In “periodic table”, the adjective is related to the noun period, and comes from Ancient Greek περίοδος through French périodique. In “periodic acid”, it is formed from the prefix per- and iodic (like peroxide and permanganate).
Wiktionary lists their respective UK pronunciations of as /pɪə(ɹ).iˈɒdɪk/
and /ˌpɜːraɪˈɒdɪk/
, markedly different: pɪə
(as in piece) vs. pɜː
(as in perfume); then i
(as in it) vs. aɪ
(as in eye).
However, is that distinction really made in practice? Would a native US/UK/Aussie speaker make the difference when talking in the lab?