Oh, wow! “The bomb” means it’s excellent. “A bomb” means it’s really bad — a complete failure. Isn’t English crazy? These are slang expressions, and to be honest, I never say “the bomb”, and rarely say “a bomb”. Actually, “it bombed” sounds a little better to me. Maybe I’m just not cool :)

"the bomb"? Americans. Just avoid it if you can.

Thank you, Maryam. My actual question is on the below. Could you tell me the difference between these 2 examples?

I think it depends on the context you could say for example A-bomb explode or the bomb was diffuse right before it went off.