I think it's important to remember that the first bomb(s) Iran would have is likely only to be a fission (atomic) bomb, not a boosted fission or fusion (hydrogen) bomb.
However, it is the mentality of the people who control Iran that is more dangerous than the other nuclear nations.
Only USA, Britain, France, Russia, and China have thermonuclear (hydrogen) bombs. Israel has boosted fission bombs. India and Pakistan have fission (atomic) bombs.
Iran does not have the technology, nor anywhere near enough HEU to completely destroy Israel. They are far more likely to use a bomb similar to the one used by USA in Hiroshima which, compared to a thermonuclear bomb, is pretty feeble.
However, it is common knowledge that by casing the bomb in certain metals (for example, gold) it can greatly intensify the neutron radiation, which would probably end up killing far more people than the flash or the blast wave.
Also, Iran does not have the tactical knowledge of atomic warfare. It is unlikely that they will have the technology or knowledge to explode a bomb in such a way to make a precursor shock wave (which would obliterate nearly everything in its path).
In fact, in reality, they are far more likely to blow themselves up than anyone else.