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Showing 1591–1620 of 1709 reference entries

PhysicsSpecular and diffuse reflection

Reflection angles are measured from the local normal.

ModelPhysicsTotal internal reflection and evanescent fields

Total reflected flux does not mean zero field beyond the interface.

ModelPhysicsSpherical-mirror images and signs

A virtual image is formed by ray extensions, not actual ray convergence.

ModelPhysicsPhotoelectric threshold and stopping potential

Photon frequency sets maximum photoelectron energy in the single-photon model.

ModelPhysicsDe Broglie wavelength and particle momentum

A momentum state has a spatial wave phase.

ModelMeasurementQuantity kind and physical dimension

Equal dimensions do not make quantities interchangeable.

DefinitionMeasurementQuantity value, numerical value and unit

A numerical value depends on the chosen unit.

DefinitionMeasurementCoherent derived units

Coherence means no extra numerical factor in the base-unit product.

DefinitionMeasurementOrdinal quantities and nominal properties

Ordering and category labels do not justify ordinary arithmetic.

DefinitionMeasurementMeasurement principle, method and procedure

A physical phenomenon, an approach and a detailed procedure have different roles.

DefinitionMeasurementMeasuring instrument and measuring system

A complete measurement system can include more than one device.

DefinitionMeasurementSensor, transducer and detector

Component names describe different measurement roles.

DefinitionMeasurementInstrument indication and measurement result

A displayed reading is only one part of a measurement result.

DefinitionMeasurementAdjustment, calibration and zero adjustment

Changing an instrument response differs from determining its calibration.

DefinitionMeasurementReference and conventional quantity values

An accepted comparison value need not be an exact true value.

DefinitionMeasurementIndication interval, measuring interval and span

Possible readings differ from a validated interval of measurement.

DefinitionMeasurementRated operating conditions

Instrument properties are specified within a domain of use.

DefinitionMeasurement · ChemistrySelectivity of a measuring system

Responding to a target differs from excluding interference.

DefinitionMeasurementDiscrimination threshold and dead band

An undetected input change differs from an interval with no output change.

DefinitionMeasurementStability of a measuring instrument

Time stability differs from short-term repeatability.

DefinitionMeasurementMaximum permissible error and accuracy class

A specification limit is not an uncertainty estimate.

DefinitionMeasurementDefinitional uncertainty

More repetitions do not resolve an underspecified measurand.

DefinitionMeasurementRelative standard uncertainty

Relative uncertainty uses a nonzero measured magnitude.

DefinitionMeasurementBlank indications and null measurement uncertainty

A zero or blank reading can still have uncertainty.

DefinitionMeasurementTarget measurement uncertainty

An intended-use target differs from achieved uncertainty.

DefinitionMeasurementIntermediate precision conditions

Within-location changed conditions differ from strict repeatability.

DefinitionMeasurementCommutability of a reference material

Certification alone does not establish equivalence across procedures.

DefinitionChemistrySubstances, elements, compounds and mixtures

Chemical composition and spatial uniformity describe different features of matter.

DefinitionChemistryPhysical change and chemical change

A change of phase need not change chemical identity.

DefinitionChemistryEmpirical, molecular and structural formulas

A formula's subscripts represent different information in different chemical models.

Definition
DefinitionSource checked

Empirical, molecular and structural formulas

Chemistry

A formula's subscripts represent different information in different chemical models.

Key knowledge

An empirical formula gives the simplest whole-number ratio of constituent atoms. A molecular formula gives their counts in a specified molecule; different structures may share it. A structural formula also represents connectivity. For an extended ionic solid, a formula unit expresses a stoichiometric ratio rather than claiming that the crystal consists of discrete molecules.

Conditions and scope

  • Specify whether the entity is a molecule, ion or extended solid.
  • A molecular formula alone does not specify bonding, stereochemistry or every isotopic composition; nonstoichiometric solids need a composition range.
Sources · 2

References

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

  1. Paul Flowers, Klaus Theopold, Richard Langley and William R. Robinson · OpenStax/Rice University2 sources
    1. Chemistry 2e (2019), §2.4 Chemical formulasPaul Flowers, Klaus Theopold, Richard Langley and William R. Robinson · OpenStax/Rice UniversityTextbook chapteropenstax.orgRead source
    2. Chemistry 2e (2019), §3.1 Formula mass and the mole conceptPaul Flowers, Klaus Theopold, Richard Langley and William R. Robinson · OpenStax/Rice UniversityTextbook chapteropenstax.orgRead source

Evidence and review

Inspected §2.4 molecular/structural/empirical formula definitions and §3.1 ionic formula-unit discussion. Excludes §2.4's illustration calling crystalline TiO2 a molecule; the record explicitly separates extended solids from molecules.

2026-10-10 · Independent Mathematics and Information editor (/root/core_math_information). Independent AI editorial review of this entry's statements, equations, applicability conditions and source coverage. Source checks apply to these claims within their stated limits.

Related reference entries

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

Showing 1–3 of 3 relationships

Scientific relationships · 3 / 3

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

  1. Related to
    Relationship sources · 2

    Linked statements

    • An empirical formula gives the simplest whole-number ratio of constituent atoms. A molecular formula gives their counts in a specified molecule; different structures may share it. A structural formula also represents connectivity. For an extended ionic solid, a formula unit expresses a stoichiometric ratio rather than claiming that the crystal consists of discrete molecules.

    Balanced reaction accounting uses formulas for the specified molecular or formula-unit entities, not a formula as a complete structural description.

  2. Related to
    Relationship sources · 2

    Linked statements

    • An empirical formula gives the simplest whole-number ratio of constituent atoms. A molecular formula gives their counts in a specified molecule; different structures may share it. A structural formula also represents connectivity. For an extended ionic solid, a formula unit expresses a stoichiometric ratio rather than claiming that the crystal consists of discrete molecules.

    A structural representation conveys connectivity/electron accounting beyond the composition shown by a molecular formula.

  3. Uses definition
    Relationship sources · 1

    Linked statements

    • For a pure compound of fixed composition, convert each constituent element's measured mass to amount using its appropriate molar mass. Divide those amounts by a common reference amount and identify a smallest whole-number atom ratio consistent with the measurements. This gives an empirical formula, not atom connectivity. For a molecular compound with independently known molar mass, the ratio to the empirical formula-unit molar mass gives the positive integer multiplier for molecular subscripts, within measurement uncertainty.

    Empirical ratios and molecular atom counts remain distinct from a structural connectivity model.

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