Formula for lithium nitride

alingy1
We have to give the formula for lithium nitride.

I know the answer is Li3N. But, I do not understand why it is not Li2N2. I drew the lewis structure and there is a way to arrange that molecule. Why don't we specify with the prefixes the number of atoms trilithium nitride?

In a same way, why does CO2 exist, but not C2O4. I know about oxalate. But, why does it need 2- charge when it could form a molecule without those two other electrons?

I feel like I am forgetting a fundamental concept. Please help me. I feel like 2+2=/=4 anymore :S

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alingy1
I'm also wondering about F2, Cl2 and O2.

Do we call them difluorine, dichlorine and dioxygen? I never heard that. But, how do we then distinguish oxygen (O) from O2?

Staff Emeritus
Homework Helper
Well, CO is carbon MONoxide and CO2 is carbon DIoxide.

H2O could be called DIhydrogen MONoxide, but 'water' is just fine.

C has a valence of 4. Oxygen has a valence of 2. Ordinarily, burning carbon would produce CO2. When carbon is burned where there is insufficient oxygen, CO is formed instead of CO2. Carbon is capable of forming compounds with single, double, or triple bonds. Carbon monoxide has a triple bond, so its structure is: C $\equiv$ O

Lithium has a valence of 1, while nitrogen can have multiple valences (3 is a common valence for nitrogen). It takes 3 Li atoms and 1 nitrogen atom to make lithium nitride, Li3N. Li2N2 doesn't form because the valences are out of whack [+2 for the 2 lithium atoms, -6 for the 2 nitrogen atoms].

The valence of different atoms usually controls how many of each form a given compound. Perhaps this is the chemical principle you are overlooking.

Staff Emeritus