= Molecule
{wiki}
A <molecule>, using the <International Union of Pure and Applied Chemistry> (IUPAC) convention, is an electrically neutral entity containing two or more <chemical atoms>. Its constituents form a bound unit through <chemical bonds>; merely drawing a boundary around neighboring particles does not make a <molecule>. The neutrality requirement concerns total <electric charge>, not the absence of internal charge separation: <water> is a <polar molecule> with zero total <electric charge>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry/molecule>[Gold Book, fifth edition, molecule: neutral entity with more than one atom; bound-state qualification]{ref}
One <water> <molecule>, $\mathrm{H_2O}$, contains two <hydrogen> and one <oxygen> <chemical atom>. One <molecular oxygen> <molecule>, $\mathrm{O_2}$, contains two <oxygen> <chemical atoms>; a <molecule> need not contain different <chemical elements>. One <carbon dioxide> <molecule>, $\mathrm{CO_2}$, contains one <carbon> and two <oxygen> <chemical atoms>. The <molecular formula> records these inventories, while <molecular geometry> describes their arrangement. <Water> and <carbon dioxide> are <chemical compounds>; <molecular oxygen> is a form of one <chemical element>.
\Image[/molecules-and-extended-ionic-arrangement.png]
{title=Discrete neutral <molecules> versus an extended <ionic crystal>}
{id=image-molecules-and-extended-ionic-arrangement}
{description=Original explanatory cartoons of <water>, <molecular oxygen>, <carbon dioxide>, and a patch of <sodium chloride>. The right panel uses a two-dimensional alternating arrangement to explain counting, not the full three-dimensional <ionic crystal>. Dotted neighbor guides are not localized <covalent bonds>; the box selects a $1:1$ count without isolating a <molecule>. Neither panel is a measured molecular image; positions, sizes, and connections are schematic.}
Ordinary crystalline <sodium chloride>, $\mathrm{NaCl}$, contains an extended arrangement of <sodium> <cations> $\mathrm{Na^+}$ and <chlorine> <anions> $\mathrm{Cl^-}$. Its <formula unit> specifies one of each for compositional accounting, without selecting a unique isolated pair. Bulk neutrality therefore does not imply a material consists of neutral <molecules>. This statement concerns the <ionic crystal>; the physical state and conditions matter when deciding what particles a <chemical formula> represents.<Chemistry 2e by Paul Flowers and others/2.6 Ionic and Molecular Compounds>[Ionic Compounds: NaCl composition and the absence of discrete molecules in the ordinary crystal]{ref}
A <polyatomic ion> is a charged bound group of <chemical atoms>, such as <hydronium>, $\mathrm{H_3O^+}$. A <molecular ion> formed by removing an <electron> from intact <water>, $\mathrm{H_2O^{\bullet+}}$, retains the same atom inventory but has net <electric charge> $+e$. Neither satisfies the neutral-<molecule> definition. Distinguishing these entities makes <molecular count and amount of substance> and <mass spectrometry> interpretable: say whether the count is of <chemical atoms>, neutral <molecules>, <ions>, or <formula units>.
The useful surprise is that a <molecule> can be treated as one particle for motion and counting while retaining internal structure. Its <molar mass> connects a particle inventory to bulk <mass>; <molecular rotation> and <molecular vibration> reveal internal organization. The <continuum approximation> uses bulk fields in place of individual molecular positions and motions.
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