= Molecule bookkeeping
Four <water> <molecules> contain eight <hydrogen> and four <oxygen> <chemical atoms>, twelve constituent <chemical atoms> altogether. Counting the constituents does not mean they have been separated by breaking <chemical bonds>. The same sample has different counts because the specified entities differ.
\Image[/molecule-atom-and-amount-bookkeeping.png]
{title=One <water> sample: <molecule>, constituent <chemical atom>, and <amount of substance> bookkeeping}
{id=image-molecule-atom-and-amount-bookkeeping}
{description=An original four-<molecule> inventory cartoon, not a measurement or a <chemical reaction>. Dashed boundaries identify whole <water> <molecules>; each contains two <hydrogen> and one <oxygen> <chemical atom>. All three counts describe the same bound sample. The <amount of substance> uses the exact <Avogadro constant>; the displayed decimal is rounded.}
For $N$ <water> <molecules>, their <amount of substance> is $n=N/N_{\mathrm A}$, while the constituent <hydrogen> and <oxygen> inventories correspond to $2n$ and $n$ respectively. The exact <Avogadro constant> is $N_{\mathrm A}=6.02214076\times10^{23}\,\mathrm{mol^{-1}}$. The <mole> applies to specified entities, including <chemical atoms>, <molecules>, and <ions>.<SI base unit: mole by Bureau International des Poids et Mesures>[SI base unit: mole, definition and the requirement to specify the elementary entity]{ref} Thus four <water> <molecules> correspond to approximately $6.6422\times10^{-24}\,\mathrm{mol}$ of <water>, but twice that <amount of substance> of constituent <hydrogen> <chemical atoms>. A macroscopic conversion through measured <mass> additionally needs the relevant <molar mass>, as in <water mass to molecular count>.
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