[eason123deng]

If i have a molecule, say BNH6, the nitrogen has a formal positive charge whereas the boron has a formal negative charge.

1)What are the implications/effects when nitrogen has a formal positive charge? What does it mean to have a formal positive charge?
My thought: Does it mean that the nitrogen-boron bond is more polar (because N has formal positive charge, N will attracts the electron clouds in the nitrogen-boron bond toward itself. thus making the bond more polar than expected (the 'expected' one is calculated based on EN difference)). Is my thought correct? Are there other implications of formal charge??

2) Another question: How and why would formal postively charged N attracts the electron cloud toward itself? please explain in detail

3) Since N is formal positively charged, does it mean that it really behaves as if it were positively charged (for instance, Will formal positively charged N attracts electron clouds better than 'normal, neutral' N?). please explain in detail

4) Why smaller formal charge better structure? please explain in detail
Thank you for your help 

[SirRoderick]

The reason why N gets a positive charge and B a negative one is due to the amount of electrons in the bonds. B usually only has 3 electrons, in that structure it has 4, hence the neg. charge. Vice Versa for N. It's a formal charge and so doesn't reflect EN value.

I'm not sure what exactly you mean by 2 and 3.

As for 4, the more charges are in a certain structure, the more unstable it becomes. If you have two possible structures, one of which has a pos. and a neg. charge and the other no charge, then the last is more stable.

[Honclbrif]

Formal charges can be useful but you have to be careful with them because as their name suggests they are in reality a formalism. Another way of saying this is that they are a tool we have created to explain certain observations, i.e they are imaginary.

First of all, formal charges tend to ignore electronegativity. In your example, boron has an electronegativity of about 2, and nitrogen has an electronegativity of about 3. The electron density should be greatest around the nitogen, giving it a partial negative charge. Yet the properly drawn structure gives it a positive formal charge. Another example would be the BF₄^- anion. Boron has a negative formal charge in this structure, yet fluorine has twice the electronegativity, and the electron density should be greatest around fluorine, which has a formal charge of 0.

When you run into situations like this where things break down, I recommend considering oxidation states instead. In the BF₄^- case, boron has an oxidation number of +3, and, and the fluorines an oxidation number of -1. Makes a lot more sense.

[Honclbrif]

This is not to say that formal charge is all bunk. It describes many things quite well. Furthermore, oxidation numbers are a formalism also. The truth, as it so often does, lies somewhere MO theory but since we don't have all day to crank through DFT calculations, we write ourselves these convenient fictions. The key is to understand what their limitations are, and how they are best applied.

[SirRoderick]

This is not to say that formal charge is all bunk. It describes many things quite well. Furthermore, oxidation numbers are a formalism also. The truth, as it so often does, lies somewhere MO theory but since we don't have all day to crank through DFT calculations, we write ourselves these convenient fictions. The key is to understand what their limitations are, and how they are best applied.

+1 on this.

[eason123deng]

1) so, does the formal charge play a role in making a bond more polar? For instance, in the case of BNH6, would the B-N bond be more polar than the expected one.

2) Formal charge always tell us the electron distribution, right?(a positive formal charge means electron deficient site, +ve formal charge implies electron rich site, right?).

3) Since N(in BNH6) has a formal positive charge, does N really behaves as if it were positively charged (for instance, if N in moleculue A has formal positive charge and another nitrogen in molecule B has 0 formal charge, Will N attracts bonding electrons better than 'normal, neutral' nitrogen?). please explain in detail

4) What is the role of formal charge (other than telling which lewis dot structure is better)?

[eason123deng]

Sorry,another question regarding the stability of a molecule.

If i have a charged molecule, why would some people say 'if the charge is spread out, then the molecule is stable'? what does it mean? Why would a charged molecule has less potential energy if the excess electrons (i.e the charge) is delocalized?

please explain in great detail, thank you

[SirRoderick]

Sorry,another question regarding the stability of a molecule.

If i have a charged molecule, why would some people say 'if the charge is spread out, then the molecule is stable'? what does it mean? Why would a charged molecule has less potential energy if the excess electrons (i.e the charge) is delocalized?

please explain in great detail, thank you

If the two charges are located further away from each other, they interact less and this leads to a more stable molecule. This still is less stable than if there were no charges at all.

[orgopete]

1) Formal charges are for electronic bookkeeping. Protons are positive and electrons negative. That doesn't change.

2) If we look at the electrons of boron, they are held less tightly than C,N,O, or F. Because it is planar, the inner shell of electron cannot shield the nuclear proton field of +5, therefore it can attract electrons from ammonia. Upon addition, the electrons surrounding boron are held even less tightly while those of nitrogen are held more tightly.

3) No, the charge of the nitrogen doesn't change. Because it is surrounded by four pairs of electrons, it is difficult for another pair of electrons to reach it. Because the electrons are pulled more tightly toward the nitrogen, by the boron pulling them toward it. The protons of the nitrogen are more easily removed. This is exactly the same as NH₄⁺ being more acidic than NH₃ or H₃O⁺ being more acidic than H₂O.

4) Not necessarily true. HCl + H2O  H₃O⁺ + Cl^-
If smaller charge were preferred, the reaction arrow should be right to left.

[SirRoderick]

4) Not necessarily true. HCl + H2O  right arrow H3O+ + Cl-
If smaller charge were preferred, the reaction arrow should be right to left.

On 4) I have to say that isn't really the point. IN THE SAME basic structure less charge = more stability.

[Honclbrif]

4) Not necessarily true. HCl + H2O  right arrow H3O+ + Cl-
If smaller charge were preferred, the reaction arrow should be right to left.

On 4) I have to say that isn't really the point. IN THE SAME basic structure less charge = more stability.

How do you explain zwitterions such as free amino acids at neutral pH?

[SirRoderick]

Below pH 2.2, the predominant  amino acid form will have a neutral carboxylic acid group and a positive α-ammonium ion (net charge +1), and above pH 9.4, a negative carboxylate and neutral α-amino group (net charge -1).

So there. the other structures are not neutral.

[orgopete]

If i have a molecule, say BNH6, the nitrogen has a formal positive charge whereas the boron has a formal negative charge.

4) Why smaller formal charge better structure? please explain in detail

4) Not necessarily true. HCl + H2O  right arrow H3O+ + Cl-
If smaller charge were preferred, the reaction arrow should be right to left.

On 4) I have to say that isn't really the point. IN THE SAME basic structure less charge = more stability.

Am I missing something? The chlorine in HCl is uncharged and in Cl^-, it is charged. If a smaller formal charge is better, shouldn't HCl be more stable than Cl^- (which is true in a unimolecular or gaseous state)?

I had believed the poster was generalizing about formal charges and that smaller charges are preferred. I was simply pointing out that this generalization is not necessarily true, hence the example. Honclbrif gave another. The reason I suggested this is that if a smaller formal charge is preferred, then one would incorrectly predict that Cl^- is a base and HCl is not an acid.

I actually thought I did a reasonable job of providing an answer which the poster was seeking ("please explain in detail").