Edexcel IGCSE化学 周期表 卤素 惰性气体
Edexcel IGCSE 化学（4CH1）把周期表放在第一部分的原理化学里，第1族与第7族则放到无机化学。很多学生以为周期表只是一张要背的图，其实它是用来推性质的工具。下面按 2024 年 9 月发布的 Issue 3 官方 Specification 的顺序，把族、周期、碱金属、卤素、惰性气体与过渡金属串一遍。读完你应该能从元素的位置说出它大概怎么反应、颜色怎样、需不需要加热。
The Edexcel IGCSE Chemistry specification (4CH1) treats the Periodic Table as a tool rather than a poster, and the two exam papers lean on that reading skill again and again. This walk-through follows the order of content in Issue 3, published in September 2024, from the meaning of groups and periods through to the halogens and transition metals. The aim is simple: given a symbol you have never met, you should be able to say how it behaves and how it looks.
周期表按原子序数递增排列
现代周期表从左到右按原子序数递增排列，原子序数就是质子数，也等于中性原子的电子数。门捷列夫在 1869 年按相对原子质量排列，遇到位置与性质冲突时选择留空位，并预测了镓和锗的性质。后来同位素被发现，才弄清楚决定元素身份的是质子数而不是质量。Edexcel 的题目常给出一个元素符号，要你说出原子序数、所在族与周期。这一步做错，后面的趋势推理全部跟着错。查表时先确认原子序数，再数族数（从左往右）与周期数（从上往下）。
The modern table runs left to right in order of increasing atomic number, which counts protons and, in a neutral atom, electrons as well. Mendeleev worked with relative atomic mass in 1869, left gaps where the pattern broke, and predicted the properties of gallium and germanium before either was isolated; isotopes later showed why proton number is the real ordering principle. In an exam the first move is always to read the atomic number, count across for the group and down for the period, because every trend question afterwards depends on that position being right.
族决定最外层电子数 周期决定电子层数
族是竖列，同一族的元素最外层电子数相同，化学性质因此相似。周期是横行，同一周期元素占据的电子层数相同。钠的排布是 2,8,1，位于第三周期第1族，最外层一个电子。氯是 2,8,7，位于第三周期第7族，最外层七个电子。氦只有一层，两个电子就填满，所以它在第0族而不是第1族。这种排法把电子排布和化学行为直接连起来，答题时把最外层电子数写出来，考官看的就是这一句。
A group is a column, and every element in it carries the same number of outer-shell electrons, which is why the chemistry repeats as you go down. A period is a row, so atoms across it have the same number of occupied shells while gaining protons one at a time. Sodium, 2,8,1, sits in Group 1 of Period 3; helium fills its single shell with two electrons and is therefore placed in Group 0 rather than Group 1. Answers score when the outer-shell count is stated explicitly, not merely implied by the word similar.
用位置预测陌生元素的性质
周期表左侧是金属，右侧是非金属，靠分界线近的元素性质介于两者之间。同族向下金属性增强，同周期向右非金属性增强。给一个不熟悉的元素，先看它在哪一族，再套用同族已知元素的反应。镁需要水蒸气才反应，它下面的钙与冷水就能反应，而且更快。很多学生以为陌生元素只能靠记忆，其实考纲只要求你会迁移。答这类题时把参照元素写出来，说明你用的是同族相似这条规律。
Metals sit on the left and non-metals on the right, and elements near the dividing line behave like something in between. Metallic character grows down a group and shrinks across a period, so an unfamiliar element can be handled by borrowing from its neighbours. Magnesium needs steam to react, calcium below it reacts with cold water and does so faster, and that comparison is what earns the mark rather than a memorised sentence about reactivity.
用族数推离子电荷与化学式
第1族元素失去一个电子，形成正一价离子；第2族失去两个，形成正二价；第6族得两个电子，形成负二价；第7族得一个，形成负一价。化合物整体不带电，所以钠与氯按一比一结合成 NaCl，镁与氯按一比二结合成 MgCl2。铝是正三价，氧化铝写成 Al2O3，两个正三价刚好抵三个负二价。这套推法不用背离子表，只要族数记准。很多学生以为化学式只能靠记忆，其实按电荷配平一步步来就不会错。
Charge follows the group: one electron lost in Group 1 gives 1+, two lost in Group 2 gives 2+, while Groups 6 and 7 gain electrons to reach 2- and 1-. A compound carries no overall charge, so sodium pairs one to one with chlorine in NaCl, magnesium takes two chloride ions in MgCl2, and aluminium oxide balances as Al2O3. Working from charges instead of memory removes most formula slips, and each step can be written out so the examiner can follow it.
第1族碱金属的物理性质
锂、钠、钾都是银白色、质地软的金属，可以用刀切开，切面几秒内变暗。它们的密度小，钠与钾都能浮在水面，熔点随原子序数增大而降低。锂约 180 摄氏度，钠约 98 摄氏度，钾约 63 摄氏度。因为太活泼，实验室把它们泡在煤油里，隔绝空气与水汽。常见的错误是把它们直接放在空气中称量，表面那层氧化物会让结果偏重。取用时用镊子夹取，滤纸吸干煤油再切。
Lithium, sodium and potassium are soft silvery metals that a knife will cut, and a fresh face tarnishes within seconds. They are less dense than water and their melting points fall steadily down the group, from roughly 180 degrees Celsius for lithium to about 63 for potassium. That reactivity is why the samples live under oil, and why a piece left in the open gains mass as the oxide layer builds up.
第1族与水的反应
钠与水的方程式是 2Na + 2H2O -> 2NaOH + H2，产物是氢氧化钠与氢气。锂反应较慢，钠熔成小球并在水面游动、发出嘶嘶声，钾会燃烧并呈淡紫色火焰。反应后的溶液使万能指示剂变紫变蓝，说明生成了碱。检验氢气用点燃的木条，会听到爆鸣声。三个现象要分开写，熔成小球说明放热且熔点低，游动说明氢气推动，火焰颜色说明反应更剧烈。写方程式时记得配平，氢气是双原子分子。
All three metals follow one pattern with water, giving a hydroxide and hydrogen, but the vigour climbs steeply down the group. Sodium melts into a ball that skitters across the surface, potassium ignites with a lilac flame, and lithium merely fizzes. Link the melting to the heat released and the movement to escaping hydrogen, then balance the equation properly, remembering that hydrogen is diatomic.
为什么第1族向下更活泼
第1族元素都只有一个最外层电子，反应时把它失去，形成正一价离子。向下走，原子半径增大，最外层电子离核更远，内层电子的屏蔽也更强，所以这个电子更容易失去。因果链要写完整，只写原子更大所以更活泼，阅卷时基本不给分。反过来看第7族，原子变大后吸引外来电子的能力变弱，活泼性向下减弱。同一个半径变化，在两族里推出相反的趋势，这是最容易考的一处对比。
Group 1 metals all lose their single outer electron to form a 1+ ion, and the ease of that loss is the whole story. Going down the group the atom gets larger, the outer electron sits further from the nucleus and is shielded by more inner shells, so less energy is needed to remove it. Group 7 runs the opposite way, because a bigger atom pulls an incoming electron in less strongly, which is why the same size trend produces opposite chemistry.
第7族卤素的颜色与状态
卤素以双原子分子存在，氯是 Cl2，溴是 Br2，碘是 I2。常温下氯是浅黄绿色气体，溴是深红棕色液体，碘是有光泽的灰黑色固体。向下走熔点和沸点升高，因为分子变大后分子间作用力增强。碘固体加热会直接变成紫色蒸气，这个升华现象常出现在实验题里。卤素都有刺激性气味且有毒，闻气味要用扇动法。溶液颜色也要记，氯水浅黄绿，溴水橙黄，碘水棕黄。
Halogens exist as diatomic molecules, and their appearance changes sharply down the group, from a pale green-yellow gas through a deep red-brown liquid to a shiny grey-black solid. Melting and boiling points rise with molecular size because the forces between molecules get stronger. Iodine sublimes to a purple vapour on gentle heating, a detail that turns up in practical questions next to the smell test, where the vapour is wafted towards the nose rather than sniffed directly.
卤素置换反应与氧化性顺序
活泼的卤素能把不活泼的卤素从它的盐溶液里置换出来。氯气通入溴化钾溶液，Cl2 + 2KBr -> 2KCl + Br2，溶液由无色变成橙色。氯气通入碘化钾溶液会析出碘，颜色变棕。溴也能置换碘，碘不能置换溴。这套反应说明氧化性顺序是氯大于溴大于碘。答题时要写清颜色变化作为证据，只写发生反应不够。碘遇淀粉变蓝黑，是另一个常用的检验方法。
A more reactive halogen displaces a less reactive one from solution, so chlorine turns colourless potassium bromide orange and liberates iodine from potassium iodide. Bromine will displace iodine, iodine cannot return the favour, and that fixes the oxidising order as chlorine, then bromine, then iodine. The mark comes from the colour change used as evidence, and iodine with starch giving a blue-black colour is the standard confirming test.
第0族惰性气体
第0族包括氦、氖、氩、氪、氙，最外层电子已经填满，氦是两个，其余是八个。因为既不容易得电子也不容易失电子，它们几乎不参与化学反应，所以叫惰性气体。用途直接来自性质，氦密度小且不燃，用于气球与低温冷却；氖通电发红光，做霓虹灯；氩用于焊接与灯泡填充，隔绝氧气。向下走原子变大，沸点升高。这一族的题多是性质与用途配对，写用途时把性质一起写。
The noble gases have full outer shells, two electrons for helium and eight for the rest, so they neither gain nor lose electrons readily and barely react at all. That inertness explains the uses: helium for balloons and for cooling because it is light and unreactive, neon in discharge tubes, argon as a shielding gas and as a filler in lamps. Down the group the atoms grow and the boiling points rise, which is the one trend worth quoting.
过渡金属的特征
过渡金属位于周期表中部，第2族与第3族之间那一块。它们密度大、熔点高，只有汞在常温下是液体。过渡金属能形成有颜色的化合物，铜离子是蓝色，亚铁离子是浅绿色，铁离子是黄棕色。它们还常作催化剂，哈伯法合成氨用铁，接触法制硫酸用五氧化二钒。同一种金属可以有不同电荷的离子，铁就有正二价与正三价两种。这些特征在第1族身上一个也找不到。
Transition metals fill the block in the middle of the table, between Groups 2 and 3, and they are dense, hard and high melting, with mercury the obvious exception. Their compounds are usually coloured, copper ions blue, iron(II) pale green and iron(III) yellow-brown, and several of the metals work as catalysts, iron in the Haber process and vanadium(V) oxide in the Contact process. One metal can also form ions of more than one charge, as iron does with 2+ and 3+.
第1族与过渡金属的对比
把这两块放在一起考，是因为差别足够明显。第1族软、密度低、熔点低，与冷水就剧烈反应，必须泡在煤油里。过渡金属硬、密度高、熔点高，多数连冷水都不反应，可以露天放置。颜色上，第1族的化合物通常无色或白色，过渡金属的化合物带颜色。催化上，第1族基本没有催化作用，过渡金属常见。按这四个维度对比，每一条配一个具体例子，四行就够。
Group 1 and the transition metals are set side by side because the contrast is easy to test. Group 1 metals are soft, light and low melting, react violently with cold water and have to be stored under oil, while transition metals are hard, dense and high melting, mostly ignore cold water and can be left in the open. Their compounds differ as well, white or colourless against coloured, and catalysis is almost exclusively a transition metal behaviour.
Paper 1 与 Paper 2 的覆盖差别
Edexcel 官方 Specification 把内容分成核心与加深两层。Paper 1 是 2 小时 110 分，占 61.1%，只考核心条目；Paper 2 是 1 小时 15 分钟 70 分，占 38.9%，可以考全部内容，包括加粗的 C 条目。两卷都允许使用计算器，两卷都会出选择题、短答、计算与长答。周期表这一块的核心要求是读位置、说趋势、写方程式，加粗部分才涉及更细的解释。复习时先把核心条目练到不假思索，再回头补 C 条目。
Pearson splits the content into core statements and the bold C statements, and the papers follow that split: Paper 1 runs 2 hours over 110 marks and draws only on core material, while Paper 2 lasts 1 hour 15 minutes for 70 marks and can reach everything. Calculators are permitted in both. For this topic that means reading positions, quoting trends and writing equations come first, with the finer explanatory detail attached to the C statements only Paper 2 can use.
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