Formal Charges (Chemistry)

[Pcac]

Can someone please explain how to calculate the formal charges of the atoms of the following substances?
a) KCN

b) NaOH

c) (CH3)4NF
 

I have found 0 as the answer to all of them, but the answer of the book doesn’t match!!
It is not a homework, just an exercise. 

Edited March 20, 2020 by Pcac

[hypervalent_iodine]

Formal charges of the atoms is not the same as the net charge of the molecule. For example, H₂O has a net charge of zero, but the formal charges for the atoms are +1 (hydrogen) and -2 (oxygen). 

[Pcac]

15 hours ago, Pcac said:

8 hours ago, hypervalent_iodine said:

Formal charges of the atoms is not the same as the net charge of the molecule. For example, H₂O has a net charge of zero, but the formal charges for the atoms are +1 (hydrogen) and -2 (oxygen). 

8 hours ago, hypervalent_iodine said:

Formal charges of the atoms is not the same as the net charge of the molecule. For example, H₂O has a net charge of zero, but the formal charges for the atoms are +1 (hydrogen) and -2 (oxygen). 

isn’t that the oxidation number?

 

 

[hypervalent_iodine]

Lord, yes, you’re very right and that was a very stupid mistake on my part. Working from home has fried my brain. 
 

To your question, you need to consider that the molecules are all salts. Have you constructed molecular diagrams for the molecules to know how many bonds and lone pairs they have?

[studiot]

Here is a good article with two ways to calculate the formal charge, one without lone pairs.

 

I can't do any better than this.

 

https://www.masterorganicchemistry.com/2010/09/24/how-to-calculate-formal-charge/

Edited March 21, 2020 by studiot

[Pcac]

13 hours ago, hypervalent_iodine said:

Lord, yes, you’re very right and that was a very stupid mistake on my part. Working from home has fried my brain. 
 

To your question, you need to consider that the molecules are all salts. Have you constructed molecular diagrams for the molecules to know how many bonds and lone pairs they have?

Yes, I could understand how to solve the first two questions! Thank you very much. However, I am still struggling with the letter “c”. How can the fluor make an ionic bond with the nitrogen if those elements are both non metals and the disparity of electronegativity between them is very small?
Here is what I have done to calculate the formal charge:

(FC = Valence Electrons - Unbonded Electrons - 1/2 Electrons from the bonds)

 

Edited March 22, 2020 by Pcac

[hypervalent_iodine]

Nitrogen can’t accommodate 5 covalent bonds. It can and does form ammonium salts, with the halogen acting as a counter ion.

[BabcockHall]

Something to bear in mind is that formal charges and oxidation numbers are both tools to help us keep track of electrons and charges.  They make different assumptions about which atom owns the electrons.