(a)Zr and Hf have almost identical radii : Give reason.
(b)Name the gas liberated when Lanthanoids (Ln) react wtih acids.

Explanation

(a)Poor shielding of 4f electrons decreases the size of Hf due to lanthanoid contraction. Hence it maintain the size equiavlent to Zr.
(b)With water lanthanoids gives hydroxides with the liberation of hydrogen.
`6Ln + 6H_(2)O to 2 Ln(OH)_(3) + 3H_(2)`
With dil acid they liberate hydrogen
`Ln + dil acid to H_(2)` gas.