Third period means the element contains 3 shells that is K,L,M Shells.
Group 16 means having 6 electrons in valence shell.That is Sulphur.
Molecular formula of sulphur reacting with hydrogen will be.
Sulphur's valency-2(2,8,6 so 8-6=2)
Formula will be H2S
(Interchange their valencies)
-LoVE ME :p