Imagine a star 30 times thr mass of the sun made of iron. Explode that star into an expanding sphere of iron atoms and by the time it reaches a radius of 1 light year, I bet you would not have any two iron atoms within an AU of each other. Not to mention that thc atoms would be going .99 times the speed of light and the escape velocity of a single atom of iron aint that much.
Let's do the math. The amount of iron ejecta from a Type II supernova isn't known for certain, but one rough estimate claims it to be about half the mass of our own sun, or about 1030 kg. One light year is about 1013 km. The volume of a sphere of that size (4/3 pi r2) is about 1026 cubic kilometers. Divide that by the mass of the ejecta (1030 kg) and you get 10,000 kg of iron per cubic kilometer.
This of course assumes a uniform distribution. Actual supernovas produce complex wavefronts (see the Crab Nebula) where the local density might be might higher.
Anyway, at 10,000 kg per cubic km you get about 10 micrograms per cubic meter. The atomic weight of iron is 55g/mol. Multiply by Avogadro's number (6.02 x 1023 atoms/mol) and you get 100,000,000,000,000,000 atoms per cubic meter.
That's a teeny weeny bit larger than 1 atom per AU.