Lewis Dot Structures: Understanding Mistakes for Chemistry Tests

[umair21]

So 3 days ago I had a chemistry test and part of the test included Lewis dot structures. I counted my valance electrons and distributed them equally but I have no Idea why I got these wrong.The second one I know why I got wrong but the rest of them look fine to me. I would really appreciate it someone can tell me what I did wrong or draw it out.

 

[Qube]

Valence electrons do not need to be distributed equally. For PClF₄, P should be the central atom. The central atom is generally the most electropositive element.

For the carbon one you should remember that carbon generally forms four bonds to complete its octet. Carbon may also have a lone pair and two bonds - this being an alternate way for carbon to complete its octet - but this is relatively rare.

For NO₃^- you have a positive two formal charge on the central nitrogen and negative formal charges on all the attached oxygen atoms. A better Lewis structure would show less formal charge separation and less formal charge in general.

I should add that you should not be drawing these figures with 90 degree bond angles - i.e. with right angles. Right angles in the world of molecular geometry is relatively rare. Tetrahedral molecules such as CCl₂F₂ definitely do not have any 90 degree bond angles.

 

[Dr Uma Sharma]

PClF₄ ---In this you have taken Cl as the central atom ..that is wrong ..it should be P (most electropositive atom or least electronegative atom.EN (electronegativity) trend for these atoms are
P<Cl<F
Remember F never makes a double bond …

In CO₃2- C is the central atom and is least electronegative so negative formal charge should be on oxygen …make four bonds around carbon so that its formal charge is zero .This is done by making a double and two single bonds with three oxygens.Two oxygens which are single bonded must have three lone pairs on them with a negative formal charge .However doubly bonded oxygen is with two lone pairs and no formal charge .

In NO₃- N is the central atom and is least electronegative so negative formal charge should be on oxygen …Nitrogen can have maximum four bond with 8 electrons and a formal positive charge on it.This is done by making a double and two single bonds with three oxygens.Two oxygens which are single bonded must have three lone pairs on them with a negative formal charge .However doubly bonded oxygen is with two lone pairs and no formal charge .

So while writing the lewis dot structure ..take care of the EN of the elements bonded in the structure with their formal charge and octet rule . Also remember C ,N ,F and O can never have expanded octet.