HBr>H2S>H2Se>BH3 so basically
1. HBr
2. H2S
3. H2Se
4. BH3
H₂Se is more acidic than H₂S, and HBr is more acidic than BH₃. In a group, from top to bottom, the acidic strength increases. H₂Se is present down in the group than H₂S.
HBr is more acidic than H₂S, and HBr is more acidic than BH₃. In a period from left to right, there is an increase in the acidic strength. Between HBr and H₂S, HBr is present at the extreme right in the period.
Hence, the decreasing order of acidic strength is as follows:
HBr > H₂Se > H₂S > BH₃
