When do you check for stability in a Lewis dot structure?

My professor gave an example of Ca2+ N3- Ca3N2 and we have to draw a lewis structure for that.
The number of v. electrons all in all is 19 (correct me if I'm wrong) and how will it look like and how will each element have 8 electrons each if u have an odd number?

Question

When do you check for stability in a Lewis dot structure? 

My professor gave an example of Ca2+ N3-  > Ca3N2 and we have to draw a lewis structure for that. 
The number of v. electrons all in all is 19 (correct me if I'm wrong) and how will it look like and how will each element have 8 electrons each if u have an odd number?

jfraser · Answer

you won't have 19 valence electrons in $Ca_3N_2$

vheah · Answer

My bad, it should be 16 shouldn't it?

jfraser · Answer

each calcium $atom$ has 2 valence electrons, and each nitrogen \(atom) has 5

jfraser · Answer

there will be 16