cyclopentaarsine

The story

1. Proto-Indo-European*kʷelh₁-redup.
2. Proto-Indo-European*kʷékʷlos
3. Proto-Hellenic*kʷókʷlos
4. Proto-Hellenic*kúklos
5. Ancient Greekκύκλος (kúklos)
6. Late Latincyclus
7. Middle Englishcicle
8. Englishcycle
9. Ancient Greekκύκλος (kúklos)
10. Ancient Greekκυκλῐκός (kuklĭkós)
11. Classical Latincyclicu
12. Middle Frenchcyclique
13. Frenchcyclique
14. Classical Latincyclicus
15. Englishcyclic
16. Englishcyclo-
17. Proto-Indo-European*pénkʷe
18. Proto-Hellenic*pénkʷe
19. Ancient Greekπέντε (pénte)
20. Ancient Greekπεντᾰ- (pentă-)
21. Englishpenta-
22. Proto-Indo-European*ǵʰelh₃-
23. Proto-Indo-European*ǵʰl̥h₃enyom
24. Proto-Indo-Iranian*ȷ́ʰr̥Hanyam
25. Proto-Iranian*jr̥Hanyam
26. Old Median*zaranyam
27. Old Median*kā
28. Old Median*zaraniyakā
29. Middle Persian*zarnīk
30. Classical Syriacܙܪܢܝܟܐ (zarnīḵā)
31. Arabicالزَرْنِيخ (az-zarnīḵ)
32. Medieval Greekἀρσενικόν (arsenikón)
33. Latinarsenicum
34. Middle Frencharsenic
35. Middle Englisharsenik
36. Englisharsenic
37. Englisharsine
38. English todaycyclopentaarsine

From cyclo- + penta- + arsine.