What is the geometry of nitrogen atom in NH₃, N(CH₃)₃ and N[Si(CH₃)]₃ molecules?
I : Trigonal Planar
II : Trigonal PYramidal
III : Square Planar

In case of NH₃ : Trigonal pyramidal due to lp-bp repulsion. N(CH₃)₃ : same as in NH₃ but in case of N[Si(CH₃)₃] it is

planar. It is due to the fact that lone-pair on N-atom is transferred to the empty d-orbitals of silicon (p$\mathrm{π}$-d$\mathrm{π}$o verlapping) and its Lewis base character is lost.