The first ionization enthalpy values (in kJ mol–1) of group 13 elements are: B=801, Al=577, Ga=579, In=558, Tl=589, how would you explain this deviation from the general trend?
Answers
When going down a group in periodic table, the ionization enthalpies usually decrease due to an increase in the atomic size and screening effect, which is more than compensating for the effect of an increase in nuclear charge. Consequently, the electron is kept less tightly.
In the case of Ga, 10 d electrons are in its electronic configuration, since the d electrons shield the “nuclear charge” less efficiently than the s & p electrons, the “outer electron” is strongly held by the nucleus.
As a result, the ionization enthalpy increases slightly as we move from “Al to Ga”, despite the “increase” in “atomic size”. The “similar increase” is also observed from “In to Tl”, which has a poor shielding effect due to the presence of “14f electrons” in “electronic configuration” of “Tl”."
