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Stoichiometry relates the quantities consumed and formed in a chemical reaction. Its coefficients are ratios of entity counts, or equivalently of amount of substance, rather than ratios of masses. For the water chemical equation, consuming two molecular hydrogen molecules per molecular oxygen molecule gives the same $2:1$ ratio as consuming two moles of molecular hydrogen per mole of molecular oxygen. Multiplying every entity count by the same factor preserves the ratio. See Flowers et al., Chemistry 2e, reaction stoichiometry.

Coefficients and subscripts do different jobs. The coefficient in $2{\mathrm{H}}_2\mathrm{O}$ counts two water molecules; the subscript says that each contains two hydrogen chemical atoms. Changing coefficients scales quantities. Changing a subscript changes the specified chemical substance.

This distinction catches a common counting error: one mole of water molecules contains two moles of hydrogen chemical atoms and one mole of oxygen chemical atoms. Always specify what is counted.

Stoichiometry records reactant and product multiplicities in a chemical reaction or a chemical reaction network. The change caused by one reaction is its stoichiometric vector; multiplying by the reaction rate gives its contribution to the concentration equations. For two identical reactants the rate-constant convention absorbs any combinatorial factor.

A sulfide ion is formally ${\mathrm{S}}^{2-}$. Relative to oxide it is larger, more polarizable and more readily oxidized; the ability of a metal to support a high oxidation state need not survive replacement of oxygen by sulfur.

A cage ${\mathrm{R}}_{10}\mathrm{S}{\mathrm{i}}_{10}{\mathrm{O}}_{15}$ with a common pentagonal-prismatic silicon skeleton and an oxygen bridge on each edge.

A cage ${\mathrm{R}}_8\mathrm{S}{\mathrm{i}}_8{\mathrm{O}}_{12}$ represented by a cube of eight silicon vertices with one bridging oxygen on each of twelve edges.

Tetrahydrofuran is a cyclic ether solvent. Its oxygen can coordinate lithium ions and alter the aggregation of organolithium reagents.

A cyclic acetal protecting group made from an alcohol and dihydropyran. It protects the original alcohol oxygen and can be removed under suitable acidic hydrolysis conditions.

The reactive early-metal alkylidene $\mathrm{C}{\mathrm{p}}_2\mathrm{Ti}=\mathrm{C}{\mathrm{H}}_2$. carbonyl methylenation proceeds through an oxatitanacyclobutane; alkene capture gives a titanacyclobutane. Its oxygen affinity makes ester methylenation feasible.

Triflic anhydride is $(\mathrm{C}{\mathrm{F}}_3\mathrm{S}{\mathrm{O}}_2{)}_2\mathrm{O}$, an electrophilic sulfonylating reagent. carbonyl oxygen attack can form an O-triflylated intermediate; adjacent nitrogen donation gives an imidoyl-triflate description.

The cyclic anion ${\mathrm{P}}_3{\mathrm{O}}_9^{3-}$ comprising three tetrahedral $\mathrm{P}{\mathrm{O}}_4$ groups linked by three P–O–P bridges. Each phosphorus has two terminal oxygen atoms. It can bind a metal through one terminal oxygen from each phosphorus.

A valence electron belongs to the electronic arrangement used to describe a chemical atom's chemical bonds and ordinary chemical reactions. For the neutral hydrogen, carbon, nitrogen, and oxygen chemical atoms used here, the valence electrons occupy the outermost occupied electron shell. Their respective electron configurations are $1s^1$, $1s^{2}2s^{2}2p^2$, $1s^{2}2s^{2}2p^3$, and $1s^{2}2s^{2}2p^4$, giving one, four, five, and six valence electrons. See Chemistry 2e, 6.4: electron configurations and valence electrons.

Lewis structures show these valence electrons while omitting the inner electrons. Consequently, eight drawn valence electrons in water do not mean eight total electrons: neutral water has ten, with two inner oxygen electrons omitted. Neutral ammonia likewise has ten total electrons but eight valence electrons; neutral carbon dioxide has twenty-two total electrons but sixteen valence electrons.

This distinction makes the small counts useful: connections and possible lone pairs can be organized without representing every electron or solving the entire many-electron wavefunction. The outermost-electron shell shortcut applies to the examples above, rather than serving as a universal definition for all chemical elements; the electronic arrangement relevant to chemical bonds can also involve other electron subshells.

The VSEPR model predicts approximate molecular geometry by arranging the electron domains around a central chemical atom to reduce their mutual repulsion. Start with a Lewis structure, count the domains, and then distinguish the resulting electron pair geometry from the positions of the chemical atoms themselves. A lone pair counts as a domain even though it is not another chemical atom.^[ref]

Three worked counts have the same total of eight valence electrons. In methane, carbon's four plus the contributions of four hydrogen chemical atoms give $4+4=8$: four bond pairs, no lone pairs, and tetrahedral molecular geometry. In ammonia, nitrogen's five plus three from hydrogen give $5+3=8$: three bond pairs and one lone pair, producing trigonal pyramidal molecular geometry. In water, oxygen's six plus two from hydrogen give $6+2=8$: two bond pairs and two lone pairs, producing bent molecular geometry. All three have four electron domains; their molecular geometries differ.

The regular tetrahedral directions can be represented by the unit vectorsEach pair has dot product $-1/3$, so the ideal bond angle is $\theta =\arccos (-1/3)\approx 109.47^{\circ }$. This is a geometric calculation, rather than a prediction that every four-domain molecule has that exact bond angle. Lone pairs and different kinds of chemical bonds distort the ideal arrangement; water's H–O–H angle is about $104.5^{\circ }$.^[ref]

The model is attractive because small-number bookkeeping produces a spatial prediction without first solving for every molecular orbital. Shape then helps explain chemical polarity and which hydrogen bond donor or hydrogen bond acceptor sites are accessible. It is a qualitative model, not a full calculation of the electronic energy or a universal solution for every chemical compound. Gillespie and Nyholm's 1957 review ^[ref] is a historical anchor for its development.

An ether with oxygen directly attached to a vinylic carbon, $\mathrm{R}-\mathrm{O}-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$ in the simplest terminal example. oxygen resonance donation strongly differentiates the two alkene carbon shifts.

Choose approximate atomic weights $A_{\mathrm{r}}(\mathrm{H})=1.008$ and $A_{\mathrm{r}}(\mathrm{O})=15.999$, and use $M_{\mathrm{u}}\approx 1\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$. The chemical formula ${\mathrm{H}}_2\mathrm{O}$ then gives water molar massThese are deliberately chosen rounded inputs, not exact atomic weights or a universal value for every water sample. Different isotopic abundances, notably those in heavy water, change the mean molar mass; chemical formula alone does not specify them. Adding atomic weights also neglects the tiny mass change associated with chemical bond energies, immaterial at this rounding precision.

For an illustrative pure water sample of mass $4.50375\mathrm{g}$, this convention givesThe count refers to water molecules. Their hydrogen chemical atom count is approximately $2N=3.01107038\times 10^{23}$, and their oxygen chemical atom count is approximately $N$. The mean mass per water molecule is $M/N_{\mathrm{A}}\approx 2.99146\times 10^{-26}\mathrm{kg}$. Extra displayed digits expose the arithmetic; they do not imply that the sample composition or mass was measured that precisely.

The same conversion makes stoichiometry a mass calculation. For the chemical equation $2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}$, complete conversion of $0.250000\mathrm{mol}$ of molecular hydrogen and $0.125000\mathrm{mol}$ of molecular oxygen gives $0.250000\mathrm{mol}$ of water. With the chosen approximate atomic weights, the reactant masses are $0.50400\mathrm{g}$ and $3.99975\mathrm{g}$, summing to $4.50375\mathrm{g}$. The $2:1:2$ ratio belongs to molecule counts and amount of substance, not to masses; this bookkeeping neglects the much smaller reaction energy contribution to mass.

A molecule containing two hydrogen chemical atoms and one oxygen chemical atom. An isolated ground-state water molecule has a bent equilibrium geometry; ordinary bulk water contains interacting water molecules rather than a collection of independent rigid objects.

A Wittig reaction with $\mathrm{P}{\mathrm{h}}_3\mathrm{P}=\mathrm{C}{\mathrm{H}}_2$ that replaces a carbonyl oxygen by $=\mathrm{C}{\mathrm{H}}_2$. On a cyclic ketone it installs an exocyclic methylene without inserting a new ring atom.

Reaction of a phosphorus ylide with a carbonyl compound to form an alkene. The carbonyl carbon and ylidic carbon become the two alkene atoms; the oxygen is transferred to phosphorus.

A dibenzo-gamma-pyrone framework with two outer benzene rings, an endocyclic ether oxygen and an exocyclic ketone oxygen. The outer rings are strongly aromatic; the central ring has a more polarized, partial aromatic description.

Zirconia stabilized by addition of yttria. At suitable elevated temperatures, oxygen vacancies allow oxide-ion transport, making the material useful in an oxygen concentration cell. Its oxide-ion transference number must remain sufficiently close to one for a simple ionic Nernst equation interpretation.

A hydride reagent that can coordinate nearby oxygen donors while reducing a carbonyl.