What is p nuclei?

The p nuclei are the stable isotopes on the proton-rich side of the valley of stability, beyond the iron peak, that neither the s nor the r process can make: they are shielded from the r process by a stable isobar of lower $Z$ and lie off the s path. They are ascribed to a separate p process, probably in supernova explosions, and with few exceptions their abundances are much smaller than those of the neighbouring s and r isotopes.