After a discussion about the structures of hydrides of the group – 1

1.	After a discussion about the structures of hydrides of the group – 15 elements, Neethu wrote the order of bond angles as NH3 < PH3 < AsH3a) is this the correct order?b) Justify your answer.c) Give the hybridization and shape of these hydrides.d) Also arrange the above hydrides in the increasing order of their thermal stability. Justify your answer.
Answer» a) No. b) From top to bottom in the group the bond angle of group 15 hydrides decreases. As the electronegativity of the central atom decreases on moving down the group, the bond pair-bond pair repulsion decreases. Hence the bond angle decreases ¡n the order NH₃> PH₃ > AsH₃. C) In all hydrides the central atom is sp³ hybridised. The molecules assume trigonal pyramidal geometry with a lone pair on the central atom. d) Thermal stability of the group 15 hydrides increases BiH₃ < AsH₃ < PH₃ < NH₃ This is due to the fact that moving up the group the EH bond dissociation enthalpy (‘E’ is a group 15 element) increases due to a decrease in size of the central atom and the molecules will decompose only at higher temperatures.

After a discussion about the structures of hydrides of the group – 15 elements, Neethu wrote the order of bond angles as NH3 < PH3 < AsH3a) is this the correct order?b) Justify your answer.c) Give the hybridization and shape of these hydrides.d) Also arrange the above hydrides in the increasing order of their thermal stability. Justify your answer.

Answer»

a) No.
b) From top to bottom in the group the bond angle of group 15 hydrides decreases. As the electronegativity of the central atom decreases on moving down the group, the bond pair-bond pair repulsion decreases. Hence the bond angle decreases ¡n the order NH₃> PH₃ > AsH₃.

C) In all hydrides the central atom is sp³ hybridised. The molecules assume trigonal pyramidal geometry with a lone pair on the central atom.

d) Thermal stability of the group 15 hydrides increases BiH₃ < AsH₃ < PH₃ < NH₃ This is due to the fact that moving up the group the EH bond dissociation enthalpy (‘E’ is a group 15 element) increases due to a decrease in size of the central atom and the molecules will decompose only at higher temperatures.

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