[xstrae]

The first ionisation enthalpies of group 13 elements in kj/mol are as follows :

B        Al        Ga        In        Tl
801   577      579      558     589

How would you explain this deviation from the general trend??
Ionisation enthalpies generally decrease down a group. However Ga and Tl seem to be exceptions. Why is this so?

[Yggdrasil]

It has to do with the fact that electrons in d- and f- orbitals are poorly shielding.  Therefore, when the d-orbitals are filling from Sc to Zn, the effective nuclear charge actually increases as you proceed from left to right on the periodic table.  The same goes with filling the f-orbitals from La to Lu (for the lanthanides, this concept is known as the Lanthanide contraction).

[xiankai]

It has to do with the fact that electrons in d- and f- orbitals are poorly shielding.

is it in anyway also related to the coulomb's inverse square law too? just wondering...

[xstrae]

yes nuclear charge definitely increases from Sc to Zn and La to Lu. not just because of poor shielding but decrease in nucelar size as more electrons are added to the same shell.
i am sorry but i dont see how that matters here. isnt the valence electron added to the 4p orbital in case of Ga and 5p in case of In? so why does the poor shielding of d and f orbitals affect the first ionisation enthalpy of these elements??

and also why is that only Ga and Tl show this deviation in behaviour? In beloning to the same group behaves as expected (its first ionisation enthalpy is considerably lower than that of Ga).

can you please explain?

[tamim83]

As Yggdrasil has mentioned, d and f orbitals are far less shieling than s and p orbitals.  You would think that they shield better, for example for scanadium, you fill the 4s then the 3d orbitals and 3d orbitals are closer to the nucleus since they have a lower principle quantum number.  But d- orbitals are not perfect shielders due to where their electron densities are, take a look at pictures of them and you will see what I mean.  So, once they are filled up and you begin putting an electron in p-orbitals, the effective nuclear charge (this charge takes sheilding from other electron orbitals into account) actually increases due to the d orbitals inperfection in shielding.  Since the 3d orbitals are introduced in period four, you see this trend breaker when you hit Galium.  For the same reason you see this again in period six for thalium when you introduce f orbitals. 

This is a pretty complicated explanation, I actually consulted with my research advisor before answering your question and this is what he told me pretty much.  There are alot of other factors that contribute to this, like relativity and I don't wish to go into it since I don't really get it myself.  So, simply put, when d and f orbitals get filled, their shielding effects do not do a great job in sheilding the increase in effective nuclear charge, so you get an break in the downward IE trend in, not only group 3A but also the group 2B metals too. 

Hope this helped, even in the tiniest bit 

[Yggdrasil]

yes nuclear charge definitely increases from Sc to Zn and La to Lu. not just because of poor shielding but decrease in nucelar size as more electrons are added to the same shell.
i am sorry but i dont see how that matters here. isnt the valence electron added to the 4p orbital in case of Ga and 5p in case of In? so why does the poor shielding of d and f orbitals affect the first ionisation enthalpy of these elements??

and also why is that only Ga and Tl show this deviation in behaviour? In beloning to the same group behaves as expected (its first ionisation enthalpy is considerably lower than that of Ga).

can you please explain?

p and s orbitals can shield well, so when you increase atomic number from say Ga to Ge, you add a proton, but the additional charge from that proton is more than shielded by the addition of a 4p electron.  So from Ga to Kr, effective nuclear charge decreases as you move to the right.

[xstrae]

ok thanks tamim and yggdrasil. one more question though.. why is this behaviour not applicable to In which lies just below Ga and above Tl ?

[tamim83]

why is this behaviour not applicable to In which lies just below Ga and above Tl ?

Well, you really only see trend deviations when something er "dramatic" happens (like a new allowable angular momentum number).  SInce nothing like that happens between galium and indium, the trend just continues, that is the IE of indium is indeed lower than Galium. 

[xstrae]

thats confusing. i thought In too will behave out of the normal because it is poorly shielded by the 4d orbitals. so shouldnt the effective nuclear charge be increasing again? so i thought it will have a greater IE than Ga. Please correct me where i am wrong. thanks for your time.

[Yggdrasil]

While In does have 4d electrons, the 3d electrons in Ga have the same effect on the atomic radius and ionization energy.