a
\[Na+N{{H}_{3}}\left( g \right)\xrightarrow{\Delta }\underset{\left[ X \right]}{\mathop{NaN{{H}_{2}}}}\,\xrightarrow{{{N}_{2}}O}\underset{\left[ Y \right]}{\mathop{Na{{N}_{3}}}}\,\xrightarrow{\Delta }\underset{\left[ Z \right]}{\mathop{{{N}_{2}}}}\,\uparrow \]