Electropositive character for the elements of group 13 follows the order

Solution :
As we move from $$B$$ to $$Al,$$  the sum of $${\Delta _i}{H_1} + {\Delta _i}{H_2} + {\Delta _i}{H_3}$$     decreases substantially $$\left( {6887\,kJ\,mo{l^{ - 1}}\,{\text{to}}\,5137\,kJ\,mo{l^{ - 1}}} \right)$$       due to increase in the atomic size and hence $$Al$$  has a high tendency to lose electrons. Since the electrode potentials $$E_{\frac{{{M^{3 + }}}}{M}}^ \circ $$  increase from $$Al$$  to $$Tl,$$  therefore, their electropositive character decreases, i.e., $$Al\left( { - 1.66\,V} \right)$$   to $$Ga\left( { - 0.56\,V} \right)$$   to $$In\left( { - 0.34\,V} \right)$$   to $$Tl\left( { + 1.26\,V} \right)$$   accordingly.