Atomic structure questions

Some interesting questions in the unit atomic structure

Q. Nitrogen does not form NCl5 but Phosphorous forms PCl5 Explain?

Atomic number of nitrogen is 7 therefore the valence shell electrons are in the principle quantum number 2 – 1s22s22px1=2py1=2pz1 This limits the number of orbitals in the valence shell to 2s and 2p however in P since the valence shell has the potential of having 3 orbitals 3s, 3p and an unfilled 3d. This makes it possible for the 3s electron to be excited and promoted to the 3d level thereby opening the possibility for hybridization involving the sp3 and d orbital leading to the sp3d hybridized orbital which has a trigonal bipyrimidal shape. The equatorial orbitals are trigonal with a bond angle of 120 degrees and the axial orbitals have a bond angle of 90 degrees with the equatorial plane.

The bondangles helps us in easily explaining the easy conversion of PCl5 to PCl3 too……

Q Oxygen is a diatomic stable molecule, however valence bond theory and the octet rule is not able to explain its paramagnetic behavior. So how can you explain this situation?

VBT fails here however the MO theory by Hund and Mullikan can be used to explain this observation.

Q Using exchange energy explain the stable electronic configuration of elements like Cr, Mo, W, Cu, Ag etc.