Difference between formal charge and oxidation number?


Moderators: Chem_Mod, Chem_Admin

Claire_Kim_2F
Posts: 67
Joined: Wed Sep 30, 2020 10:02 pm

Difference between formal charge and oxidation number?

Postby Claire_Kim_2F » Mon Nov 16, 2020 3:27 pm

What is the difference between a formal charge and a oxidation number?

Chem_Mod
Posts: 19524
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 881 times

Re: Difference between formal charge and oxidation number?

Postby Chem_Mod » Mon Nov 16, 2020 3:31 pm

One key difference is that, when assigning oxidation numbers, you arbitrarily assign the electrons of each covalent bond to the more electronegative atom in the bond. Whereas, in formal charge, each bonding pair is divided equally between the two atoms in the bond without concerning electronegativity.

Courtney Situ 2B
Posts: 104
Joined: Wed Sep 30, 2020 10:03 pm

Re: Difference between formal charge and oxidation number?

Postby Courtney Situ 2B » Mon Nov 16, 2020 3:34 pm

Hi!

Formal charge refers to the charge that an atom would have if the electrons in their shared bonds were split evenly. Therefore, electronegativity and polarity have no bearing on the formal charge.

In comparison, oxidation number takes this electronegativity and polarity in mind when dividing the electrons. The bonded electrons are not split evenly.

The best way to illustrate the difference is to draw the lewis structure of CH4.
H
|
H-----C-----H
|
H
The formal charges of all of the atoms are 0. In comparison, the oxidation number for H is +1, and the oxidation number for O is -4. I guess an important similarity though is that the sum of the formal charges and the sum of the oxidation numbers should equal the overall charge of the compound. For example, a polyatomic ion CO3 with a charge of 2- means that the sum of the formal charges will be -2 and the sum of the oxidation numbers will also be -2.

Hope this helps!

Courtney Situ 2B
Posts: 104
Joined: Wed Sep 30, 2020 10:03 pm

Re: Difference between formal charge and oxidation number?

Postby Courtney Situ 2B » Mon Nov 16, 2020 3:35 pm

Courtney Situ 3D wrote:Hi!

Formal charge refers to the charge that an atom would have if the electrons in their shared bonds were split evenly. Therefore, electronegativity and polarity have no bearing on the formal charge.

In comparison, oxidation number takes this electronegativity and polarity in mind when dividing the electrons. The bonded electrons are not split evenly.

The best way to illustrate the difference is to draw the lewis structure of CH4.
H
|
H-----C-----H
|
H
The formal charges of all of the atoms are 0. In comparison, the oxidation number for H is +1, and the oxidation number for O is -4. I guess an important similarity though is that the sum of the formal charges and the sum of the oxidation numbers should equal the overall charge of the compound. For example, a polyatomic ion CO3 with a charge of 2- means that the sum of the formal charges will be -2 and the sum of the oxidation numbers will also be -2.

Hope this helps!


oops that lewis structure didn't translate well
.........H
.........|
H-------C--------H
.........|
.........H

Crystal Hsueh 2L
Posts: 114
Joined: Wed Sep 30, 2020 9:35 pm
Been upvoted: 1 time

Re: Difference between formal charge and oxidation number?

Postby Crystal Hsueh 2L » Mon Nov 16, 2020 3:35 pm

Formal charge is assigned to atoms using the number of bonds it has and the number of nonbonded electrons. And it is under the assumption that the electrons are equally shared, emphasizing covalent character. Oxidation numbers are assigned to the more electronegative atom and emphasizes ionic character

shevanti_kumar_1E
Posts: 90
Joined: Wed Sep 30, 2020 9:40 pm
Been upvoted: 1 time

Re: Difference between formal charge and oxidation number?

Postby shevanti_kumar_1E » Mon Nov 16, 2020 3:36 pm

Formal charge is calculated for each molecule by using the formula formal charge = (valence electrons)- (bonded electrons/2 + lone pair electrons). Formal charge of an atom indicates gain or loss of electrons while forming covalent bonds.

I don't think we've got over oxidation numbers in lecture yet but they are obtained by a certain set of rules:

1. The oxidation number of a free element is always 0.
2. The oxidation number of a monatomic ion equals the charge of the ion.
3. The oxidation number of H is +1, but it is -1 in when combined with less electronegative elements.
4. The oxidation number of O in compounds is usually -2, but it is -1 in peroxides.
5. The oxidation number of a Group 1 element in a compound is +1.
6.The oxidation number of a Group 2 element in a compound is +2.
7.The oxidation number of a Group 17 element in a binary compound is -1.
8.The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.
9.The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.

Ximeng Guo 2K
Posts: 96
Joined: Wed Sep 30, 2020 9:54 pm

Re: Difference between formal charge and oxidation number?

Postby Ximeng Guo 2K » Wed Nov 18, 2020 3:10 am

formal charge = (# of valence electrons)- (# of bonded electrons/2 + # of lone pair electrons); I think we use them to determine how stable a resonance structure of a covalent molecule is.

In other words, oxidation number is more to do with "loss and gain" of electrons when forming a compound. It's more to do with ionic characters. I think there is also one sampling HW which compares formal charge and oxidation number which you could refer to.


Return to “Formal Charge and Oxidation Numbers”

Who is online

Users browsing this forum: No registered users and 1 guest