The difference between a nuclear weapon detonation and a conventional explosion is?

Answer 1

The source of the energy. In nuclear weapons it comes from the nucleus of the atoms, in conventional weapons it comes from the electrons orbiting the atoms.

Or, another way to say it is that nuclear weapons depend on release of sub-atomic energy while conventional explosives rely on chemical energy. In a conventional explosive, the energy comes from breaking the bonds of complex multi-atom molecules, taking one complex molecule and turning it into several smaller, simple molecules. In a nuclear weapon, the bonds holding individual neutrons and protons together inside a single atom's nucleus are broken or changed, resulting in a whole new atom (or several new atoms). Nuclear fission takes a single, large atom and breaks it into two smaller atoms (plus several neutrons). Nuclear fusion takes two very small atoms and creates a slightly larger atom (plus a free neutron or proton).

The differences is that the bonds between atoms in a molecule are much, much weaker than the bonds between subatomic particles. Several thousand times, in fact, so breaking just one sub-atomic bond results in the same amount of energy released as from breaking thousands of molecular bonds.

Besides the difference in energy source, a chemical weapon really only produces two effects: a blast wave and a thermal wave. A nuclear weapon, however, produces four effects: blast wave, thermal wave, "pure" radiation (gamma/X-Rays, etc., plus the associated EMP), and radioactive by-products.