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.



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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