Q.What is the action of a. Sodium on arsenic? b. Magnesium on bismuth?

Reactions of Group 15 Elements with Metals

The Group 15 (nitrogen family) elements — N, P, As, Sb, Bi — show a common oxidation state of −3 when they combine with more electropositive metals. Because they are three electrons short of a noble-gas configuration, they accept electrons from active metals to form ionic or partly-ionic binary compounds: nitrides, phosphides, arsenides, antimonides and bismuthides.

General reaction with a metal M:

Concrete examples:

• Sodium heated with arsenic gives sodium arsenide: $3Na + As \longrightarrow Na_3As$.
• Magnesium burns in nitrogen to give magnesium nitride: $3Mg + N_2 \longrightarrow Mg_3N_2$.
• Magnesium with bismuth gives magnesium bismuthide: $3Mg + 2Bi \longrightarrow Mg_3Bi_2$.
• Calcium with phosphorus gives calcium phosphide: $3Ca + 2P \longrightarrow Ca_3P_2$. …