lanthanide contraction across the period....plz helpp

cracked_doc · Junior Member Joined: 01 May 2007 Posts: 7

I cant understand lanthanide contraction....as we go along the period if the number of electrons increase in the f-subshell, then the shielding effect should be more pronounced..instead they say that the shielding effect is reduced and hence the nucleus can attract the outer shelll electrons more strongly and ultimately the size decreases...but WHY the addition of the electrons in f-subshell across the period reduce the shielding effect??

Hix3r · Regular Joined: 24 Apr 2007 Posts: 43 Location: Hungary

Don't try to think of the shielding effect as a complex attribution of an atom. The shielding effect is an attribution of a subshell. The s subshell has the best shielding attributions and the f subshell has the worst.

So try to think of it this way: electrons in the f subshell do not shield the increased nuclear charge as much as an s subshell would. And because of this the nuclear charge can affect electrons on outer layers. This I think is because of the shape of the subshell. You know s subshell is a sphere, p is three dumbell shaped things, etc. So they start out with a sphere covering the nucleus. Three dumbells can't really cover a spheric nucleus that well so the shielding effect is reduces. And as you move along the shape covers less of the still spheric nucleus, therefore allowing the nuclear charge to have its effect on outer electrons.

cracked_doc · Junior Member Joined: 01 May 2007 Posts: 7

thanks...that really cleared the concept Smile

Novin · Probably a bot. Joined: 20 Apr 2008 Posts: 1

searching goolge for 15 minutes trying to find a good answer for this lanthanide contraction and this response really cleared up the concept. makes perfect sense now. thanks for the indirect answer!

-Kris

cracked_doc · Junior Member Joined: 01 May 2007 Posts: 7