Question

The first ionization enthalpy values $\left(KJ{mol}^{-1}\right)$ of group 13 elements are;

B Al Ga In

801 577 579 558

How would you account for their deviation from the general trend?

Answer 1

• 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 an increase in size. The observed discontinuity in the ionization enthalpy values between Al and Ga, and between In and Tl are due to the inability of d- and f-electrons, which have low screening effect, to compensate the increase in nuclear charge.