Fluorine is extremely rare in biological systems. (I was going to say “never” but I looked it up and apparently there are a couple of exotic compounds that have it.) However, fluorine is a component of many man-made drugs and poisons. Halogens are generally not incorporated directly into bio-molecules (with exceptions, the chief one being iodine in thyroid hormones) but chlorine plays an essential role in all living things as a free, negatively charged ion.

Some heavier, metallic elements in the form of ions are necessary for the function of many enzymes, but biological systems can’t work with chemically bound metals the way that human technology can. I looked up what the heaviest element with a biological role is and the answer is apparently tungsten (although I’ve never come across an enzyme incorporating tungsten during the course of my work) but even heavier metals can act as poisons by taking the place of lighter, catalytically active metals in enzymes.

It can be fun to look at the Wikipedia article of some weird element that never seems to be mentioned and see what strange uses humans have actually found for it.