Symbols, formulae & representing substances
1.2 · Matter
Symbols, formulae & representing substances
By the end of this section:
- I can show a pure substance (a compound or an element) with a symbol or a formula, for example Au or H₂O
- I can find the symbol for an element’s atoms in the periodic table
1 · Read
The notes, the key idea and the words to knowChemists write substances in a shorthand that is the same in every language. Each element has a chemical symbol of one or two letters — the first always a capital, the second (if there is one) always lowercase: C, He, Na, Fe. Compounds are written as a chemical formula: symbols with subscripts that show how many of each atom are present.
- H₂O: Water
- O₂: Oxygen
- CO₂: Carbon dioxide
- NaCl: Sodium chloride (common salt)
- H₂SO₄: Sulphuric acid
- Ca(OH)₂: Calcium hydroxide
- 2 H₂O: Water
- 2 CO₂: Carbon dioxide
- 2 H₂SO₄: Sulphuric acid
Tap a number or a symbol in the formula. No big number in front means one molecule.
Atoms counted
Each box is one molecule. Circles are atoms, coloured by element.
Try changing a number
The formulae come from the unit booklet (page 12) and the notes for 1.2.
The formula is just a count of the atoms in the picture
Why the rules are written so strictlyTap to open
The capital-then-lowercase rule is not fussiness — it removes ambiguity. Co is the element cobalt, but CO is the compound carbon monoxide (one carbon, one oxygen). One letter case changes the meaning entirely, so the convention has to be exact.
Subscripts and coefficients do different jobs, and mixing them up is the commonest slip:
- Subscript (small, after a symbol): changes what the substance is — H2O and H2O2 are different substances.
- Coefficient (big, in front): changes how many copies you have — 2 H2O is two of the same water molecule.
How many oxygen atoms are in 2 H2SO4?
- 4 is the oxygen in one molecule. The big 2 in front means there are two molecules: how many oxygen atoms is that?
- 6 is 4 + 2, but a big number does not add. It multiplies: two whole copies of H₂SO₄ means two lots of the oxygen.
- One H2SO4 has 4 oxygen: the small 4 counts the O just before it.
- The big 2 in front multiplies the whole formula: 4 × 2.
- 8 oxygen atoms (and 4 hydrogen, 2 sulphur).
-
Not thisCO and Co mean the same thing.
ThisNo — CO is carbon monoxide (two atoms); Co is the element cobalt. Letter case is part of the meaning.
-
Not thisA subscript can multiply a whole formula.
ThisA subscript only counts the symbol before it. To multiply the whole formula, use a coefficient in front (2 H2O).
-
Not this2 H2O has 2 oxygen atoms in each molecule.
ThisEach water molecule still has 1 oxygen. The 2 means two molecules, so 2 oxygen atoms in total.
-
Not thisYou can change a subscript to balance an equation.
ThisNever — changing a subscript changes the substance. You only adjust coefficients (see 1.7).
- chemical symbol
- one or two letters for an element; first letter capital, second lowercase
- chemical formula
- symbols and subscripts showing the atoms in a substance
- subscript
- the small number after a symbol; counts the atoms of that element only
- coefficient
- the large number in front of a formula; multiplies the whole formula
- diatomic molecule
- a molecule of two atoms, e.g. O2, H2, Cl2
2 · Watch
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Videos in other languagesPortuguês · 中文 · 日本語
We try to help you in other languages. Learn the idea here in your own language, then learn the science words from the English or French video. Not all of these videos have been independently checked. If something does not make sense, or does not match what we did in class, ask your teacher.
Scroll sideways for more videos
- Booklet pp. 7–10
- Knowledge Book pp. 7–10
- In your class workbook: Chapter 1.2 Atomic structure and classification of matter (pp. 8–15) · Revise this section
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