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The ionization enthalpy for elements along a group generally reduces. But there is an exception for group 13 elements and the order is not uniform.

The order is: B>Tl>Ga>Al>In

According to my textbook, the reason for this is the weak screening effect of electrons in d and f orbitals. What does this mean?

The following is from the textbook i referred:

The ionization enthalpy values as expected from the general trends do not decrease smoothly down the group. The decrease from B to Al is associated with increase in size. The observed discontinuity in the ionization enthalpy values between Al and Ga, and between In and Tl are due to inability of d- and f-electrons ,which have low screening effect, to compensate the increase in nuclear charge.

Source

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As we know radius is inversely proportional to I.E but in group 13 there is exception due to poor screening effect/sheilding effect . Means d &f orbital has poor screening effect due to which nucleus take advantage and attract outermost valance electron due to which size decrease.

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