You're probably thinking of arsenic, which used to be used in various pesticides. However, all industrialized nations have stopped arsenic use for this purpose, as the collateral damage is way too serious.
So, today, no "element" makes up rat poison.
Barium is used for rat poison.
bromadiolone is found in rat poisons
Barium is used as a rat poison.
iodine
Arsenic
Bromine
arsenic
Arsnic and Warfrin
arsenic
arsenic
Arsenic is used in rat poison and is in the title "Arsenic and Old Lace."
Arsenic is a common rat poison often found in mystery titles. Arsenic and Old Lace