Answer 1

1) In the Lanthanoid series with increasing atomic number, the atomic and ionic radii decrease from one element to another but a decrease is very small.

2) Increase in nuclear charge in a period increases force of attraction, towards outer electrons. Hence
atomic size or radius decreases

3) Filling of the electrons in the inner shells increases atomic size as inner electrons repel outer electrons. The shielding effect of electrons decreases in the order s > p > d > f

4) The new electrons are added to the same inner 4f - orbital. Hence Lanthanoids contraction is due to

• poor shielding effect due to 4f electrons
• Increase in nuclear charge.
• Non-introduction of new outer shells.