pentaarsenic

The story

1. Proto-Indo-European*pénkʷe
2. Proto-Hellenic*pénkʷe
3. Ancient Greekπέντε (pénte)
4. Ancient Greekπεντᾰ- (pentă-)
5. Englishpenta-
6. Proto-Indo-European*ǵʰelh₃-
7. Proto-Indo-European*ǵʰl̥h₃enyom
8. Proto-Indo-Iranian*ȷ́ʰr̥Hanyam
9. Proto-Iranian*jr̥Hanyam
10. Old Median*zaranyam
11. Old Median*kā
12. Old Median*zaraniyakā
13. Middle Persian*zarnīk
14. Classical Syriacܙܪܢܝܟܐ (zarnīḵā)
15. Arabicالزَرْنِيخ (az-zarnīḵ)
16. Medieval Greekἀρσενικόν (arsenikón)
17. Latinarsenicum
18. Middle Frencharsenic
19. Middle Englisharsenik
20. Englisharsenic
21. English todaypentaarsenic

From penta- + arsenic.