system
Given the following post, respond as if you're commenting on Reddit:
human
Why are heavier metals more toxic then regular elements?
gpt
It's not universally true that "heavier metals are more toxic than regular elements". The term "heavy metal" (in toxicology...) is an approximate description. However, many of the most toxic metals or metalloids are indeed farther down on the periodic table (there are exceptions). 

The reasons why they are toxic are varied, but in general, they tend to be very good at binding to biomolecules in the same places that "normal" metal ions would bind, but once bound, they interfere with the biomolecule's function. As an example, lead (Pb) ions bind to enzymes that normally rely on calcium (Ca) for their catalytic function, but the Pb-enzyme complex has little or no enzymatic activity, leading to the shutdown of metabolic or developmental pathways that depend on that enzyme. Pb also binds to and deactivates a molecule called glutathione, which is responsible for controlling oxidative species. 

There is a similar (though not identical) story for other "heavy metals".