📚 Naming Compounds and Writing Formulae | 化合物命名与化学式书写
In KS3 chemistry, you learn to read and write the language of chemical formulae. A formula uses element symbols and small subscript numbers to show which elements are present and in what ratio the atoms or ions are combined. Naming compounds follows a clear set of rules, so you can work backwards from a name to a formula and forwards from a formula to a name.
在 KS3 化学中，你将学习阅读和书写化学式这门语言。化学式使用元素符号和较小的下标数字，来表示含有哪些元素以及原子或离子以什么比例结合。化合物命名遵循一套清晰的规则，因此你既能从名称反推化学式，也能从化学式正推名称。
This guide covers element symbols, binary compounds, ionic formulae, polyatomic ions, covalent prefixes, transition metals and Roman numerals, plus a checklist of the mistakes that cost the most marks.
本指南涵盖元素符号、二元化合物、离子化学式、多原子离子、共价前缀、过渡金属与罗马数字，并附上最容易丢分的错误检查清单。
1. Elements, Symbols and the Periodic Table | 元素、符号与周期表
Every element has a chemical symbol, usually made of one or two letters. The first letter is always a capital letter, and the second letter, if there is one, is always lower case. Hydrogen is H, helium is He and lithium is Li.
每种元素都有一个化学符号，通常由一个或两个字母组成。第一个字母始终大写；如果有第二个字母，则始终小写。氢是 H，氦是 He，锂是 Li。
The periodic table arranges elements in order of atomic number, which is the number of protons in the nucleus. At KS3 you mainly need the first 20 elements, plus common metals such as iron, copper, zinc, silver, tin, gold, mercury and lead.
周期表按原子序数排列元素，原子序数就是原子核中质子的数目。在 KS3 阶段，你主要需要掌握前 20 号元素，以及铁、铜、锌、银、锡、金、汞和铅等常见金属。
Some symbols do not match the English name at all, because they come from older Latin names. Sodium is Na (natrium), potassium is K (kalium), iron is Fe (ferrum), copper is Cu (cuprum), silver is Ag (argentum), gold is Au (aurum), mercury is Hg (hydrargyrum), tin is Sn (stannum) and lead is Pb (plumbum).
有些符号与英文名称完全对不上，因为它们来自更早的拉丁名称。钠是 Na（natrium），钾是 K（kalium），铁是 Fe（ferrum），铜是 Cu（cuprum），银是 Ag（argentum），金是 Au（aurum），汞是 Hg（hydrargyrum），锡是 Sn（stannum），铅是 Pb（plumbum）。
Learn symbols in both directions: name to symbol and symbol to name. This single skill underpins every formula question you will meet later.
符号要双向记忆：由名称写符号，也由符号说出名称。这一项技能是之后所有化学式题目的基础。
2. From Names to Symbols: Common Elements | 从名称到符号：常见元素
Before writing formulae, you must be confident changing element names into symbols and back again. Most names match the first one or two letters of the symbol, but the Latin-derived ones must simply be memorised.
在书写化学式之前，你必须熟练地将元素名称转换为符号，并能反向转换。大多数名称与符号的前一两个字母一致，但那些源自拉丁语的符号只能靠记忆。
Here is a useful table of common KS3 elements:
下面是一个很有用的 KS3 常见元素表：
| Element | Symbol | 中文名称 |
|---|---|---|
| hydrogen | H | 氢 |
| helium | He | 氦 |
| lithium | Li | 锂 |
| beryllium | Be | 铍 |
| boron | B | 硼 |
| carbon | C | 碳 |
| nitrogen | N | 氮 |
| oxygen | O | 氧 |
| fluorine | F | 氟 |
| neon | Ne | 氖 |
| sodium | Na | 钠 |
| magnesium | Mg | 镁 |
| aluminium | Al | 铝 |
| silicon | Si | 硅 |
| phosphorus | P | 磷 |
| sulfur | S | 硫 |
| chlorine | Cl | 氯 |
| argon | Ar | 氩 |
| potassium | K | 钾 |
| calcium | Ca | 钙 |
| iron | Fe | 铁 |
| copper | Cu | 铜 |
| zinc | Zn | 锌 |
| silver | Ag | 银 |
| tin | Sn | 锡 |
| gold | Au | 金 |
| mercury | Hg | 汞 |
| lead | Pb | 铅 |
A quick self-test: cover the middle column and write the symbol for each element from memory, then cover the first column and name each symbol out loud.
一个快速自测法：遮住中间一列，凭记忆写出每种元素的符号；然后遮住第一列，把每个符号的名称大声读出来。
3. What a Formula Tells You | 化学式告诉了你什么
A chemical formula shows which elements are joined together and the ratio in which their atoms or ions are present. The small number written after a symbol, slightly below the line, is called a subscript, and it applies only to the symbol immediately before it.
化学式表示哪些元素结合在一起，以及它们的原子或离子以什么比例存在。写在符号后面的小数字、位置略低于基线，称为下标，它只作用于紧挨着它前面的那个符号。
In H₂O, the subscript 2 means there are two hydrogen atoms for every one oxygen atom. In CO₂, there are two oxygen atoms for every one carbon atom. A symbol with no subscript means there is just one of that atom.
在 H₂O 中，下标 2 表示每一个氧原子对应两个氢原子。在 CO₂ 中，每一个碳原子对应两个氧原子。没有下标的符号表示该原子只有一个。
Brackets work the same way. In Ca(OH)₂ the bracket contains one oxygen and one hydrogen, and the subscript 2 outside the bracket multiplies everything inside it, giving two oxygen atoms and two hydrogen atoms.
括号的作用原理相同。在 Ca(OH)₂ 中，括号内是一个氧和一个氢，括号外的下标 2 会乘到括号内所有原子上，因此共有两个氧原子和两个氢原子。
Ca(OH)₂ → 1 Ca + 2 O + 2 H
Never change a subscript to balance an equation, because that changes the substance itself. Balancing is done by writing numbers in front of formulae, not by editing the formulae.
千万不要为了配平方程式而改动下标，因为那会改变物质本身。配平靠的是在化学式前面写系数，而不是修改化学式。
4. Naming Binary Compounds | 二元化合物的命名
A binary compound contains exactly two elements. When a metal combines with a non-metal, the metal is named first and the non-metal takes the ending -ide. So NaCl is sodium chloride and MgO is magnesium oxide.
二元化合物只含两种元素。当金属与非金属结合时，金属名称写在前面，非金属名称改为 -ide 结尾。因此 NaCl 是氯化钠，MgO 是氧化镁。
The most common -ide endings you need are oxide (O), chloride (Cl), bromide (Br), iodide (I), fluoride (F), sulfide (S), nitride (N), phosphide (P) and carbide (C).
最常用的 -ide 结尾包括：oxide（O，氧化物）、chloride（Cl，氯化物）、bromide（Br，溴化物）、iodide（I，碘化物）、fluoride（F，氟化物）、sulfide（S，硫化物）、nitride（N，氮化物）、phosphide（P，磷化物）和 carbide（C，碳化物）。
In Chinese, binary compounds are usually named as 某化某, with the electropositive element first and the electronegative element second, exactly mirroring the English order.
在中文里，二元化合物通常命名为“某化某”，正电性元素在前、负电性元素在后，与英文顺序完全一致。
- NaCl is sodium chloride.
NaCl 是氯化钠。 - MgO is magnesium oxide.
MgO 是氧化镁。 - CaO is calcium oxide.
CaO 是氧化钙。 - KBr is potassium bromide.
KBr 是溴化钾。 - ZnS is zinc sulfide.
ZnS 是硫化锌。 - Al₂O₃ is aluminium oxide.
Al₂O₃ 是氧化铝。
Notice that the formula does not always show a 1:1 ratio, but the name still uses the same two words. The name tells you the elements, and the formula tells you the ratio.
注意，化学式并不总是 1:1 的比例，但名称仍然只用这两个词。名称告诉你有哪些元素，化学式才告诉你比例。
5. Writing Formulae for Ionic Compounds | 离子化合物化学式的书写
Metal and non-metal compounds are usually ionic. Metals lose electrons to form positive ions (cations), and non-metals gain electrons to form negative ions (anions). The formula of an ionic compound must have no overall charge.
金属与非金属形成的化合物通常是离子化合物。金属失去电子形成正离子（阳离子），非金属得到电子形成负离子（阴离子）。离子化合物的化学式必须整体不带电荷。
Use this four-step method every time:
每次都使用下面这个四步法：
- Step 1: write the symbols of the two ions side by side, metal first.
第一步：并排写出两种离子的符号，金属写在前。 - Step 2: write the charge on each ion as a superscript.
第二步：在每个离子符号的右上角标出电荷。 - Step 3: swap the numbers of the charges to become the subscripts.
第三步：把两个电荷数交叉写到对方的下标位置。 - Step 4: simplify the ratio and drop the number 1, and add brackets if a group ion is multiplied.
第四步：把比例化简，去掉数字 1；若某个离子团被乘，需加括号。
Worked example one: magnesium and chlorine. Mg²⁺ and Cl⁻ swap to give Mg₁Cl₂, simplified to MgCl₂.
例一：镁与氯。Mg²⁺ 与 Cl⁻ 交叉得 Mg₁Cl₂，化简为 MgCl₂。
Mg²⁺ + 2Cl⁻ → MgCl₂
Worked example two: aluminium and oxygen. Al³⁺ and O²⁻ swap to give Al₂O₃, which is already the simplest whole-number ratio.
例二：铝与氧。Al³⁺ 与 O²⁻ 交叉得 Al₂O₃，这已经是最简整数比。
2Al³⁺ + 3O²⁻ → Al₂O₃
Worked example three: calcium and chlorine. Ca²⁺ and Cl⁻ swap to give Ca₁Cl₂, simplified to CaCl₂.
例三：钙与氯。Ca²⁺ 与 Cl⁻ 交叉得 Ca₁Cl₂，化简为 CaCl₂。
Common ion charges worth memorising: group 1 metals are 1+, group 2 metals are 2+, aluminium is 3+, oxygen and sulfur are 2−, and the halogens chlorine, bromine and iodine are 1−.
值得记住的常见离子电荷：第 1 族金属为 1+，第 2 族金属为 2+，铝为 3+，氧和硫为 2−，卤素氯、溴、碘为 1−。
6. Compounds with Polyatomic Ions | 含多原子离子的化合物
Some ions are made of several atoms joined together and carry a single overall charge. These are called polyatomic ions or group ions. You must learn their formulae, charges and names as whole units.
有些离子由多个原子结合在一起，整体只带一个电荷，这类离子称为多原子离子或离子团。你必须把它们作为一个整体来记忆化学式、电荷和名称。
| Ion | Formula | Charge | 中文名称 |
|---|---|---|---|
| ammonium | NH₄ | 1+ | 铵根 |
| hydroxide | OH | 1− | style="border:1px solid #ccc;padding:8px
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