— Why is lead Neurotoxic?
It seems lead has the ability to replace divalent cations like Calcium and Zinc (divalent, 2 charge — cation 2+ charge) ions in the Protein Kinase C Enzymes , which is definitely not good since they help out with a ton of things…
…metal transport, energy metabolism, apoptosis, ionic conduction, cell adhesion, inter- and intracellular signaling, diverse enzymatic processes, protein maturation, and genetic regulation. Membrane ionic channels and signaling molecules seem to be one of the most relevant molecular targets contributing to lead’s neurotoxicity; the developing central nervous system is particularly susceptible. 
— Why are heavy metals Neurotoxic?
So the characteristic that makes them neurotoxic, or just toxic in general, is because they have that ability to replace those divalent ions. If you look at this periodic table, from my understanding of the Inert Pair Effect, any metal that is stable in a 2+ config will be able to replace those cations. But for Lead, it can do this because it has the 2+ charge AND it is able to out-compete the Calcium or Zinc because it can bind more strongly to the proteins (Fig.1)
Because of its larger ionic radius…and greater electronegativity (in the Pauling scale)…
The metal is allowed to interact with the proteins that Calcium would have normally, but because there is the Inert Pair present, the interactions with the proteins are changed. In this case, the lead is not able to interact with a water molecule (like the Calcium is). The protein is inactive because the electron pair pushes the protein residues away in a different configuration.
So stars can go supernova because it was large enough to begin with and when it runs out of hydrogen, it starts moving up the periodic table fusing heavier and heavier elements. Once it started making Nickel-56, it would decay into Iron-56 and this is the end. At this point it no longer gets energy from nuclear fusion and the stars gravity starts to win the fight, meaning it collapses.
This is where all of the other heavier elements in the universe come from and is the R (rapid)-Process. The S-Process is different in that it only captures one neutron before beta-decay takes place. But it was recently found from Harvard-Smithsonian that Earth’s gold and “substantial amounts of heavy elements” came from colliding Neutron stars. That just makes the phrase “we are all star stuff’ even cooler.