📚 IB Chemistry: Periods and Groups in the Periodic Table | IB化学：元素周期表的周期与族
The periodic table is the central organising framework in chemistry. By arranging elements in order of increasing atomic number, it reveals recurring trends in physical and chemical properties. For IB Chemistry students, understanding the meaning of periods and groups is essential for predicting reactivity, bonding, and structure.
元素周期表是化学中核心的组织框架。按原子序数递增排列元素，可以揭示物理和化学性质的周期性变化。对于IB化学学生而言，理解周期与族的含义，是预测反应性、成键方式和结构的关键基础。
1. Structure of the Periodic Table | 周期表的结构
The periodic table arranges elements into rows called periods and columns called groups. Each element's position is determined by its electron configuration, specifically the highest occupied principal quantum number and the number of valence electrons.
周期表将元素排列为水平行（称为周期）和垂直列（称为族）。每个元素的位置由其电子构型决定，特别是最高占据主量子数和价电子数。
Elements in the same group share the same outer-shell electron arrangement, which gives them similar chemical behaviour. Moving across a period, the number of protons increases while electrons are added to the same principal energy level.
同一族元素具有相同的外层电子排布，因此表现出相似的化学行为。沿周期向右移动时，质子数增加，而电子被填入同一主能级。
2. Periods | 周期
A period corresponds to a horizontal row in the periodic table. The period number equals the principal quantum number (n) of the highest occupied energy level in the ground-state electron configuration.
周期对应周期表中的水平行。周期数等于基态电子构型中最高占据能级的主量子数（n）。
For example, sodium (Na) is in period 3 because its electron configuration ends at n = 3: 1s² 2s² 2p⁶ 3s¹. The first 18 elements span periods 1–3, while periods 4 and 5 contain 18 elements, and periods 6 and 7 contain 32 elements due to the f-block.
例如，钠（Na）位于第三周期，因为其电子构型终止于n = 3：1s² 2s² 2p⁶ 3s¹。前18种元素跨越第1至第3周期，第4和第5周期各含18种元素，而第6和第7周期因f区元素的存在各含32种元素。
As we move from left to right across a period, the atomic number increases, the nuclear charge increases, and electrons are added to the same shell. This leads to a gradual change in properties such as atomic radius, ionisation energy, and metallic character.
从左到右穿过一个周期时，原子序数增加，核电荷增大，而电子被加到同一电子壳层中。这导致原子半径、电离能和金属性等性质发生渐变。
3. Groups | 族
A group is a vertical column in the periodic table. Elements in the same group have the same number of valence electrons, which is the principal reason for their similar chemical properties.
族是周期表中的垂直列。同一族元素具有相同的价电子数，这是它们化学性质相似的主要原因。
For main-group elements, the group number (using the IUPAC system) often matches the number of valence electrons. For example, Group 1 elements (alkali metals) have one valence electron, forming +1 ions; Group 17 elements (halogens) have seven valence electrons, typically forming -1 ions.
对于主族元素而言，族号（采用IUPAC系统）通常与价电子数对应。例如，第1族元素（碱金属）有一个价电子，形成+1离子；第17族元素（卤素）有七个价电子，通常形成-1离子。
Group 18 elements (noble gases) have full outer shells and are extremely unreactive. However, heavier noble gases like xenon can form compounds with highly electronegative elements, showing that "inertness" is relative.
第18族元素（稀有气体）具有全充满的外层电子壳层，因此极不活泼。然而，氙等较重的稀有气体可与电负性极强的元素形成化合物，说明“惰性”是相对的。
4. Blocks: s, p, d, and f | 区：s区、p区、d区与f区
Periods and groups are further organised into blocks based on the type of atomic orbital being filled. The s-block includes Groups 1 and 2, plus helium; the p-block includes Groups 13 to 18; the d-block corresponds to the transition metals; and the f-block contains the lanthanides and actinides.
周期和族还可以根据被填充的原子轨道类型划分为不同区。s区包括第1族和第2族以及氦；p区包括第13至18族；d区对应过渡金属；f区包含镧系和锕系元素。
- s-block: valence electrons in s orbitals, reactive metals and helium.
- p-block: valence electrons in p orbitals, includes nonmetals, metalloids, and some metals.
- d-block: d orbitals are progressively filled, typical metals with variable oxidation states.
- f-block: inner transition metals, often with complex chemistry.
- s区：价电子位于s轨道，包括活泼金属和氦。
- p区：价电子位于p轨道，包括非金属、类金属和一些金属。
- d区：d轨道被逐渐填充，是典型金属，具有可变氧化态。
- f区：内过渡金属，化学性质复杂多样。
5. Main-Group Elements | 主族元素
Main-group elements (Groups 1, 2, and 13–18) display a wide variety of properties that vary systematically across periods and down groups. Their chemistry is largely determined by the number of valence electrons in s and p orbitals.
主族元素（第1、2族和第13–18族）显示出一系列随周期和族系统变化的性质。它们的化学行为主要由s和p轨道中的价电子数决定。
Across a period, the metallic character decreases from left to right. For instance, sodium (Na) is a reactive metal, silicon (Si) is a metalloid, and chlorine (Cl) is a nonmetal. Down a group, metallic character increases, as seen in Group 14: carbon (nonmetal), silicon (metalloid), tin and lead (metals).
沿周期从右向左，金属性从左到右减弱。例如，钠（Na）是活泼金属，硅（Si）是类金属，氯（Cl）是非金属。沿族向下金属性增强，如第14族：碳（非金属）、硅（类金属）、锡和铅（金属）。
6. Transition Metals | 过渡金属
The d-block transition metals are found in Groups 3–12. These elements are all metals, typically hard, shiny, and good conductors of heat and electricity. Their properties arise from the partially filled d subshells.
d区过渡金属位于第3至第12族。这些元素全部为金属，通常坚硬、有光泽，是良好的热和电导体。它们的性质源于未完全充满的d亚层。
Transition metals often show multiple oxidation states, form coloured compounds, and act as catalysts. For example, iron can exist as Fe²⁺ and Fe³⁺, and its compounds are often coloured. The similarity in atomic radii across a transition series causes some surprising similarities, such as the nearly equal sizes of Zr and Hf.
过渡金属通常呈现多种氧化态，形成有色化合物，并可用作催化剂。例如，铁可以以Fe²⁺和Fe³⁺存在，其化合物往往有颜色。过渡系列中原子半径的相似性导致一些令人惊讶的相似之处，如锆和铪的原子大小几乎相同。
7. Atomic Radius Trends | 原子半径的趋势
Atomic radius is half the distance between the nuclei of two identical atoms. In IB Chemistry, you should understand both the period and group trends.
原子半径是两个相同原子核间距离的一半。在IB化学中，你需要理解周期趋势和族趋势。
- Across a period: Atomic radius decreases from left to right, because increasing nuclear charge pulls the outer electrons closer to the nucleus. Shielding by inner electrons remains roughly constant.
- Down a group: Atomic radius increases, because each new period adds a new principal energy level, making the outermost electrons farther from the nucleus.
- 沿周期：原子半径从左到右减小，因为核电荷增加，将外层电子拉得更靠近原子核。内层电子的屏蔽效应大致保持不变。
- 沿族向下：原子半径增大，因为每个新增周期都引入一个新的主能级，使最外层电子离原子核更远。
r(period 2) : Li > Be > B > C > N > O > F
This trend is important for explaining other periodic properties, such as ionisation energy and electronegativity.
这一趋势对于解释其他周期性性质（如电离能和电负性）至关重要。
8. Ionisation Energy | 电离能
Ionisation energy is the minimum energy required to remove one electron from a gaseous atom or ion in its ground state. The first ionisation energy (IE₁) corresponds to removing the first electron.
电离能是从基态的气态原子或离子中移去一个电子所需的最小能量。第一电离能（IE₁）对应移去第一个电子所需的能量。
- Across a period: IE₁ generally increases, because the nuclear charge increases while the outer shell remains the same, so electrons are more strongly attracted.
- Down a group: IE₁ decreases, because the outermost electron is farther from the nucleus and more shielded by inner electrons.
- 沿周期：IE₁总体增大，因为核电荷增加而外层壳层不变，电子受到更强的吸引。
- 沿族向下：IE₁减小，因为最外层电子离核更远，且受到内层电子更强的屏蔽。
There are two notable exceptions in each period: Group 13 has a lower IE₁ than Group 2, and Group 16 has a lower IE₁ than Group 15. These arise from the stability of half-filled and full s orbitals, and from electron–electron repulsion in paired p orbitals.
每个周期中都有两个值得注意的例外：第13族的IE₁低于第2族，第16族的IE₁低于第15族。这源于半充满和全充满轨道的稳定性，以及成对p轨道中的电子-电子排斥作用。
9. Electronegativity | 电负性
Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. The Pauling scale is commonly used in IB Chemistry.
电负性是原子在化学键中吸引共用电子对能力的量度。IB化学中常用鲍林标度。
Across a period, electronegativity increases from left to right, because the nuclear charge increases and the atomic radius shrinks. Down a group, electronegativity generally decreases, because the atomic radius increases and the shared electron pair is farther from the nucleus.
沿周期从左到右，电负性增大，因为核电荷增加且原子半径缩小。沿族向下，电负性通常减小，因为原子半径增大，共用电子对离核更远。
Fluorine is the most electronegative element, with a Pauling value of 3.98. Cesium and francium have the lowest values. Nonmetals have high electronegativity, while metals have low values.
氟是电负性最强的元素，鲍林值为3.98。铯和钫的电负性最低。非金属的电负性高，而金属的电负性低。
10. Electron Affinity | 电子亲和能
Electron affinity is the energy change when an electron is added to a gaseous atom. The first electron affinity is often negative, indicating that energy is released when neutral atoms gain an electron.
电子亲和能是指向气态原子加入一个电子时的能量变化。第一电子亲和能通常为负值，表明中性原子获得电子时会释放能量。
Across a period, electron affinity becomes more negative from left to right, as the nucleus becomes more positively charged and attracts the incoming electron more strongly. Down a group, it becomes less negative because the added electron goes into a higher principal energy level, farther from the nucleus.
沿周期从左到右，电子亲和能变得更负，因为核正电荷增加，对进入电子的吸引力更强。沿族向下，电子亲和能变得不那么负，因为加入的电子进入更高的主能级，离核更远。
Group 15 elements have unexpectedly low electron affinities because the added electron must enter the already half-filled p subshell, causing increased repulsion.
第15族元素的电子亲和能出人意料地低，因为加入的电子必须进入已经半充满的p亚层，导致排斥作用增强。
11. Metal, Nonmetal, and Metalloid | 金属、非金属与类金属
The position in the periodic table determines whether an element is classified as a metal, nonmetal, or metalloid. Metals are found on the left and in the middle, nonmetals on the upper right, and metalloids along the zigzag line between them.
元素在周期表中的位置决定了它被归类为金属、非金属还是类金属。金属位于左侧和中部，非金属位于右上角，类金属位于二者之间的斜线地带。
| Category | Properties |
| Metal | Shiny, malleable, ductile, good conductor; forms cations |
| Nonmetal | Dull, brittle, poor conductor; forms anions or covalent bonds |
| Metalloid | Intermediate properties; often semiconductors |
| 类别 | 性质 |
| 金属 | 有光泽、可延展、可锻，良导体；形成阳离子 |
| 非金属 | 暗淡、脆、不良导体；形成阴离子或共价键 |
| 类金属 | 性质介于金属与非金属之间；常为半导体 |
Metalloids such as silicon and germanium are important in electronic devices because their conductivity can be tuned by doping.
硅和锗等类金属在电子器件中具有重要价值，因为它们的导电性可以通过掺杂来调控。
12. Summary of Periodic Trends | 周期规律的总结
Understanding periods and groups allows chemists to predict a wide range of physical and chemical properties. The table below summarises key trends for main-group elements.
理解周期和族能够让化学家预测广泛的物理和化学性质。下表总结了主族元素的关键趋势。
| Property | Across a period (→) | Down a group (↓) |
| Atomic radius | Decreases | Increases |
| First ionisation energy | Increases (with small drops) | Decreases |
| Electronegativity | Increases | Decreases |
| Metallic character | Decreases | Increases |
| 性质 | 沿周期（→） | 沿族（↓） |
| 原子半径 | 减小 | 增大 |
| 第一电离能 | 增大（有小幅下降） | 减小 |
| 电负性 | 增大 | 减小 |
| 金属性 | 减弱 | 增强 |
The periodic table is thus not just a list of elements but a map of periodic behaviour. By connecting position to electron configuration, students can rationalise many otherwise disconnected facts.
因此，周期表不仅是元素的清单，更是周期行为的图谱。通过将位置与电子构型联系起来，学生可以将许多原本零散的事实系统化地理解。
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