均值。
Relative atomic mass = Σ (isotope mass × abundance) / 100
For chlorine, about 75% is chlorine-35 and about 25% is chlorine-37, so the relative atomic mass is roughly 35.5. This is a very common exam calculation.
对氯而言，约75%为氯-35，约25%为氯-37，所以相对原子质量约为35.5。这是考试中非常常见的计算题。
8. Ions, Charges and the Balance of Particles | 离子、电荷与粒子平衡
An ion is a charged particle formed when an atom loses or gains electrons. The nucleus never changes during ordinary chemical reactions, so the proton number stays the same while the electron number changes. This breaks the balance between positive and negative charge.
离子是原子失去或得到电子后形成的带电粒子。在普通化学反应中原子核不会改变，所以质子数保持不变，改变的只是电子数。这样一来，正负电荷的平衡被打破。
| Ion / 离子 | Protons / 质子数 | Electrons / 电子数 | Charge / 电荷 |
|---|---|---|---|
| Na⁺ | 11 | 10 | +1 |
| Mg²⁺ | 12 | 10 | +2 |
| Cl⁻ | 17 | 18 | -1 |
| O²⁻ | 8 | 10 | -2 |
A useful shortcut is that the charge on a simple ion equals the number of electrons gained or lost. Metals in Groups 1 and 2 lose electrons and become positive; non-metals in Groups 6 and 7 gain electrons and become negative.
一个好用的捷径是：简单离子的电荷数等于得到或失去的电子数。第1、2族的金属失去电子带正电，第6、7族的非金属得到电子带负电。
9. Metals, Non-metals and the Shape of the Table | 金属、非金属与周期表的格局
The periodic table is roughly divided into metals on the left and centre, and non-metals on the right. A staircase-shaped line separates them. Elements close to that line, such as silicon, have properties in between and are called metalloids.
元素周期表大致分为左侧和中间的金属区，以及右侧的非金属区，二者之间由一条阶梯状的分界线隔开。靠近这条线的元素（例如硅）性质介于两者之间，被称为类金属。
As you move across a period from left to right, the proton number increases and one outer electron is added each time. Elements on the left have few outer electrons and tend to lose them; elements on the right have nearly full shells and tend to gain electrons. Down a group, the atoms get larger and the outer electrons are further from the nucleus.
在同一周期中从左向右移动，质子数递增，外层电子每次增加一个。左侧元素外层电子少，倾向于失去电子；右侧元素外层接近满层，倾向于得到电子。同族自上而下，原子半径增大，外层电子离核更远。
Noble gases in Group 0 have full outer shells. Because they already have the arrangement that other atoms are trying to reach, they have no need to react, which is why they are described as inert.
第0族的稀有气体外层电子已满。由于它们已经具备其他原子努力想达到的排布，无需再发生反应，因此被称为惰性气体。
10. Common Misconceptions and Exam Tips | 常见误区与答题技巧
Many students lose marks not because they do not know the content, but because of small slips in wording or logic. Below are the mistakes that appear most often in KS3 and GCSE chemistry papers.
许多学生丢分并不是因为不懂知识，而是因为措辞或逻辑上的小疏漏。以下是KS3和GCSE化学试卷中最常出现的错误。
- Mistake 1: Saying an electron has a mass of 0. It has a very small mass, treated as 0 at this level. 误说电子质量为0——它只是质量极小，在这个阶段才视为0。
- Mistake 2: Confusing mass number with relative atomic mass. Mass number counts protons and neutrons in one atom; relative atomic mass is an average. 把质量数与相对原子质量混淆——前者是单个原子内质子与中子数之和，后者是平均值。
- Mistake 3: Thinking isotopes have different chemical properties. They have the same electron arrangement, so they react the same way. 以为同位素化学性质不同——它们电子排布相同，反应行为一致。
- Mistake 4: Writing the electron arrangement starting from the outer shell. Always fill from the innermost shell outwards. 电子排布从外层写起——必须由内层向外层依次填充。
- Mistake 5: Saying protons move during a chemical reaction. The nucleus stays unchanged; only electrons are transferred or shared. 认为化学反应中质子会移动——原子核不变，只有电子转移或共用。
When answering questions, always state the number of outer electrons and link it to the group number, then explain the resulting ion charge. This structure earns method marks even if the final answer is wrong.
答题时，要写出外层电子数，将其与族号联系起来，再说明由此形成的离子电荷。即使最终答案有误，这种结构也能拿到方法分。
11. Quick Revision Summary | 快速复习总结
Use the summary below as a last-minute check before a test. If you can reproduce every line from memory, you have the core of this topic secure.
把下面的总结当作考前最后一遍检查。如果每一行都能凭记忆写出，这个专题的核心内容就已经掌握牢固了。
- An atom has a nucleus of protons and neutrons, with electrons in shells. 原子由质子和中子组成的原子核以及分层排布的电子构成。
- 1, charge +1. Neutron: relative mass 1, charge 0. Electron: relative mass ≈ 0, charge -1. 质子：相对质量1，电荷+1；中子：相对质量1，电荷0；电子：相对质量约为0，电荷-1。
- The proton number defines the element and equals the atomic number. 质子数决定元素种类，即原子序数。
- In a neutral atom, protons = electrons. 中性原子中质子数等于电子数。
- Electron shells fill 2, 8, 8 for the first 20 elements. 前20号元素电子层填充为2、8、8。
- Group number relates to outer electrons; period number relates to the number of shells. 族号与外层电子数相关，周期号与电子层数相关。
- Elements react to gain a full outer shell, usually 8 electrons. 元素反应是为了获得满外层，通常为8个电子。
- Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions. 金属失电子形成正离子，非金属得电子形成负离子。
- Isotopes have the same protons but different neutrons. 同位素质子数相同，中子数不同。
- The periodic table is ordered by increasing proton number. 元素周期表按质子数递增排列。
Mastering these points gives you a reliable foundation for topics such as bonding, reactivity trends and quantitative chemistry. Revise the particle table and the electron arrangement rules until they are automatic, then practise applying them to unfamiliar elements.
掌握这些要点，就能为化学键、反应性规律以及定量化学等后续内容打下可靠基础。把粒子数据表和电子排布规律练到脱口而出，再尝试把它们应用到陌生的元素上。
Published by TutorHao | KS3 Chemistry 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