A chemical formula represents a substance using element symbols and subscripts to show the number of each kind of atom, for example H2O or CO2.
A chemical formula represents a substance using element symbols and subscripts to show the number of each kind of atom present, for example H2O or CO2. Malay: formula kimia · Chinese: 化学式.
The symbols come from the Periodic Table, and the small subscript number after a symbol tells you how many atoms of that element are in the formula. If there is no subscript, it means one atom. A chemical formula is a compact code for a substance: it names the elements present and gives their ratio in a single short expression, which is why it is used everywhere in Chemistry.
Example. In H2O the subscript 2 shows two hydrogen atoms, while the oxygen has no subscript, so there is one oxygen atom. In sulfuric acid, H2SO4, there are two hydrogen atoms, one sulfur atom and four oxygen atoms, seven atoms in all. Where a group of atoms repeats, brackets are used, as in calcium hydroxide Ca(OH)2. Reading a formula correctly is the first step in working out a relative formula mass.
Confusion to avoid. “Chemical formula” is the general term, and empirical, molecular and ionic formulae are all particular kinds of it, so do not treat them as unrelated. Take care too to distinguish a subscript, which is written small and after a symbol, from a coefficient, the large number written in front of a whole formula in an equation. Changing a subscript changes the substance itself, while a coefficient only changes how many units you have.
In this chapter the chemical formula is the raw material for every calculation: you read it to find a relative mass, to work out percentage composition, and to build and balance equations, so mastering it underpins the rest of SPM Chemistry.
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