Which of the two ions `Ca^(2+) " or " Zn^(2+)` is more stable and why ?

Which of the two ions, Ca^(2+) and Zn^(2+) is more stable? Why? For ionic substances which is the proper term to use 'Formula weight' or "Molecular weight'?

Which one is more stable and why?

Which of two : O_2N < H_2CH_2O or CH_3CH_2O^(-) is more stable and why?

Why Fe^3+ is more stable than Fe^2+ ?

(i) State Hund's rule. (ii) Between two ions Fe^(2+) and Fe^(3+) , which one is more stable and why?

Why PbX_2 is more stable than PbX_4 ?.