WJEC GCSE化学 原子结构 同位素 电子排布
原子结构是 WJEC GCSE 化学 Unit 1 的第一个理论主题，也是后面键合、摩尔计算、电解的前提。这篇把 1.2 这个主题的要求拆成十个知识点，每条都配一个数值例子和一类常见扣分，中英对照写，方便你直接对上卷面。
Atomic structure is the first theoretical topic in WJEC GCSE Chemistry Unit 1, and nearly every later calculation rests on it. The sections below follow the order of specification topic 1.2, with a worked value and a typical lost mark attached to each point.
1. 原子模型的修订：实心球到核式模型
道尔顿把原子想成不可再分的实心小球，这个图像撑了大约一百年。汤姆孙发现电子之后，原子内部必须既有负电也有正电，模型改成带正电的球体里嵌着电子。卢瑟福用阿尔法粒子打金箔，绝大多数粒子直接穿过，极少数被大角度弹回。这两件事合起来，只能解释成原子中心有一个体积很小、集中了几乎全部质量的正电核，电子在外面绕行。WJEC 考纲 1.2(a) 要求你描述带正电的原子核与绕核运动的电子。(b) 再加一条，原子核由质子和中子组成。答题时把这两条分开写，比笼统说原子由三种粒子组成更稳。模型演变的题目常问哪一次实验推翻了哪个模型，把实验现象和结论对上就能拿满。
Dalton pictured the atom as a hard sphere with nothing inside, and that image lasted about a century. Thomson's discovery of the electron forced a positive sphere with electrons embedded in it; Rutherford's alpha-particle scattering then broke that model, because most particles went straight through the gold foil while a few came back at large angles. A tiny positive nucleus holding almost all the mass, surrounded by orbiting electrons, is the only arrangement that fits both observations. WJEC's specification 1.2(a) and (b) ask for those two statements separately, which is how examiners award the marks.
2. 质子中子的相对质量与电子可忽略的质量
质子的相对质量记为 1，电荷 +1。中子相对质量也是 1，但不带电。电子的相对质量约是质子的 1/1840，WJEC 教学指导写成大约小两千倍，电荷是 -1。质量数只把质子和中子相加，电子不计入，因为它的质量小到不会改变整数结果。很多学生以为电子既然带电、又在核外，就该算进质量数，这个念头一起，后面的中子数全错。原子不带电的原因是质子数与电子数相等，正负数量上一抵一。WJEC 1.2(c) 和 (d) 就把相对质量、相对电荷、整体不带电这三条并排列出。所以问到电荷的那一问，答案要落到电子数等于质子数。
Relative mass is 1 for a proton and 1 for a neutron, with charges of +1 and 0 respectively; an electron carries -1 and a relative mass of about 1/1840, described in WJEC teaching guidance as roughly two thousand times lighter. Electrons are left out of the mass number because their mass cannot shift an integer total. An atom is neutral because the proton count equals the electron count, not because the charges cancel in some vague sense. Specification points 1.2(c) and (d) list relative mass, relative charge and neutrality side by side, so an answer about charge should name the electron-proton balance.
3. 原子序数与质量数决定四个数字
原子序数就是质子数，WJEC 考纲 1.2(e) 直接这么定义，元素的一切判断都从这里出发。质量数是质子数与中子数之和，减去原子序数就得到中子数。以氟为例，原子序数 9、质量数 19，于是 9 个质子、10 个中子，中性原子里电子也是 9 个。原子序数决定元素身份，改了就换成别的元素。质量数变化只是同位素不同，化学性质基本照旧。常见的错误是把 19 减 9 之后又拿 19 去当电子数。另一个错误是看到符号上下两个数字，把上面那个当成原子序数。上面是质量数，下面是原子序数，位置写反是阅卷时最容易看到的失分。
The atomic number is the proton count, and every other figure about that atom follows from it. Subtract the atomic number from the mass number and you have the neutrons; fluorine, with 9 protons and a mass number of 19, therefore holds 10 neutrons and 9 electrons when neutral. Changing the atomic number creates a different element, while changing the mass number only produces an isotope. A frequent slip is to reuse the mass number as an electron count, or to read the top figure of a symbol as the atomic number when it is the mass number.
4. 离子的电子数算法：Na+ 到 O2-
离子是电子数被改动过的原子，质子数一动不动。钠原子 11 个质子 11 个电子，失掉一个电子变成 Na+，质子还是 11，电子只剩 10，净电荷 +1。氯原子 17 个质子 17 个电子，得一个电子变成 Cl-，电子 18，净电荷 -1。镁失两个电子是 Mg2+，氧得两个电子是 O2-，WJEC 教学指导点名的就是这四个。算离子的电子数只改电子那一栏，中子数完全不变。很多学生会在这里栽：题目给 Al3+、质量数 27、原子序数 13，正确是 13 个质子、14 个中子、10 个电子。把质子、中子、电子画成三列的小表，比心算可靠。
An ion is an atom whose electron count has changed while its protons stay put. Sodium loses one electron to become Na+ with 11 protons and 10 electrons; chlorine gains one to become Cl- with 17 protons and 18 electrons. WJEC's teaching guidance names Na+, Mg2+, Cl- and O2- as the ions learners should handle with confidence. For Al3+ with mass number 27 and atomic number 13, the correct row is 13 protons, 14 neutrons and 10 electrons; neutrons never change when an ion forms.
5. 同位素与相对原子质量的加权计算
同位素是质子数相同、中子数不同的同种元素原子。WJEC 考纲 1.2(e) 和 (g) 要求你能用丰度算相对原子质量。氯有两种主要同位素，质量数 35 的约占 75%，质量数 37 的约占 25%。加权平均是 35 乘 0.75 加 37 乘 0.25，结果是 35.5。硼更适合练手，质量数 10 的占 20%，质量数 11 的占 80%，加权后是 10.8。丰度是百分比时先除以 100 再逐项相乘相加。直接拿 75 去乘会让答案大一百倍。常见的错误是把两个质量数简单平均，把氯算成 36，这类答案阅卷时拿不到分。
Isotopes share a proton count but differ in neutrons, and the specification asks you to turn abundances into a relative atomic mass. Chlorine's two main isotopes, mass 35 at about 75% and mass 37 at about 25%, give 35 x 0.75 + 37 x 0.25 = 35.5. Boron works the same way, with 20% at mass 10 and 80% at mass 11 producing 10.8. Convert any percentage to a decimal before multiplying, otherwise the answer lands a hundred times too large; taking a plain average of the two mass numbers is the error examiners see most often.
6. 电子排布 2,8,8 与前 20 号元素
电子按能量最低先填，第一层最多 2 个，第二层最多 8 个，第三层在 20 号以内也是 8 个。前 20 号元素只用到四层。氢是 1，氦是 2，锂是 2,1，氖是 2,8，钠是 2,8,1。氩是 2,8,8，钾是 2,8,8,1，钙是 2,8,8,2。WJEC 考纲 1.2(j) 只要求到前 20 号元素，过渡金属的排布不用写。用分层数字表示最省事，画方框也可以。很多学生以为第三层永远只能放 8 个，看到钾的 2,8,8,1 就慌。20 号以内确实停在 8，第十九个电子进第四层。
Electrons fill the lowest available shell first, with 2 in the first shell and 8 in each of the next two up to calcium. Hydrogen is written 1, helium 2, lithium 2,1, neon 2,8, sodium 2,8,1, argon 2,8,8, potassium 2,8,8,1 and calcium 2,8,8,2. Specification point 1.2(j) limits the requirement to the first twenty elements, so there is no need to describe transition metal arrangements at GCSE. Learners often expect the third shell to keep filling beyond 8, yet within the first twenty elements it stops there and the nineteenth electron opens the fourth shell.
7. 周期表位置读法：族看最外层、周期看层数
周期表按原子序数递增排列，WJEC 考纲 1.2(h) 写明性质相近的元素落在同一族。族号对最外层电子数，第一族 1 个，第二族 2 个，第七族 7 个。周期号对占有的电子层数，钠 2,8,1 就在第三周期。0 族最外层是满的，2 或 8，所以几乎不反应。金属在左边和中间，非金属在右边，这一条在 1.2(i)，卷面常让你从几个元素里挑出金属。用电子排布推位置比背整张表可靠。先数层数定周期，再数最外层定族，两步就够。
The periodic table runs in order of increasing atomic number, and elements with similar properties fall into the same group. Group number matches outer-shell electrons and period number matches the number of occupied shells, so sodium with 2,8,1 sits in group 1 of period 3. Group 0 atoms have full outer shells and react hardly at all; metals occupy the left and centre of the table while non-metals sit to the right. Reading position off an electron configuration is more reliable than memorising the whole table, and specification 1.2(i) uses exactly that kind of question.
8. 原子与原子核的尺度：标准形式与单位
原子半径大约 0.1 nm，写成标准形式是 1 x 10^-10 m。原子核半径约 1 x 10^-14 m，两者差四个数量级。换句话说，核比原子小一万倍以上。WJEC 化学卷不专考数学，但科学素养题要求你把 nm 与 m 换对。1 nm 等于 1 x 10^-9 m，这一步漏掉就全错。原子几乎全部质量集中在核里，因为质子中子相对质量都是 1，电子只有约 1/1840。体积小、质量大这两件事放一起，才解释了卢瑟福实验里为什么极少数粒子被弹回。常见的错误是把 10^-10 念成 10 的十次方。
An atom's radius is roughly 0.1 nm, or 1 x 10^-10 m, while a nucleus measures about 1 x 10^-14 m. Four orders of magnitude separate the two, which is why the nucleus is often described as smaller than the atom by a factor of ten thousand or more. Remember that 1 nm equals 1 x 10^-9 m, since unit conversion is where the arithmetic usually breaks. Nearly all the mass sits in the nucleus because protons and neutrons each count 1 while an electron counts about 1/1840; small volume plus large mass is what sends a few alpha particles back in Rutherford's experiment.
9. WJEC 卷面术语与 Unit 1 的考法
WJEC 化学卷的术语和英格兰几家考试局不完全一样。评分只用 atomic number 和 mass number，proton number 与 nucleon number 在化学卷里不出现，教学指导写得很明确。英文题里用错词，数值对也可能不达标。Unit 1 是 1 小时 45 分、80 分、占 45% 的那张卷，WJEC 官网的 spec 写的是 2016 年起施教，Unit 3 从 2026 年起换新版。1.2 这个主题就在 Unit 1 里。威尔士的资格框架也在动，从 2026 年 9 月起 WJEC 增加 GCSE The Sciences 双奖与 GCSE Integrated Science 单奖，现行 GCSE Chemistry 仍然保留。按学校实际教的 spec 复习，别照着英格兰的卷子背。阅卷时最常见的是把原子序数写成质量数、单位漏掉、离子电荷符号写反这三类。
WJEC chemistry papers use atomic number and mass number only; proton number and nucleon number do not appear on the papers, as the guidance for teaching states. Using the wrong label in an extended answer can cost marks even when the numbers are right. Unit 1 is the 1 hour 45 minute paper worth 80 marks and 45% of the qualification, and topic 1.2 sits inside it; the specification covers teaching from 2016, with a new Unit 3 for first teaching from 2026. Wales is also changing its qualification landscape, since from September 2026 WJEC offers GCSE The Sciences (Double Award) and GCSE Integrated Science (Single Award) alongside the existing GCSE Chemistry, so check which specification your school follows.
10. 从电子排布到 Unit 2 的键合
Unit 2 的 2.1 讲键合，出发点还是电子排布。最外层不满的元素靠得失或共用电子达到满层。钠 2,8,1 容易失一个变 Na+，氯 2,8,7 容易得一个变 Cl-。两者靠静电引力结合成氯化钠。原子结构做不顺，键合、摩尔计算、电解都会跟着卡。这是很多学生到 Unit 2 才回头补第一单元的原因。复习时把前 20 号元素的排布写在卡片正面。背面写质子数、常见同位素的中子数和族周期。说实话，这部分内容不多，真正拉分的是单位、术语和离子的电子数三项。
Unit 2 starts from the same electron configurations: atoms with incomplete outer shells gain, lose or share electrons to reach a full shell. Sodium's 2,8,1 loses one electron and chlorine's 2,8,7 gains one, and the resulting ions hold together by electrostatic attraction in sodium chloride. Struggles with atomic structure usually resurface in bonding, mole calculations and electrolysis later in the course. A single revision card per element, with configuration on one side and protons, neutrons, group and period on the other, catches most of the slips before the exam.
11. 相对式量与质量守恒：与 1.1 的接口
相对式量把化合物里每个原子的相对原子质量加起来，CO2 就是 12 加 2 乘 16 等于 44。WJEC 卷面会附周期表上的相对原子质量，不必背这些数字。质量守恒说的是反应前后各元素原子的总数不变。所以 12 克碳在 32 克氧气里完全燃烧，得到 44 克二氧化碳。1.1 里的化学式与方程式计算，到 Unit 2 仍然要用，两边的标准是同一套。算相对式量时漏乘括号外的数字，是最常见的失手。Ca(OH)2 是 40 加 2 乘 17，不是 40 加 17。把每个元素的原子个数先在纸上点一遍，再乘起来，比直接心算错得少。
Relative formula mass adds up the relative atomic masses in a formula, so CO2 comes to 12 + 2 x 16 = 44. Periodic tables supplied in the exam carry the relative atomic masses, which is why WJEC does not expect them to be memorised. Mass is conserved in a reaction, so 12 g of carbon burning completely in 32 g of oxygen gives 44 g of carbon dioxide. The formulae and equation work from topic 1.1 is still examined in Unit 2 and the same conventions apply; the usual slip is forgetting to multiply everything inside a bracket, so Ca(OH)2 is 40 + 2 x 17 rather than 40 + 17.
12. 填表题与符号题的答法
填表题一般给原子序数、质量数、质子数、中子数、电子数五列，缺哪列就补哪列。符号题的写法是左上角写质量数、左下角写原子序数，离子上标电荷，例如质量数 27、原子序数 13 的铝离子上标 3+。题目给了符号却没给数字时，先读两个角上的数，再用质量数减原子序数得到中子数。图表里的电子层画法要和数字排布对得上，画三层就得是 2,8,x 的写法。很多学生把符号的角标当作装饰，跳过不读，结果整道题的前提就错了。答题时把五列一次性填完，别只答题目问的那一格。多填的格子不扣分，漏填的那一格才扣分。
Table questions usually supply five columns (atomic number, mass number, protons, neutrons, electrons) and expect the missing ones to be filled in. In symbol notation the mass number goes top left and the atomic number bottom left, with any charge written as a superscript such as 3+ on an aluminium ion of mass number 27 and atomic number 13. Shell diagrams must match the numerical configuration, so a three-shell drawing has to agree with the 2,8,x pattern. Many learners treat the corner figures as decoration and skip them, which throws away the starting data for the whole question; filling all five columns costs nothing even when only one cell is marked.
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