Question

What is the structure of `"Si"_3"N"_4`?

Answer 1

I would expect that the three silicon atoms are each in between a pair of nitrogen atoms. Silicon can make two double bonds, or one triple bond and one single bond.

Here is what this looks like:

There are two lone valence electrons on the lewis structure at each nitrogen, and no lone valence electrons on each silicon.

The `"N"-="Si"-"N"` equilibrium geometry is linear (similar to acetylene), the `"Si"-"N"="Si"` equilibrium geometry is bent (due to the lone pair on nitrogen creating three bonding groups), and the `"N"="Si"="N"` equilibrium geometry is linear (similar to carbon dioxide).