Question

What is lanthanoid contraction? write the cause and consequences of it?

Answer 1

The effect results from poor shielding  of nuclear charge (nuclear attractive force on electrons) by 4f electrons; the 6s electrons are drawn towards the nucleus, thus resulting in a smaller atomic radius.
lanthanide contraction is the greater-than-expected decrease in ionic radii of theelements in the lanthanide series from atomic number 57, lanthanum, to 71, lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements starting with 72, hafnium.