The electropositive character increases from B to Al and then decreases from Al to Tl because of

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Decrease in the ionization energy of the elements

Decrease in the electronegativity from boron to aluminium and then increases

Increase in the size of the elements

Effective shielding of the nuclear charge by d-electrons in the case of Ga, In and Tl

answer is C.

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Detailed Solution

As we move from boron to aluminium in the periodic table, the electronegativity decreases due to the increase in atomic size and shielding effect of inner electrons. This results in a decrease in the attraction of the nucleus for electrons, making it easier for the outermost electron to be removed, resulting in an increase in electropositivity.

However, from aluminium to thallium, the electronegativity increases due to the decrease in atomic size and incomplete shielding of the nuclear charge by d-electrons, making it more difficult to remove an electron and resulting in a decrease in electropositivity.

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