= Chemical equation
{wiki}
A <chemical equation> represents a <chemical reaction> using <chemical formulas>, with <reactants> on the left and <reaction products> on the right. A balanced <chemical equation> has the same number of <chemical atoms> of each <chemical element> on both sides. For <molecular hydrogen> and <molecular oxygen> forming <water>,
$$
2\mathrm{H}_2+\mathrm{O}_2\longrightarrow2\mathrm{H}_2\mathrm{O}.
$$
The coefficients $2,1,2$ give the relative numbers of <molecules>; the coefficient $1$ is normally omitted. Count <hydrogen> as $2\times2=4$ on the left and $2\times2=4$ on the right, and <oxygen> as $1\times2=2$ on the left and $2\times1=2$ on the right. See https://openstax.org/books/chemistry-2e/pages/4-1-writing-and-balancing-chemical-equations[Flowers et al., Chemistry 2e, §4.1: writing and balancing chemical equations].
\Image[/chemical-equation-atom-balance.png]
{title=Two <molecular hydrogen> <molecules> and one <molecular oxygen> <molecule> account for two <water> <molecules>}
{description=Exactly four <hydrogen> and two <oxygen> <chemical atoms> occur on each side. The circles and connecting lines are an invented schematic: they indicate grouping and <chemical bonds>, without representing actual <bond lengths>, <bond angles> or a <reaction mechanism>.}
To balance the initial expression $\mathrm H_2+\mathrm O_2\longrightarrow\mathrm H_2\mathrm O$, first put $2$ before <water> to match the two <oxygen> <chemical atoms>. This creates four <hydrogen> <chemical atoms> on the right, so put $2$ before <molecular hydrogen>. Changing a coefficient multiplies the entire <chemical formula>. Changing a subscript changes composition and usually the <chemical substance>: $2\mathrm H_2\mathrm O$ is two <water> <molecules>, whereas $\mathrm H_2\mathrm O_2$ is one <hydrogen peroxide> <molecule>. Never balance a <chemical equation> by changing its subscripts.
The bookkeeping follows from conservation of the <atomic nuclei> in ordinary, non-nuclear <chemical reactions>; <chemical bonds> and the distribution of <Electrons> can change. For a <chemical equation> involving <ions>, total <electric charge> must also match. For example, the existing <self-ionization of water> equation has zero total <electric charge> on each side, since the $+1$ of <hydronium> cancels the $-1$ of <hydroxide>. See <Chemistry 2e by Paul Flowers and others/2.6 Ionic and Molecular Compounds> (§2.6: nuclei and ions).
The same $2:1:2$ ratio relates changes in <amount of substance>: two <moles> of <molecular hydrogen> consumed per <mole> of <molecular oxygen> correspond to two <moles> of <water> formed. A <mole> is a defined unit, not one <molecule>; the little whole-number picture is an abstract counting model, not a claim that a macroscopic sample contains just five <molecules>. See <molecular count and amount of substance> for the exact conversion and https://www.bipm.org/en/si-base-units/mole[BIPM, SI base unit: mole] for the definition.
This is why a balanced <chemical equation> is useful: <stoichiometry> turns the same atom accounting into relations between measurable quantities. It does not determine <reaction rate>, the final conversion set by <chemical equilibrium>, or the actual yield of recovered <reaction products>. The <limiting reagent> gives an upper bound under the specified <chemical reaction>.
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