What very poisonous metalloid is often used in rat poison?

"One old fashioned poison that is safe is cheap and easy to make yourself is to mix equal parts of corn meal and plaster of Paris and place it in the rat holes. The plaster of Paris hardens in the stomach of the rat causing death."

& trust me this one always work.

Or I Just found a recipe for home-made pet friendly Rat and Mouse killer on a website from Tennessee as follows :

1 cup of flour

1 cup of sugar

1 cup of baking soda ( Bicarbonate of soda)

Mix together and place in small bowls where you notice rodent traffic.

(The bicarb will react with stomach acid and produce carbon dioxide gas, obviously in excess to create the swelling. Apparently, most of these vermin and creatures cannot pass wind, hence the internal combustion. )