16. Reactivity towards halogen: Group 15 elements form trihalides and pentahalides.
Trihalides – covalent compounds and become ionic down the group.
sp 3 hybridisation, pyramidal shape Pentahalides- sp 3 d hybridisation, TBP shape
They are lewis acids because of the presence of vacant d – orbitals.
PCl5 + Cl- → [ PCl6 ]-
PCl5 is ionic in solid state and exist as [ PCl4 ] + [ PCl6 ]-
In PCl5, there are three equatorial bonds and two axial bonds. The axial bonds are longer than equatorial bonds because of greater repulsion from equatorial bonds.
Nitrogen does not form pentahalides due to absence of d- orbitals.
17. Reactivity towards metals: All elements react with metals to form binary compounds in – 3 oxidation state.
18. Anomalous behaviour of nitrogen: The behaviour of nitrogen differs from rest of the elements.
Reason:
i. It has a small size.
ii. It does not have d – orbitals iii. It has high electronegativity
iv. It has high ionization enthalpy