Sources

Scientific sources — NIST, IUPAC, CODATA, PubChem

All data comes from official sources: NIST (Atomic Spectra Database, Chemistry WebBook), IUPAC 2021, CODATA and PubChem. Precision preserved, SI units, no invented values.

Curation principles

Precision preserved. When a source gives a value with 8 significant digits, we keep it in full. Rounding happens only at display time, never at the source.
SI units. Data are stored in official units (u, K, pm, kJ/mol, g/cm³, S/m, W·m⁻¹·K⁻¹…) and display conversions (°C next to K) happen at render time.
No invention. Unknown values are marked as unavailable rather than estimated. This is particularly frequent for transactinides, most of whose properties have not been measured experimentally.
Bilingual FR/EN. Editorial content is written in French and translated to English. Scientific vocabulary and chemical formulas are universal. The i18n architecture handles routing and hreflang alternates for international SEO.

Sources used

NIST — Atomic Weights & Isotopic Compositions

https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl

Used for: Standard atomic weights, isotopic masses, abundances.

Values aligned with the IUPAC 2021 tables, with the published precision (up to 9 significant digits when available).

NIST — Atomic Spectra Database

https://www.nist.gov/pml/atomic-spectra-database

Used for: Successive ionisation energies I₁..Iₙ.

Values consolidated from spectroscopic measurements referenced by the NIST ASD.

NIST Chemistry WebBook

https://webbook.nist.gov/chemistry/

Used for: Thermodynamic properties: melting and boiling points, enthalpies, specific heats.

Reported values correspond to standard phases (solid/liquid at 1 atm).

IUPAC — 2021 Recommendations

https://iupac.org/what-we-do/periodic-table-of-elements/

Used for: Group classification, transactinide nomenclature, lanthanide (57–71) and actinide (89–103) convention.

The 2021 IUPAC rules settle in particular the composition of the f-block and the official names of the latest transactinides.

CODATA — Fundamental physical constants

https://codata.org/en/committees-and-groups/task-group-on-fundamental-constants

Used for: Fundamental constants used for unit conversions.

CODATA 2018 (the latest safely usable recommended values until the next revision).

PubChem

https://pubchem.ncbi.nlm.nih.gov/

Used for: Compound identifiers: CID, CAS, InChI, SMILES, molar masses. IUPAC names and compound synonyms.

Used across all compound pages. The 3D structures shown are currently hand-coded from standard geometries, not parsed from PubChem SDF files — a future iteration will automate this.

Royal Society of Chemistry

https://www.rsc.org/periodic-table

Used for: Editorial content (uses, history, etymology).

Used as a secondary reference to cross-check non-numerical information. The text on element and family pages is originally written for this site.

An error? A suggestion?

If you spot an incorrect value or a more recent source, contributions are welcome — this project is open source and curation lives on community feedback.