How these pages are checked
Two sources, nothing else
Every looked-up number on this site — an atomic weight, a molecular weight, an electronegativity value — comes from either NIST's Atomic Weights and Isotopic Compositions table or PubChem's compound and element pages. Nothing is taken from a textbook, a forum answer or another chemistry site.
What is looked up, and what is calculated
Each page marks a number one of two ways. A plain number with a link is quoted directly from NIST or PubChem. A number wrapped like this is worked out by this site from those quoted numbers — an atom-by-atom molar mass, a bond angle read off the VSEPR rules, or an electronegativity difference — and is not an independent lookup, so it carries no separate citation.
The build-time check
Every quoted number is registered in this site's source code together with the short phrases that should surround it in the archived copy of the page it came from. Before the site builds, a script reads that archived copy and confirms every phrase appears in it, in order, close enough together that it could not be two different numbers stitched together. If a phrase is missing or a number has been changed without updating this record, the build stops and nothing is published. The same check also re-derives every calculated number from the quoted ones and stops the build if they do not match, which catches a source number being edited without the numbers that depend on it being updated too.
We proved this check actually catches bad data before trusting it: a deliberately wrong copy of the source data was built, the build failed, the change was reverted, and the build passed again, byte-identical to before.
VSEPR shapes and bond angles
The molecular shape and the labelled bond angle on each page are not looked up from a source; they follow directly from VSEPR theory applied to the Lewis structure on that page, and are marked as calculated for that reason. Where the real, measured bond angle in a bent molecule differs from the idealized geometric angle because lone pairs repel more than bonding pairs do, the page says so in words without quoting a specific measured degree value, since that value is not one of the two sources this site uses.
Sources used so far
NIST's full atomic-weight table carries a compilation notice under 15 U.S.C. §290e; this site quotes only the rows for the elements its published pages use, each on the page that needs it, not the table as a whole.
A 50-item cross-check against a textbook
Before launch, 50 statements from the molecule pages written so far (CO2 and H2O, plus ten more waiting to be published one a day) were checked against Chapter 7 of OpenStax Chemistry 2e as hosted on LibreTexts (sections 7.3 and 7.6 and the end-of-chapter exercises), together with the chapter 7 answer key on openstax.org. The book is published under CC BY 4.0. The statements covered valence-electron totals, single, double and triple bonds, lone pairs on the central atom, electron-domain geometry, molecular shape, idealized bond angle and polarity.
35 of the 50 match a sentence in the book that names that molecule, for example the textbook's own count of 10 valence electrons for HCN or its answer that CS2 has two regions of electron density and a linear shape. The other 15 are not worked for that molecule in the book; for those we checked that the page follows a general rule the book states, such as "Two of the electron regions are lone pairs, so the molecular structure is bent." All 50 agreed; none needed a correction. Two kinds of fact were left out because the book gives them only in figures, not in text: the valence-electron counts of chlorine and sulfur.
The textbook is used only as a cross-check. No number on a page is taken from it; looked-up numbers still come only from NIST and PubChem. The list of 50 items, with the exact textbook wording for each, is part of the site's source code, and the build stops if any of that wording is missing from the archived chapter pages or the matching sentence disappears from a molecule page.