[shehri]

Hi all,

 In my text book about the Group 3 A elements it's said "The increase in the at. size is not regular in this group.This's due to the presence of d ens. in heaviour members whic've poor shielding effect than s & p ens."I'm unable to understand this shielding effect factor here.Can someone help me?Thanks.

[Dan]

What do you mean by "ens."?

Anyway, the d orbitals shield less efficiently than the s and p. As the d orbitals are filled as you walk across, say, from Sc - Zn, the effective nuclear charge (Z_eff) and atomic radius generally drop to a greater extent than they do when you mentally traverse the p block. As a result Ga, for example is smaller than you might expect.

The concept here is similar to the consequences of lanthanide contraction (look that up).

[Borek]

electrons?

[shehri]

Dear friends,

                   The elements which I'm concerned're 3A(B,Al,Ga,In,Ti) & 'ens' means electrons.U plz. explain me whether the less shielding effect of 'd & f' orbitals're  due to their  shapes in space or something else?Thanks.

[Dan]

It is to do with the radial distribution of the orbital - basically, the shape, yes.
The radial distribution functions (look these up) show that for a given principle quantum (n), the penetration of ns>np>nd>nf, and so the magnitude of shielding for ns>np>nd>nf.

ie. the f orbitals are generally speaking poor at shielding, and this is what causes the lanthanide contraction, and this is the reason why the 4d transition metals are very similar in size to the 5d.
The d orbitals are also generally speaking poor at shielding and cause a similar (although not as drastic) contraction accross the d block, which is particularly noticable when you compare Al and Ga's atomic radii (or for that matter, Si-Ge, P-As, S-Se or Cl-Br).
The effect of this is not only apparent in atomic radii. Take a look at, for example, electronegativity - and you will see an uneven or alternating pattern down group 3A. This general phenomenon is often referred to as the "alternation effect" or "alternating effect", thats probably worth looking up too.