photohydrogen

The story

1. Proto-Indo-European*bʰeh₂-
2. Proto-Indo-European*bʰéh₂os
3. Proto-Hellenic*pʰáwos
4. Ancient Greekφᾰ́ος (phắos)
5. Ancient Greekφῶς (phôs)
6. Ancient Greekφωτο- (phōto-)
7. Englishphoto-
8. Proto-Indo-European*wed-
9. Proto-Indo-European*wódr̥
10. Proto-Hellenic*údōr
11. Ancient Greekῡ̆̔́δωρ (hū̆́dōr)
12. Ancient Greekὑδρο- (hudro-)
13. Frenchhydro-
14. Proto-Indo-European*ǵenh₁-
15. Proto-Indo-European*ǵénh₁os
16. Proto-Hellenic*génos
17. Ancient Greekγένος (génos)
18. Frenchhydrogène
19. Englishhydrogen
20. English todayphotohydrogen

From photo- + hydrogen.