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Showing 241–270 of 1709 reference entries

Physics · Earth & cosmosNuclear binding energy

A bound nucleus has less rest mass than its separated nucleons.

ModelPhysics · Earth & cosmosExponential radioactive decay

A fixed decay probability per time gives a population timescale.

ModelPhysics · Earth & cosmosFission, fusion and reaction energy

Nuclear rearrangement can release or require energy.

ModelPhysicsScattering cross section

Normalize a scattering rate by incident flux.

ModelPhysicsStandard Model matter families

Quarks and leptons form three generations.

ModelPhysicsStandard Model interactions

Gauge fields mediate three interactions; the Higgs is a scalar sector.

ModelPhysicsInteraction-specific quantum-number balances

A balance rule must name the interaction where it applies.

ModelPhysics · ChemistryBloch states and electronic bands

A periodic crystal potential organizes energies by wavevector and band.

ModelPhysics · ChemistrySemiconductor electrons and holes

Band occupancy and doping control mobile carriers.

ModelPhysicsBose and Fermi thermal occupations

Indistinguishable particles have different equilibrium state populations.

ModelChemistryElectron screening and effective nuclear attraction

Other electrons modify the attraction experienced by an electron in an atom.

ModelChemistryMultielectron ground-state configurations

Orbital occupations connect atomic electronic structure with valence chemistry.

ModelChemistryLocalized bonds and resonance descriptions

Localized orbital overlap and resonance are complementary chemical descriptions.

ModelChemistryMolecular orbitals and bonding occupation

A molecular orbital description distributes electronic states across a molecule.

ModelChemistryIntermolecular interactions

Dispersion, dipole interactions and hydrogen bonding influence molecular materials.

ModelChemistryBonding classes of solids

Ionic, metallic, molecular and network solids organize common bonding patterns.

DefinitionChemistryPolymers and macromolecular architecture

Polymers are substances composed of macromolecules with recurring structural units.

DefinitionChemistryCoordination compounds and ligands

Ligand donor atoms surround a central metal in a coordination sphere.

DefinitionChemistryActivities and chemical potentials

Activity expresses composition relative to a specified thermodynamic standard state.

DefinitionChemistrypH and hydrogen-ion activity

pH is an activity-based quantity realized through measurement conventions.

DefinitionChemistryBuffers and finite buffering capacity

An acid–base pair can resist limited changes in solution pH.

ModelChemistrySolubility products and precipitation

An equilibrium product constrains dissolution of a specified solid phase.

ModelChemistryReference electrodes and redox potentials

Electrode potentials are specified relative to a reference half-cell.

DefinitionChemistryThe Nernst equilibrium relation

Reversible electrochemical potential depends on activities and temperature.

ModelChemistryElementary steps and reaction mechanisms

A mechanism proposes steps that account for an overall chemical transformation.

ModelChemistryArrhenius temperature dependence

Many rate constants follow an exponential temperature model over a limited range.

ApproximationChemistryHess's law and thermochemical accounting

Reaction enthalpies can be combined because enthalpy is a state function.

ModelChemistryPhase diagrams and coexistence

Phase diagrams describe equilibrium states under stated conditions.

ModelChemistryCalibration, standards and matrix effects

Quantitative analysis needs a validated relation between response and analyte amount.

MethodChemistryChromatographic retention and separation

Differential interactions with mobile and stationary phases separate mixture components.

Method
ModelSource checked

Elementary steps and reaction mechanisms

Chemistry

A mechanism proposes steps that account for an overall chemical transformation.

Key knowledge

A reaction mechanism is a proposed sequence of elementary steps whose sum yields an overall reaction. For an elementary mass-action step, molecularity determines concentration factors in its rate law. The overall rate law usually requires analysis of the full mechanism.

Conditions and scope

  • An overall balanced equation is not automatically an elementary step.
  • Agreement with stoichiometry and a rate law is necessary evidence, but may not uniquely identify a mechanism.
Sources · 1

References

Sources retain their recorded scope. A source link alone does not establish verification of every statement.

  1. Chemistry 2e, 12.6: Reaction MechanismsPaul Flowers, Klaus Theopold, Richard Langley and William R. Robinson — OpenStax, Rice UniversityTextbook chapteropenstax.orgRead source

Evidence and review

Source elementary-step rate laws and requirements for a proposed mechanism.

2026-10-10 · Checked elementary versus overall scope without a universal rate-determining-step shortcut.

Related reference entries

Explore relationships within this area of study and its connections to other areas.

Showing 1–5 of 5 relationships

Scientific relationships · 5 / 5

The relationship type describes the connection. Its linked statements, sources and conditions define the scope.

  1. Related to
    Relationship sources · 1

    Linked statements

    • A reaction mechanism is a proposed sequence of elementary steps whose sum yields an overall reaction. For an elementary mass-action step, molecularity determines concentration factors in its rate law. The overall rate law usually requires analysis of the full mechanism.

    Proposed elementary steps must be consistent with kinetic evidence.

  2. Related to
    Relationship sources · 1

    Linked statements

    • In polar-mechanism notation, a full-headed curved arrow starts at an electron pair or bond and ends where that pair forms a bond or becomes a lone pair. Nucleophiles donate electron pairs and electrophiles accept them in the described step.

    Pair arrows describe proposed polar steps, rather than just a net transformation.

  3. Related to
    Relationship sources · 1

    Linked statements

    • For the simple mechanism E + S ⇌ ES → E + P, a quasi-steady-state approximation gives a saturating initial-rate relation. The Michaelis constant is the substrate concentration at half the model's maximum rate; it includes binding and turnover rate constants.

    The rate model is derived from a specific enzyme-binding and turnover mechanism.

  4. Related to
    Relationship sources · 2

    Linked statements

    • A low-concentration intermediate can have approximately equal formation and consumption rates after a short transient, allowing its rate equation to be eliminated from a mechanism. A pre-equilibrium approximation instead assumes a fast reversible step maintains its equilibrium ratio before slower conversion. These assumptions are distinct and require compatible timescales.

    Approximate rate analysis applies to a stated elementary sequence.

  5. Related to
    Relationship sources · 1

    Linked statements

    • A radical chain has initiation steps that generate radicals, propagation steps that transform reactants while regenerating a radical carrier, and termination steps that consume carriers. Homolytic bond changes use single-electron bookkeeping. Relative propagation rates and termination influence yield and selectivity.

    A chain is a sequence of elementary steps.

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