定义Definition
化学式用元素符号和数字表示物质的组成。分子化合物用分子式表示每个分子中各原子的实际数目（如 C₆H₁₂O₆）；离子化合物用最简整数比表示（如 NaCl、CaCl₂），称为式单位；实验式只给出最简原子数比（如葡萄糖为 CH₂O）。
A chemical formula shows the composition of a substance with element symbols and numbers. A molecular formula gives the actual number of each atom in a molecule (C₆H₁₂O₆); an ionic compound is written in its simplest whole-number ratio (NaCl, CaCl₂), called a formula unit; an empirical formula gives only the simplest ratio (CH₂O for glucose).
离子化合物的写法Writing ionic formulae
使正负电荷总数相等：Al³⁺ 与 O²⁻ 组成 Al₂O₃；Ca²⁺ 与 NO₃⁻ 组成 Ca(NO₃)₂，多原子离子多于一个时加括号。
Balance the charges: Al³⁺ and O²⁻ give Al₂O₃; Ca²⁺ and NO₃⁻ give Ca(NO₃)₂, with brackets round a polyatomic ion that appears more than once.
几种化学式的比较Kinds of formula
以乙酸为例：分子式 C₂H₄O₂；实验式 CH₂O；结构简式 CH₃COOH；展开式和键线式还能显示原子的连接方式。结晶水写在点后，如 CuSO₄·5H₂O。
For ethanoic acid: molecular formula C₂H₄O₂; empirical formula CH₂O; structural formula CH₃COOH; displayed and skeletal formulae also show how the atoms are joined. Water of crystallisation follows a dot, as in CuSO₄·5H₂O.
年表Timeline
- 1813
贝采利乌斯提出用字母表示元素的化学符号体系
Berzelius proposes letter symbols for the elements