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.
