Answer:
Beryllium
Explanation:
From the information given: Beryllium has the ability to form a monoatomic divalent metal cation. The atomic number (Z) is 4 and the mass number is 9. The atomic element of (Z-1) is Li and that of (Z-2) is He. The elements they are referring to that share the same similar biological importance within Beryllium are Magnesium and Calcium because they are in the same group.
The atomic radii of He = 140
The atomic radii of Li = 182; &
The atomic radii of Be = 153 pm