interatomic

The story

1. Proto-Indo-European*h₁én
2. Proto-Indo-European*h₁n̥tér
3. Proto-Italic*n̥ter
4. Latininter
5. Latininter-
6. Englishinter-
7. Proto-Indo-European*né
8. Proto-Indo-European*n̥-
9. Proto-Hellenic*ə-
10. Ancient Greekἀ- (a-)
11. Proto-Indo-European*temh₁-
12. Ancient Greekτέμνω (témnō)
13. Ancient Greek*τομός (*tomós)
14. Ancient Greekἄτομος (átomos)
15. Latinatomu
16. Middle Frenchathome
17. Middle Englishattome
18. Englishatom
19. Englishatomic
20. English todayinteratomic

From inter- + atomic.