Chemical formulae
Introduced| English | Meaning |
|---|---|
| chemical formula/ˈkemɪkl ˈfɔːmjʊlə/ | chemical formula |
| subscript/ˈsʌbskrɪpt/ | subscript |
| valency/ˈveɪlənsi/ | valency |
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.
Formula writing route
Follow charges or valencies to a neutral chemical formula.
How many hydrogen atoms are in one molecule of H₂O?
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The subscript 2 means two hydrogen atoms.
The small number that shows how many atoms of an element are present is the ______.
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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
Atoms combine in fixed ratios because of their:
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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.
How many oxygen atoms are in Ca(OH)₂?
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The bracket subscript 2 applies to OH, giving 2 O.
How many atoms in total are in CaCO₃?
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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.
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
- 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
Write from 7 atoms in sulfuric acid
- The formula is H2SO4.
- Count every atom shown. Water's two hydrogens and the five atoms in CaCO3 are different formulae.
How many atoms are in one molecule?
H2SO4 contains seven atoms in one molecule.
How many atoms are in one H2SO4 molecule?
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Two plus one plus four is seven. Other options — 5 (the atom count in CaCO3); 4 (the oxygen atoms only).