#formal charge

[nandlalgupta2009]

[organiclearning-blog]

Formal Charge (FC): associated with any atom that does not exhibit the appropriate number of valence electrons. This means that the valence electrons (VE) are not what they normally are. +VE means that electrons were donated and -VE means that electrons were attained from another atom.  FC of atoms are a useful for electron book keeping and helpful in determining reactivity of a compound. If you struggle to determine formal charge you can draw the lewis dot structure which can help you visualize it better. With enough practice one can determine Formal charge at a glance.

In neutral compounds, there can be individual atoms with charges. The molecule remains neural when the overall charge is 0. This means that the charges canceled each other out.

Charges are useful for insight on or predicting reactivity.

Unless you have a metal involved, the FC will either be neutral or +1/-1 

There are two equations you can use to determine FC. This equations include:

 FC = (Group Number) - (#electrons in Covalent bonds) - (#LP)

Group Number is the number of VE in the neutral atom

FC = (Group Number) - (#VE around atom in molecule)

This is the equation used above

[kcthechemist]

Chemistry Majors Be Like

[actively enjoying drawing resonance structures]

[negative formal charge on a carbon]

All in Unison: Eeeewwwww…

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[organic-chemistry-notes]

Using Formal Charge To Determine Major And Minor Contributors (Among Other Things)

Now that we know how to determine the Formal Charge, we can decide what is a Major Contributor and what is a Minor Contributor when it comes to structure.

The Major Resonance Contributor is the most stable resonance structure that exists for a molecule. The other resonance structures will be Minor Contributors.

In general, the Major Resonance Contributor:

has octets on as many atoms as possible

has reduced charge separation on the molecule (formal charges of 0 are best)

has negative charges on the more electronegative elements

has positive charges on the less electronegative elements

places the charge of carbo-cations on the more substituted carbon atom

places the charge of the carbo-anions on the less substituted carbon atom

  Knowing this, the first thing you are going to want to do is look for something that does not have Formal Charges. If only one resonance structure has a Formal Charge of 0, that is the resonance structure that will be your Major Contributor, and the rest will be Minor Contributors.

If there is more than one resonance structure with a Formal Charge of 0, then you will have to look at the other bulleted points and determine which meets the criteria of a stable resonance structure best. This will be your Major Contributor. The rest will be Minor Contributors.

[organic-chemistry-notes]

Finding Formal Charge ( To Determine Major and Minor Contributors)

So now that you know what major and minor contributors are, let's show how to calculate which one of them any given resonance structure is. The first part of doing this is determining the formal charge.

Formal charge = group number of the atom - (lone pair electrons + (1/2) Bonding electrons)

What this basically means is that you go to the periodic table and find whatever atom you are trying to find the formal charge of in the molecule. How many valence electrons does it have? This is your group number. Carbon would be 4. Nitrogen would be 5. Etc.

For the second number in the equation you find it by taking the number of lone pair electrons and adding them to half the number of bonding electrons. First, look at the molecule you have been given. How many things is the atom bonded to? Each bond is two electrons, so (1/2) bonding electrons would essentially give you the same number as the number of bonds the atom has. Because of this, I generally like to just count the bonds and add them to the lone pair electrons.

The lone pair electrons will be drawn on the atom (black dots around it). Just add these up.

Let's calculate the Formal Charge of the Nitrogen in the molecule Pyrrole:

On the periodic table, Nitrogen has 5 valence electrons. This is our group number. Now, let's count the bonds. The Nitrogen here is bonded to the Hydrogen and single-bonded to the Carbons on either side of it. That's three bonds. This means six electrons total, so (1/2) of the six would be three. It also has a lone pair of electrons above it. That's 2 more electrons.

This brings the total for the second number in the equation to 5.

Remembering that the group number is 5, we fill in the numbers of our equation:

Formal Charge = (group number of 5) - (combined bonding electrons and lone pairs adding up to 5)

Formal Charge = (5) - (5)

Formal Charge = 0

In this molecule, the Formal Charge of the Nitrogen is 0, so the Nitrogen does not have a formal charge.