Standard atomic weights of the elements 2021 (IUPAC Technical Report) — IUPAC, 1960. — ISSN 0033-4545; 1365-3075; 0074-3925 — doi:10.1515/PAC-2019-0603<a href='https://wikidata.org/wiki/Track:Q33438'></a><a href='https://wikidata.org/wiki/Track:Q2425378'></a>