Actually the process you are talking about is called fusion not fission. The compacting of material until it combines on a molecular level is nuclear fusion.
fission is the spiting of the molecules which is a entirely different process and is in essence "ignited"
You need to go read up on A-Bombs 101. Fission bombs involve driving chunks of appropriate materials together with explosives, in order to achieve critical mass and a fission reaction. H-bombs typically use a heavy-hydrogen core, inside of an A-bomb, to actually fuse nuclei and create an even bigger explosion.
Actually, the process is fission. The fissionable material has to be forced together in a critical mass quick enough for the fission to be an explosive release of energy. Fusion is the forcing of hydrogen atoms together to create helium and the release of energy.
No. The "compacting" remark I made referred to the style of atomic weapon used on Nagasaki, known as "Fatman". This type of weapon contains a hollow sphere of fissionable material, which is crushed at detonation by symmetrically placed explosive charges into a lump which thereby achieves critical mass. The resulting mass fissions (the atomic nuclei split) which produces the energy of the blast.
Fusion is the process of combining light atomic nuclei (as opposed to the heavy elements which release energy during fission) to release energy. It produces more energy than fission, but in weapons requires a fission "igniter" to heat the fusible material to the required temperature (hundreds of millions of degrees, or about 10,000 times hotter than the surface of the sun). This temperature, not compaction, is what makes the fusion weapon work.
I hope that helped. :-)
Actually, "ignition" (in the sense of "starting a fire") is a chemical reaction. Both fusion and fission bombs involve compaction and both are nuclear reactions. In that sense, referring to the triggering explosion as "igniting" the reaction is equally erroneous (if you want to be technical about the chemical vs. nuclear reaction) or correct (if you want to be figurative about the explosion being the impetus for starting the nuclear reaction) for both fusion and fission bombs.