Chemical formulae
Vocabulary
| English | Chinese | Pinyin |
|---|---|---|
| chemical formula | 化学式 | huà xué shì |
| subscript | 下标 | xià biāo |
| valency | 化合价 | huà hé jià |
The shorthand of chemistry
- Writing "two hydrogen atoms joined to one oxygen atom" every time would be exhausting.
- Chemists use formulae — a compact code for what a substance is made of.
Explore
Formula writing route
Follow charges or valencies to a neutral chemical formula.
Practice
How many hydrogen atoms are in one molecule of H₂O?
The subscript 2 means two hydrogen atoms.
Practice
The small number that shows how many atoms of an element are present is the ______.
Subscripts give atom counts in a formula.
Reading a formula
- A chemical formula 化学式 shows the elements and how many atoms of each.
- The small subscript 下标 number gives the count: H₂O = 2 hydrogen + 1 oxygen.
For an ionic compound, the ion charges cross over to give the formula
Practice
Atoms combine in fixed ratios because of their:
Valency sets how many bonds an atom forms.
Why formulae are fixed
- Atoms combine in fixed ratios set by their combining power (valency 化合价).
- For example, hydrogen has valency 1 and oxygen 2, so water is H₂O — not HO.
Practice
How many oxygen atoms are in Ca(OH)₂?
The bracket subscript 2 applies to OH, giving 2 O.
Practice
How many atoms in total are in CaCO₃?
1 Ca + 1 C + 3 O = 5 atoms.
Brackets in formulae
- Brackets group atoms that repeat: Ca(OH)₂ means one calcium and two OH groups.
- So Ca(OH)₂ contains 1 Ca, 2 O and 2 H.
Example
Worked example. How many atoms are in CaCO₃? One calcium, one carbon, and three oxygen = 5 atoms in total. The subscript 3 applies only to the oxygen.
You've got it
Key idea
- a formula shows which elements and how many atoms of each (subscripts)
- atoms combine in fixed ratios set by their valency
- brackets group repeating units: Ca(OH)₂ has 2 O and 2 H