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?
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?
On moving down a group,
ionization enthalpy generally decreases due to an increase in the
atomic size and shielding. Thus, on moving down group 13, ionization
enthalpy decreases from B to Al. But, Ga has higher ionization
enthalpy than Al. Al follows immediately after s–block
elements, whereas Ga follows after d–block elements. The
shielding provided by d-electrons is not very effective. These
electrons do not shield the valence electrons very effectively. As a
result, the valence electrons of Ga experience a greater effective
nuclear charge than those of Al. Further, moving from Ga to In, the
ionization enthalpy decreases due to an increase in the atomic size
and shielding. But, on moving from In to Tl, the ionization enthalpy
again increases. In the periodic table, Tl follows after 4f
and 5d electrons. The shielding provided by the electrons in
both these orbitals is not very effective. Therefore, the valence
electron is held quite strongly by the nucleus. Hence, the ionization
energy of Tl is on the higher side.
On moving down a group, ionization enthalpy generally decreases due to an increase in the atomic size and shielding. Thus, on moving down group 13, ionization enthalpy decreases from B to Al. But, Ga has higher ionization enthalpy than Al. Al follows immediately after s–block elements, whereas Ga follows after d–block elements. The shielding provided by d-electrons is not very effective. These electrons do not shield the valence electrons very effectively. As a result, the valence electrons of Ga experience a greater effective nuclear charge than those of Al. Further, moving from Ga to In, the ionization enthalpy decreases due to an increase in the atomic size and shielding. But, on moving from In to Tl, the ionization enthalpy again increases. In the periodic table, Tl follows after 4f and 5d electrons. The shielding provided by the electrons in both these orbitals is not very effective. Therefore, the valence electron is held quite strongly by the nucleus. Hence, the ionization energy of Tl is on the higher side.
