📚 Atoms and Elements Essentials for WJEC A-Level Science | 原子与元素考点精讲
This article provides a comprehensive revision summary of atoms and elements for WJEC A-Level Science students. Covering atomic structure, electron configurations, isotopes, periodic trends and more, it will help you master these fundamental concepts and tackle exam questions with confidence.
本文为WJEC A-Level科学考生提供原子与元素的全面复习总结，涵盖原子结构、电子排布、同位素、周期趋势等核心内容。掌握这些基础概念，助你在考试中从容应对各类题型。
1. The Structure of the Atom | 原子结构
At the centre of every atom lies a tiny, dense nucleus containing protons and neutrons, surrounded by electrons moving in regions of space called orbitals. Most of the atom is empty space, and the nucleus accounts for nearly all the mass.
每个原子的中心都有一个微小而致密的原子核，内含质子和中子，核外电子在被称为轨道的空间区域中运动。原子内绝大部分是虚空，而原子核几乎占据了全部质量。
The number of protons defines the element and is called the atomic number (Z). The total number of protons and neutrons is the mass number (A). An atom is electrically neutral because the number of electrons equals the number of protons.
质子数决定了元素的种类，称为原子序数（Z）。质子与中子的总数称为质量数（A）。原子呈电中性，因为电子数与质子数相等。
A = Z + n (where n = number of neutrons)
质量数 = 质子数 + 中子数
Ions form when atoms gain or lose electrons: a loss of electrons gives a positive cation, a gain gives a negative anion. The nuclear composition remains unchanged.
原子得到或失去电子便形成离子：失电子形成阳离子，得电子形成阴离子。原子核的组成保持不变。
2. Subatomic Particles and Their Properties | 亚原子粒子及其性质
You must know the relative masses, relative charges and positions of the three subatomic particles. The proton and neutron have a relative mass of approximately 1, while the electron has a negligible mass of 1/1836. The proton carries a +1 charge, the electron –1, and the neutron is neutral.
你必须牢记三种亚原子粒子的相对质量、相对电荷和位置。质子和中子的相对质量约为1，电子的质量可忽略不计，仅为1/1836。质子带+1电荷，电子带–1电荷，中子不带电。
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 | –1 | Orbitals |
| 粒子 | 相对质量 | 相对电荷 | 位置 |
|---|---|---|---|
| 质子 | 1 | +1 | 原子核 |
| 中子 | 1 | 0 | 原子核 |
| 电子 | 1/1836 | –1 | 轨道 |
When writing nuclide notation, place the mass number as a superscript on the left and the atomic number as a subscript, e.g. ¹²₆C. For ions, write the charge as a superscript on the right: ²³Na⁺.
书写核素符号时，质量数写在左上标，原子序数写在左下标，如¹²₆C。对于离子，电荷写在右上标：²³Na⁺。
3. Electron Configuration and Energy Levels | 电子排布与能级
Electrons occupy shells (principle quantum number n), which are split into subshells: s, p, d and f. Within each subshell, electrons fill orbitals according to the Aufbau principle, Hund’s rule and the Pauli exclusion principle.
电子占据电子层（主量子数n），电子层又分为亚层：s、p、d和f。在每个亚层中，电子按构造原理、洪特规则和泡利不相容原理填入轨道。
The order of filling for the first elements is: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p. Note that the 4s subshell fills before 3d because it has a lower energy for atoms up to calcium. From scandium onwards, 3d is filled.
前几个周期的填充顺序为：1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p。注意，对于直到钙的元素，4s亚层能量低于3d，因此先填充4s。从钪开始，3d轨道开始被填充。
Electron configurations can be written in full (e.g. 1s² 2s² 2p⁶ 3s² 3p⁶ for Ar) or using noble gas shorthand ([Ne] 3s² 3p⁴ for S). Diagrammatically, use arrows in boxes to represent electrons with opposite spins.
电子排布可用完整表示法（如Ar为1s² 2s² 2p⁶ 3s² 3p⁶），也可用稀有气体简写法（如S为[Ne] 3s² 3p⁴）。图示上，用方框中的箭头表示自旋相反的电子。
For WJEC, you are expected to write configurations for elements up to krypton (Z = 36), and for ions by adding or removing electrons from the highest energy level first (e.g., Fe²⁺: [Ar] 3d⁶, not 4s² 3d⁴).
WJEC要求考生能书写到氪（Z=36）的元素电子排布，以及离子的电子排布（先移去最高能级的电子，如Fe²⁺：[Ar] 3d⁶，而不是4s² 3d⁴）。
4. Isotopes and Relative Atomic Mass | 同位素与相对原子质量
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have identical chemical properties because their electron configurations are the same, but slightly different physical properties (e.g. density, rate of diffusion).
同位素是指质子数相同而中子数不同的同种元素原子。它们化学性质相同（因电子排布一致），但物理性质略有差异（如密度、扩散速率）。
The relative atomic mass (Aᵣ) of an element is the weighted average mass of an atom relative to 1/12th the mass of a carbon‑12 atom, taking into account the relative abundances of its isotopes. The formula is:
元素的相对原子质量（Aᵣ）是考虑同位素丰度后，一个原子的加权平均质量相对于一个碳‑12原子质量的1/12的比值。计算公式为：
Aᵣ = Σ (isotopic mass × % abundance) / 100
相对原子质量 = Σ（同位素质量 × 百分丰度） / 100
You could also be given relative abundances as fractions rather than percentages; the principle is the same. Always show your working clearly in calculations.
题目中可能给出分数形式的相对丰度而非百分数，但计算原理相同。解答时务必清晰展示计算步骤。
5. Mass Spectrometry and Isotopic Abundance | 质谱法与同位素丰度
A mass spectrometer can be used to determine the relative atomic mass of an element. The sample is vaporised, ionised (usually by electron impact or electrospray), accelerated, deflected by a magnetic field according to mass‑to‑charge ratio (m/z), and finally detected.
质谱仪可用于测定元素的相对原子质量。样品经气化、电离（通常为电子轰击或电喷雾）、加速，在磁场中按质荷比（m/z）偏转，最终被检测。
The mass spectrum shows peaks corresponding to each isotope. The height or area of each peak is proportional to the relative abundance of that isotope. From the spectrum, you can calculate the Aᵣ by multiplying each m/z value by its relative abundance, summing, and dividing by the total abundance.
质谱图显示每种同位素的峰。峰高或峰面积与该同位素的相对丰度成正比。由谱图可计算Aᵣ：将每个m/z值乘以其相对丰度，求和后除以总丰度。
In WJEC exams, you may be asked to interpret spectra for elements like chlorine (³⁵Cl and ³⁷Cl) or bromine (⁷⁹Br and ⁸¹Br), where diatomic molecules give characteristic patterns due to combinations of isotopes.
WJEC考试中可能出现对于氯（³⁵Cl和³⁷Cl）或溴（⁷⁹Br和⁸¹Br）的谱图解释，其双原子分子因同位素组合而产生特征峰型。
6. The Periodic Table: Arrangement and Blocks | 元素周期表：排列与分区
The modern periodic table arranges elements in order of increasing atomic number. Periods are horizontal rows; groups are vertical columns. Elements in the same group have the same number of electrons in their outer shell, leading to similar chemical properties.
现代周期表按原子序数递增排列元素。横行为周期，纵列为族。同族元素的最外层电子数相同，因此化学性质相似。
The table can be divided into s‑block (groups 1 and 2), p‑block (groups 13 to 18), d‑block (transition metals) and f‑block (lanthanides and actinides). The block an element belongs to corresponds to the highest‑energy subshell being filled.
周期表可分为s区（第1、2族）、p区（第13至18族）、d区（过渡金属）和f区（镧系和锕系）。元素所属的区对应着其最高能级填充的亚层。
For example, sodium (1s² 2s² 2p⁶ 3s¹) is in the s‑block; aluminium ([Ne] 3s² 3p¹) is in the p‑block; iron ([Ar] 3d⁶ 4s²) is in the d‑block. Understanding this link between electron configuration and position in the table is essential.
例如，钠（1s² 2s² 2p⁶ 3s¹）属于s区；铝（[Ne] 3s² 3p¹）属于p区；铁（[Ar] 3d⁶ 4s²）属于d区。理解电子排布与周期表位置之间的关联至关重要。
7. Periodic Trends: Atomic Radius and Ionisation Energy | 周期趋势：原子半径与电离能
Atomic radius decreases across a period due to increasing nuclear charge pulling electrons closer without a significant increase in shielding. Down a group, atomic radius increases because extra electron shells are added, outweighing the increased nuclear charge.
同周期从左到右，原子半径减小，因为核电荷增大而屏蔽作用增加不多，电子被更紧密地吸引。同族从上到下，原子半径增大，因为增加了新的电子层，其影响超过了核电荷的增加。
First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous unipositive ions. It generally increases across a period (due to greater nuclear attraction) and decreases down a group (electron is further from the nucleus and more shielded).
第一电离能是指从一摩尔气态原子移去一摩尔电子形成一摩尔+1价气态离子所需的能量。它通常沿周期从左向右增大（核吸引力增强），沿族从上向下减小（电子离核更远且屏蔽更强）。
Watch out for small drops at Group 13 (B to Al) and Group 16 (N to O, P to S) due to subshell effects: the outer electron in a p‑orbital of higher energy or paired repulsion makes it easier to remove.
注意第13族（B到Al）和第16族（N到O、P到S）的微小下降，这源于亚层效应：能量较高的p轨道电子或成对电子间的排斥使得移去电子更容易。
8. Ionisation Energies and Successive Ionisations | 电离能与逐级电离
Successive ionisation energies refer to the removal of second, third, etc., electrons from an atom. The energy increases each time because the remaining electrons experience a greater effective nuclear charge and the ion becomes more positively charged.
逐级电离能指从原子中移去第二、第三等个电子所需的能量。每次电离能都增大，因为剩余电子感受到更大的有效核电荷，且离子正电荷更高。
A large jump in successive ionisation energies indicates the removal of an electron from a new inner shell. This provides evidence for the existence of electron shells and can be used to deduce an element’s group. For instance, a big jump after the 3rd ionisation energy suggests the element has 3 outer electrons and belongs to Group 13.
逐级电离能出现大幅跃升表明开始从新的内层移去电子。这为电子层的存在提供了证据，并可用来推断元素的族。例如，若第三电离能之后出现剧增，说明该元素有3个最外层电子，属于第13族。
WJEC questions often provide a table of successive ionisation energies and ask you to identify the element or explain the jumps. Practice plotting log(ionisation energy) against electron number to make the jumps more visible.
WJEC试题常提供逐级电离能数据表，要求判断元素或解释跃升原因。练习将电离能取对数后对电子序数作图，可更清楚地看到跃升。
9. Chemical Bonding and the Octet Rule | 化学键与八隅律
Atoms bond to achieve a more stable electron arrangement, often that of a noble gas with a full outer shell of eight electrons (the octet rule). The three main types of strong bonding are ionic, covalent and metallic.
原子通过键合形成更稳定的电子排布，通常达到稀有气体那样的满外层的八电子结构（八隅律）。三种主要强键类型是离子键、共价键和金属键。
Ionic bonding involves the transfer of electrons from a metal to a non‑metal, forming oppositely charged ions held together by electrostatic attraction. For example, in NaCl, Na loses one electron to become Na⁺, and Cl gains one to become Cl⁻.
离子键是金属向非金属转移电子，形成带相反电荷的离子，通过静电引力结合。例如NaCl中，Na失去一个电子成为Na⁺，Cl得到一个电子成为Cl⁻。
Covalent bonding is the sharing of electrons between non‑metal atoms. A single covalent bond contains one shared pair of electrons (e.g., H—H); double and triple bonds contain two or three shared pairs. Lewis structures help visualise the arrangement of electrons.
共价键是非金属原子间共享电子。单键包含一对共用电子（如H—H）；双键和三键分别包含两对或三对共用电子。路易斯结构有助于直观显示电子排布。
While atoms and elements alone do not fully cover bonding, WJEC expects you to relate electron configurations to the type and number of bonds an element can form. Carbon (2,4) forms four covalent bonds, nitrogen (2,5) forms three, and so on.
尽管原子与元素本身并不涵盖所有化学键内容，但WJEC要求你将电子排布与元素所能形成的键的类型和数目关联起来。碳（2,4）形成四个共价键，氮（2,5）形成三个，等等。
10. Exam Tips and Common Misconceptions | 考试技巧与常见误区
When writing electron configurations, remember that chromium and copper are exceptions: Cr is [Ar] 3d⁵ 4s¹, not [Ar] 3d⁴ 4s²; Cu is [Ar] 3d¹⁰ 4s¹, not [Ar] 3d⁹ 4s². This is due to the extra stability of half‑filled and fully‑filled d subshells.
书写电子排布时，记住铬和铜是特例：Cr为[Ar] 3d⁵ 4s¹，而不是[Ar] 3d⁴ 4s²；Cu为[Ar] 3d¹⁰ 4s¹，而不是[Ar] 3d⁹ 4s²。这是因为半充满和全充满d亚层具有额外的稳定性。
A common mistake is confusing relative atomic mass with mass number. The mass number is an integer for a specific isotope, whereas relative atomic mass is a weighted average of all naturally occurring isotopes and is often not an integer.
常见的错误是混淆相对原子质量与质量数。质量数是特定同位素的整数值，而相对原子质量是所有天然同位素的加权平均值，通常不是整数。
For ionisation energy trends, many students state that shielding increases across a period. In fact, shielding remains roughly constant because electrons are added to the same outer shell. The decrease across a period is due to increasing nuclear charge.
在解释电离能趋势时，许多学生说同周期屏蔽作用增大。实际上，由于电子被添加到同一外层，屏蔽作用大致不变。同周期电离能增大的原因是核电荷增加。
Finally, always show all steps in mass spectrometry calculations and label axes on any graph you draw. Pay attention to units: ionisation energies are usually in kJ mol⁻¹; distinctions between ‘atom’ and ‘ion’ are crucial.
最后，质谱计算要展示所有步骤，绘制的图表要标注坐标轴。注意单位：电离能通常以kJ mol⁻¹表示；区分“原子”与“离子”至关重要。
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