Question

What is Inert pair effect? What are Lanthanoid contraction properties?

Answer 1

In the heavier p-block elements, the pair of s electrons in the valence shell are pulled down so far in energy, because Zeff increases notably across the d-block (since d electrons are poorly shielding) that they tend not to participate in the atom's valence. So the atom acts like it just has the p electrons in its valence shell. Lead is a good example. In principle it has 4 electrons in its valence shell ([Xe]4f14,4d10,6s2,6p2) but in practice the 6s electrons aren't especially likely to participate, so Pb usually forms the Pb+2 cation and is found in the Pb(II) oxidation state. Thallium is another example: it's in the same group as aluminum, which exhibits the +3 oxidation state, having 3 electrons in its valence shell. But Tl often prefers the +1 oxidation state, because the 6s pair is too low in energy.

The lanthanide contraction happens for very similar reasons. The 4f and 5f electrons are quite poorly shielding, because they don't spend much time near the nucleus, so Zeff increases quite sharply through both lanthanide rows. Consequently the size of these atoms contracts notably:

Ba 215pm
La 195pm
Lu 175pm
Hf 155pm

Answer 2

Oh i think i got it!
Thanx @carl_pham!