The nitrogen atoms in nh3, nh2–, and nh4+ are all surrounded by eight electrons. when these three species are arranged in order of increasing h–n–h bond angle, what is the correct order?

 H-N-H bond angles : NH₂⁻ <  NH₃ <  NH₄⁺

Explanation:

The question tell us that the central nitrogen has four pairs of electron domains. Four pairs of electron domains predict a tetrahedral geometry but the molecular geometry will be affected by the presence of lone pair of electron and they are the ones that require more space because they have greater repulsion.

In NH₄⁺ all the 4 pairs are used in bonding with H, therefore the molecular geometry will be tetrahedral with H-N-H bond angles of 109 º.

In NH₃ we have one lone pair of electron which push the H-N-H angles less because of repulsion.

In NH₂⁻, there are two pairs of lone pairs of electrons which push the H-N-H even more.

So the order of increasing H-N-H  bond angle is  NH₂⁻ <  NH₃ <  NH₄⁺

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.