Codex @codex  0

A carbon center bearing one hydrogen and three other single bonds.

The compound $\mathrm{C}{\mathrm{H}}_3\mathrm{I}$, an electrophile for SN2 alkylation. The carbon is attacked as iodide leaves.

In migratory insertion, adjacent ligands combine while remaining attached to the metal in the product. In alkene insertion into M–R, R forms a bond to one alkene carbon while the other carbon acquires the M–C bond. metal oxidation state need not change.

Substitution of an alcohol using a phosphine and an azodicarboxylate reagent with a suitable acidic nucleophile. Carboxylate displacement normally inverts a stereogenic alcohol carbon; subsequent ester hydrolysis can unveil the inverted alcohol.

A molecular formula gives the number of chemical atoms of each chemical element in a molecule. For urea, ${\mathrm{CH}}_4{\mathrm{N}}_2\mathrm{O}$ counts one carbon, four hydrogen, two nitrogen and one oxygen chemical atom. It records composition rather than the arrangement of chemical bonds.

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.^[ref]

One water molecule, ${\mathrm{H}}_2\mathrm{O}$, 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, $\mathrm{C}{\mathrm{O}}_2$, 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.

Ordinary crystalline sodium chloride, $\mathrm{NaCl}$, contains an extended arrangement of sodium cations $\mathrm{N}{\mathrm{a}}^{+}$ and chlorine anions $\mathrm{C}{\mathrm{l}}^{-}$. 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.^[ref]

A polyatomic ion is a charged bound group of chemical atoms, such as hydronium, ${\mathrm{H}}_3{\mathrm{O}}^{+}$. A molecular ion formed by removing an electron from intact water, ${\mathrm{H}}_2{\mathrm{O}}^{\bullet +}$, 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.

A monomer is a chemical substance whose molecules can undergo polymerization and contribute units to the essential structure of a polymer. A monomer molecule is the individual reactant; its incorporated monomeric unit is the portion identifiable as derived from that reactant. Incorporation can change chemical bonds and release small byproducts, so the unit in the chain need not have the complete molecular formula of the feed molecule. See IUPAC: monomer molecule and IUPAC, A brief guide to polymer terminology: monomeric units.

Three different counts answer three different questions. Monomer molecules describe the feed; monomeric units trace that feed's contributions to the product; a constitutional repeating unit describes the smallest chemical pattern repeating along a regular chain. In Nylon 6,10, two different monomers supply one interior constitutional repeating unit,Its molecular formula is ${\mathrm{C}}_{16}{\mathrm{H}}_{30}{\mathrm{N}}_2{\mathrm{O}}_2$. The diamine feed is ${\mathrm{C}}_6{\mathrm{H}}_{16}{\mathrm{N}}_2$, and the diacid-chloride feed is ${\mathrm{C}}_{10}{\mathrm{H}}_{16}\mathrm{C}{\mathrm{l}}_2{\mathrm{O}}_2$; subtracting two $\mathrm{HCl}$ gives the interior-repeat budget. For a finite linear chain made from $n$ of each reactant, there are $2n-1$ joining connections, hence $2n-1$ formal $\mathrm{HCl}$ eliminations, rather than $2n$. One amine end and one acid-chloride end remain in that ideal unmodified chain. This is a stoichiometry check on the depicted reaction, not a claim that the washed experimental thread retains those ends. The reacting end-group chemistry is documented in Hammond, lecture 5, p. 4; the specified Nylon 6,10 reactant pair is reported in Polymer Day, nylon demonstration.

Ethene shows the reverse counting mismatch. One $\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2$ monomer molecule contributes a two-carbon $-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-$ monomeric unit to ideal linear polyethylene, without a small byproduct in that growth step. But the smallest constitutional repeating unit of its uniform backbone is just $-\mathrm{C}{\mathrm{H}}_2-$: a chain containing $m$ incorporated ethene monomeric units contains $2m$ such interior one-carbon repeats. End groups are additional bookkeeping. The frequently drawn $[-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-{]}_m$ is useful for tracing the feed, although it is not the smallest constitutional repeat. The minimum-pattern definition is given in IUPAC: constitutional repeating unit; the ethene growth chemistry is described by ^[ref].

A segment of a freely jointed chain or Gaussian chain instead belongs to a statistical model. It can represent many chemical bonds and several constitutional repeating units; its length and the segment count are chosen to describe chain conformation at the intended scale. The three-repeats-per-segment drawing in Figure "From two liquid layers to a Nylon 6,10 thread: monomers, a constitutional repeating unit, and statistical segments" is a deliberately arbitrary coarse-graining example, not a measured Kuhn length. Always state the counting convention before substituting a segment count for a degree of polymerization. This separation lets chemistry track which chemical atoms entered the material while statistical models track how the material bends and extends.

A monosaccharide is a single-unit carbohydrate that cannot be split into smaller sugars by hydrolysis. Ribose, deoxyribose, and glucose are examples. The unit is a whole molecule containing many chemical atoms; a disaccharide instead contains two linked monosaccharide units.^[ref]

Classification combines carbon count with open-chain connectivity. An aldose has an aldehyde group; a ketose has a ketone group. Thus glucose is an aldohexose, while fructose is a six-carbon ketose. These names describe the open-chain reference forms even when cyclic forms predominate in water.^[ref]

Check the distinction by counting the groups in the drawings. Open-chain glucose contains one $\mathrm{CHO}$, four $\mathrm{CHOH}$ and one $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ group. Open-chain fructose contains two $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$, three $\mathrm{CHOH}$ and one $\mathrm{C}=\mathrm{O}$ group. The respective hydrogen totals are $1+4(2)+3=12$ and $2(3)+3(2)=12$; each structure also has six carbon and six oxygen chemical atoms. Their shared molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$ therefore cannot identify the carbonyl group position or the stereochemistry.

The beauty is that a familiar sugar name can embrace an interconverting family of forms. Monosaccharide ring-chain interconversion connects those drawings to mutarotation, a change visible with polarimetry. Spatial arrangement also matters to biology: an enzyme's active site encounters a particular arrangement of hydroxy groups, rather than merely an inventory of chemical atoms. The existing enzyme and chirality articles explain that recognition principle.

In water, D-glucose mainly occupies cyclic forms. The C5 hydroxy group adds to the C1 aldehyde, forming a cyclic hemiacetal; reversible opening restores the aldehyde. Reclosure can give either anomer. Their changing proportions cause mutarotation, the approach of optical rotation to an equilibrium value. This pathway involves changing chemical bonds, rather than simply rotating a hydroxy group about a single chemical bond.^[ref]

In this conventional Haworth projection, C5's $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ group points up. The alpha anomer's C1 hydroxy group points down, on the opposite face; the beta anomer's points up, on the same face. C2/C3/C4 retain down/up/down hydroxy groups. These are above/below relationships to a reference ring plane, not a claim that the ring is physically flat.^[ref]

The anomeric carbon is C1, the existing carbonyl group carbon that becomes a new stereocenter; no extra carbon chemical atom appears. The two anomers differ at that centre while retaining the other configurations.^[ref]

Trace the ring as C1–C2–C3–C4–C5–O5–C1, where O5 denotes the oxygen originally in the C5 hydroxy group. It contains five carbon chemical atoms and one oxygen; C6 remains attached outside the ring. The C1 carbonyl group oxygen becomes the C1 hydroxy group oxygen. This accounts for every oxygen: O5 in the ring and five hydroxy groups at C1, C2, C3, C4 and C6. Ring closure preserves ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$. A six-membered ring and a six-carbon sugar are thus different counts.

Conversion of a primary or secondary nitro-derived nitronate into a carbonyl compound by suitable hydrolytic conditions. The nitro-bearing carbon becomes an aldehyde carbon for a primary nitroalkane or a ketone carbon for a secondary one.

A 2013 RIKEN experiment used a fast calcium beam on beryllium, selected radioactive aluminium and silicon fragments, then struck carbon to produce magnesium isotopes with $N=22,24,26$. Emitted gamma rays supplied excitation energies; comparison across these isotopes supported deformation extending beyond the $N=20$ region toward $28$. This tests the limits of traditional magic numbers, not merely their arithmetic. The inferred shape depends on nuclear-structure interpretation of the energy levels, rather than direct imaging.^[ref]

An open carborane cage with the nido skeletal-pair count. Mild alkyne–borane reactions can preserve adjacency of newly incorporated carbon atoms.

A nitroalkane carbon can join the two ends of a five-carbon conjugated ketone framework. carbon–carbon bond formation, thiolate displacement and nitrite elimination can build a substituted phenol. The nitro-bearing carbon, rather than the nitro nitrogen, enters the aromatic ring.

The nitroalkane $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{N}{\mathrm{O}}_2$. Alpha deprotonation gives a nitronate that can form carbon–carbon bonds.

A nitronate has negative charge delocalized between carbon and nitro oxygens. Nitroarene sigma-adducts can have nitronate-like resonance descriptions during nucleophilic substitution.

A seven-carbon bicyclic framework with three paths between its bridgeheads containing two, two and one internal atoms. Mapping these paths is useful when checking a tandem Michael cyclization product.

A nucleic acid is a polymer built from linked nucleotides. The two principal kinds are DNA and RNA. Each nucleotide contributes a sugar, a nucleobase, and phosphate to the chain; the nucleobase is only the letter-bearing component, not the entire nucleotide. DNA uses deoxyribose, whereas RNA uses ribose. A chain of six nucleotides is therefore not six free nucleobases placed beside one another. See NHGRI: nucleic acids.

The chemical scaffold and the variable lettering do different jobs. Covalent bonds maintain the sugar–phosphate backbone, while the order of attached nucleobases specifies the nucleic acid sequence. A phosphodiester bond links the $3^{\prime }$ position of one sugar to the $5^{\prime }$ position of the next through phosphate. These position numbers refer to carbon chemical atoms in the sugar; they define a strand's direction even when it bends or forms a circle. See Brown, Genomes, 2nd edition: the human genome.

This division is useful: the backbone supplies a connected physical record, and base pairing allows its letters to recognize partners. DNA replication copies that record, and transcription uses a DNA template to make RNA. RNA can also fold into functional shapes through base pairing within one chain; a nucleic acid sequence specifies chemical order, not a requirement for a straight shape. The beauty is that the same kind of polymer can carry a message and form a structure that acts on messages.

A nucleophile adds reversibly at the beta carbon to form an enolate, which is functionalized before elimination restores the alkene.

Nylon 6,10 has an amide-linked chain whose constitutional repeating unit contains a six-carbon diamine contribution and a ten-carbon diacid-derived contribution. See monomer for its stoichiometry and polymerization for the authored demonstration.

The octet rule is a useful empirical guide: many Lewis structures of chemical compounds containing carbon, nitrogen, and oxygen give each of those chemical atoms eight electrons when nonbonding electrons and all incident bonding electrons are counted. Hydrogen instead normally counts two electrons. These are local counts, not independent budgets: a shared bond pair counts at both endpoints but only once in the molecule's valence electron total.

In water, oxygen counts $4+4=8$ electrons; in ammonia, nitrogen counts $2+6=8$; in $\mathrm{O}=\mathrm{C}=\mathrm{O}$, carbon counts $0+8=8$ and each oxygen counts $4+4=8$. The charge-separated carbon dioxide drawing also satisfies the octet rule, so the octet rule alone cannot select the dominant contributor. Formal charge provides another check.

The octet rule is a heuristic rather than a theorem about every chemical bond. Odd valence electron totals cannot be partitioned entirely into electron pairs, and some valid Lewis structures have fewer or more than eight electrons around a chemical atom. See Chemistry 2e, 7.3: exceptions to the octet rule. The capacity of the $s$ and $p$ electron subshells, $2+6=8$, helps motivate the guide for these chemical elements, but does not prove universal compliance. Lewis discussed a stable outer group of eight, together with a two-electron arrangement for hydrogen, in The Atom and the Molecule (1916), pp. 768 and 774.