What makes a H bomb more destructive than an A bomb?

Answer 1

it is a 2 (or more) stage device, the first stage is an A bomb while the remaining stage(s) contain fusion fuel. typically lithium deuteride.

the amount of fission fuel (uranium or plutonium) that can be put in an A bomb is inherently limited. if you put too much in the bomb it will spontaneously begin to fission (possibly even during assembly or storage, definitely long before use and delivery to a target is possible) causing the bomb to heat up, melt, and produce a low yield fizzle explosion; producing mostly contamination not damage. it has been computed that the limit to the yield of an A bomb is about 1 megaton, the highest yield A bomb ever tested had a yield of 500 kilotons.

the amount of fusion fuel that can be put in an H bomb has no such limit, fusion cannot begin spontaneously. also the yield can be raised by using as many stages as you want. also the fusion reaction produces many very high energy neutrons, which are capable of fissioning depleted uranium (which is not fissionable under ordinary conditions), if the outer casing/tamper of an H bomb is made of depleted uranium it is possible to raise the yield of the H bomb by up to a factor of 10. the first H bomb tested had a yield of 10 megatons. the highest yield H bomb tested had a yield of just over 50 megatons, but had it used the uranium casing/tamper it was designed for the yield was predicted to be 100 megatons.