Why is `Tl^(+1)` is more stable than `Tl^(+3)`?

Why PbX_2 is more stable than PbX_4 ?.

Why H_(2) is more stable than H_(2)^(+) while He_(2)^(+) is more stable than He_(2) ?

Why is a ._(2)^(4)He nucleus more stable than a ._(3)^(4)Li nucles ?

Assertion: CHCl_(3) is more acidic than CHF_(3) . Reason: The conjugate base of CHCl_(3) is more stable than CHF_(3) .

Why arre Mn^(2+) compounds more stable than Fe^(2+) toward oxidation to their +3 state?