Answer to Question #105162 in General Chemistry for Kenrick Lee

Question

Why does it make sense that both nitrogen and phosphorus form compounds with hydrogen in the same 1:3 ratio (i.e., NH3 and PH3, respectively)? Based on your answer, what would you predict for the formula for the molecule formed when arsenic (As) reacts with hydrogen?

Answer 1

First of all, let us look on the electron structure of nitrogen and phosphorous atoms:

N - 1s²2s²2p³;

P - 1s²2s²2p⁶3s²3p³3d⁰.

Thus, only three electrons are missed on the outer (valency) shell of the both atoms. This means that it would take them out from the electropositive monovalent atom like hydrogen. In other words, the valency of phosphorous as well as nitrogen is equal to III.

As for the Arsenic, the situation would be simply the same. Its electron structure is 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p³. It would be three valent atom and its compound with hydrogen would also 1 to 3 composition or AsH₃.

PS. You cannot produce it directly from As and H₂ and it is quite unstable.