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Here are the approximate bond angles for first three members of group15.

$\ce{NH3}$: $107°$

$\ce{PH3}$: $94°$

$\ce{AsH3}$: $92°$

As we move down the group the bond angles approach towards $90°$. I don't understand why there is such a major deviation from the standard angle of $109°28'$.

In the case of $\ce{NH3}$, the deviation is justified because of the presence of lone pair(lp-bp repulsions) , but for others, this much deviation is quite unexpected.

What causes this large deviation in angle?


marked as duplicate by Ben Norris, Todd Minehardt, pentavalentcarbon, airhuff, paracetamol Jun 20 '17 at 14:09

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  • $\begingroup$ In the same direction ($\ce{N -> P -> As}$), not only the atoms include more nucleons and electrons (increase of mass), but the atomic radii increase, too ($65, 100, \pu{115 pm}$) which contributes to the reason. (consulted source: en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(data_page)) $\endgroup$ – Buttonwood Jun 20 '17 at 11:53
  • $\begingroup$ Also incorrect is the assumption that VSEPR works in general. $\endgroup$ – Zhe Jun 20 '17 at 13:43