Question

the arsenic substitution failed partly because the arsenate compounds were not stable and arsenic is toxic. what is a reason (other than valence electrons) why a chemically similar element might still fail to substitute for an essential element in living organisms?

Explanation:

In living organisms, elements need to be compatible with biological processes. Even if elements are chemically similar, differences in atomic radius can lead to improper fit in biological molecules. For example, arsenic has a larger atomic radius than phosphorus (a common essential element). This size difference can disrupt the structure and function of biomolecules (like enzymes, nucleic acids) that are specifically adapted to interact with the smaller - sized essential element. Also, differences in bond strengths (arsenic - containing bonds might be weaker or stronger in an inappropriate way compared to the bonds of the essential element) can affect the stability and reactivity of biological compounds.

Answer:

Differences in atomic radius and bond strengths compared to the essential element.