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
/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?