Thallium(I) sulfate
Thallium sulfate, like all thallium compounds, is very toxic. This is because the size of the thallium ion is very similar to the size of the potassium ion. After it goes into the cells, it cannot be used like potassium is used and so kills the cell. It also gets absorbed through the skin. Eating more than 1/2 gram can kill. It gets deposited in the kidney, liver, brain, and similar places. It has been blamed for the extinction of the Brown Fish Owl.