Arsenic is AS, Phosphorus is P. They're directly above/below one another in the periodic table, which means they have the same number of valence (bonding) electrons.. so in many ways, they're very similar, though Arsenic is a heavier element. Yes, other elements could also be replaced... one that's been hypothesized for a long time (but not found in the wild yet) is replacing Carbon (C) with Silicon (Si). Like arsenic and phosphorus, they're also directly above/below one another in the table. I would not at all be surprised to see that come up at some point as well.
