The first ionization enthalpy values (in kJmol–1) of group 13 elements are B Al Ga In Tl





Q.19:- The first ionization enthalpy values (in kJmol–1) of group 13 elements are :

B

Al

Ga

In

Tl

801

577

579

558

589

How would you explain this deviation from the general trend?

 

Answer:- 
The decrease in ∆iH1 value from B to Al is due to the bigger size of Al.
In Ga there is 10 3d electrons which do not screen as is done by S and P electrons. Therefore, there is an unexpected increase in the magnitude of effective nuclear charge resulting in increased ∆iH1 values. The same is with into Tl. The later has fourteen ∆f electrons with very poor shielding effect. This also increases, the effective nuclear charge thus the value of ∆iH1 increases.