Electronic structure is really the KEY to chemical behavior.
It is the configuration of valence electrons that define the characteristic observations that we call "chemical reactions".
Related to that are masses themselves, as heavy elements may accumulate in biological tissues instead of being purged out of the body by normal metabolism processes.
Both lead and arsenic are heavy metals. Their accumulation can lead to disruption of essential biological functions. Arsenic's reactivity makes it directly poisonous in relatively small amounts.