Pumpkin seeds are hard to eat in quantity, and they work best over a multi-week timeframe similar to pyrantel pamoate through paralysis not direct killing.
An easier way to get good quantities down is to make pumpkin seed butter with a blendtec or similar high speed blender or buy pumpkin butter or cold pressed pumpkin seed oil. Better yet is pumpkin seed pesto with raw garlic.
Arsenic in anything higher a dose than a few seeds is one of the worst ideas ever. Trace amounts of arsenic are likely beneficial.
As shares the same column as P on the periodic table.