The decreasing values of bond angle from 107º in NH3 to 91º in SbH3 down the group 15 elements of the periodic table owes to the decreasing electronegativity of the elements down the group.
The bond angle is the angle between any two bonds to a single atom.
The hydrides of group 15 elements form hydrides of the sort AH3 where A is the particular group 15 element.
Electronegativity is a periodic trend that decreases down the group and increases across the period.
As we move down group 15, atoms of elements become less electronegative and as such, repulsion between lone pair and bond pair and the repulsion between two bond pairs decreases. Ultimately, the bond angles of the hydrides decrease also.
This accounts for the decreasing values of bond angle from 107º in NH3 to 91º in SbH3 down the group 15 elements of the periodic table.
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