Name the elements of group $15$. What is their valence shell electronic configuration?

Write the shapes of the hydrides of group $15$ elements. Arrange them in order of decreasing basic strength, increasing bond angle  and decreasing reducing character.

Account for the following observation:

${\mathrm{NH}}_3$ has higher boiling point than ${\mathrm{PH}}_3$

Account for the following observation:

Nitrogen $\left({\mathrm{N}}_2\right)$ is a fairly inert gas.

Account for the following observation:

Nitrogen is a gas but phosphorous is a solid.

Account for the following observation:

Nitrogen forms trihalides and not pentahalide.

Complete the following reaction:

$({\mathrm{NH}}_4{)}_2{\mathrm{Cr}}_2{\mathrm{O}}_7 \stackrel{\mathrm{Heat}}{\to }$