Why is the formula for lithium nitride Li3N and not Li2N2?

[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

 

[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?

 

[SteamKing]

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.

 

[SteamKing]

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?

All elemental gases (except for the noble gases) form molecules in the pure state. These atoms do this so they can share valence electrons. So F2 is a molecule of fluorine gas, Cl2 is a molecule of chlorine gas, and O2 is a molecule of oxygen gas. A single O, for instance, is just an oxygen atom.

 

[Borek]

Welcome to chemistry

Just because you can draw a Lewis structure (most likely linking N=N) doesn't mean such a molecule will exist. There is no simple rule that will help here - other than "apart from carbon, most elements don't create chains in typical molecules". But there are plenty of counterexamples, so even this rule doesn't have to work always.

Seriously, problem is, at the beginning you are trying to learn several simplified rules that will help you deal with the most basic compounds. These rules have to be simplified, but as they are simplified, they work only for a small subset of compounds. And the world around is freaking complicated, it doesn't want to be described by simple rules. Even complicating these rules doesn't help much. The most general rules are those saying something around the lines "this is a way atoms arrange to obtain minimum energy". We even know methods to calculate this minimum energy, but it is not something that you can do with a pen and paper (unless for an isolated hydrogen atom), you need fast computer and sophisticated software for that. And even then sometimes the reality shows the middle finger - which only makes our efforts to understand what is going on more interesting!