Which element from following lanthanoids has half-filled $$\mathrm{f}$$- orbital in observed and expected electronic configuration?

How many atoms of niobium are present in $$2.43 \mathrm{~g}$$ if it forms bcc structure with density $$9 \mathrm{~g} \mathrm{~cm}^{-3}$$ and volume of unit cell $$2.7 \times 10^{-23} \mathrm{~cm}^3$$ ?

Identify the reagent R used in following conversion.

Glucose $$\mathrm{\buildrel R \over \longrightarrow}$$ n - hexane

Buna-S is obtained from