# formal charge

Formal charge in ozone and the nitrate anion
In chemistry, a formal charge (FC) is a partial charge on an atom in a molecule assigned by assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity [1] or in another definition the charge remaining on an atom when all ligands are removed homolytically [2].

The formal charge can be calculated by the following equation: FC = number of valence electrons of the atom - number of lone pair electrons on this atom - half the total number of electrons participating in covalent bonds with this atom.

When determining the correct Lewis structure (or predominant resonance structure) for a molecule, the structure is chosen such that the formal charge on each of the atoms is minimized.

Examples:
• carbon in methane: FC = 4 - 0 - 8/2 = 0
• Nitrogen in the nitro group NO2-: FC = 5 - 2 - 6/2 = 0
• double bonded oxygen in NO2-: FC = 6 - 4 - 4/2 = 0
• single bonded oxygen in NO2- FC = 6 - 6 - 2/2 = -1
An alternative method for assigning charge to an atom taking into account electronegativity is by oxidation number. Other related concepts are valence which counts number of electrons that an atom uses in bonding and coordination number, the number of atoms bonded to the atom of interest.

## Examples

Ammonium NH4+ is a cationic species. By using the vertical groups of the atoms on the periodic table it is possible to determine that each hydrogen contributes 1 electron, the nitrogen contributes 5 electrons, and the charge of +1 means that 1 electron is absent. The final total is 8 total electrons (1 Ã— 4 + 5 − 1). Drawing the Lewis structure gives an sp3 (4 bonds) hybridized nitrogen atom surrounded by hydrogen. There are no lone pairs of electrons left. Thus, using the definition of formal charge, hydrogen has a formal charge of zero (1-(0 + Â½ Ã— 2)) and nitrogen has a formal charge of +1 (5−(0 + Â½ Ã— 8)). After adding up all the formal charges throughout the molecule the result is a total formal charge of +1, consistent with the charge of the molecule given in the first place.

Note: The total formal charge in a molecule should be as close to zero as possible, with as few charges on the molecule as possible
• Example: CO2 is a neutral molecule with 16 total valence electrons. There are three different ways to draw the Lewis structure
• Carbon single bonded to both oxygen atoms (carbon = +2, oxygens = -1 each, total formal charge = 0)
• Carbon single bonded to one oxygen and double bonded to another (carbon = +1, oxygendouble = 0, oxygensingle = −1, total formal charge = 0)
• Carbon double bonded to both oxygen atoms (carbon = 0, oxygens = 0, total formal charge =0)
Even though all three structures gave us a total charge of zero, the final structure is the superior one because there are no charges in the molecule at all

## Alternative method

Although the formula given above is correct, it is often unwieldy and inefficient to use. A much quicker and still accurate method is to do the following:
• Draw a circle around the atom for which the formal charge is requested (as with carbon dioxide, below)
• Count up the number of electron in the atom's "circle." Since the circle cuts the covalent bond "in half," each covalent bond counts as one electron instead of two.
• Subtract the number of electrons in the circle from the group number of the element (the roman numeral from the older system of group numbering, NOT the IUPAC 1-18 system) to determine the formal charge. (aka: old group number minus electrons in circle)
• The formal charges computed for the remaining atoms in this Lewis structure of carbon dioxide are shown below.

Again, this method is just as accurate as the one cited above, but is much easier to use. It is important to keep in mind that formal charges are just that-formal, in the sense that this system is a formalism. Atoms in molecules do not have "signs around their necks" indicating their charge. The formal charge system is just a method to keep track of all of the valence electrons that each atom brings with it when the molecule is formed.

• Formal charge @ Georgia Southern University Link
• Formal charge exercise @ Michigan State University Link
• Even more formal charge exercises @ the University of Southern Maine Link

## References

1. ^ Lewis Structure Representation of Free Radicals Similar to ClO Hirsch, Warren; Kobrak, Mark. J. Chem. Educ. 2007, 84, 1360. Abstract
2. ^ Valence, Oxidation Number, and Formal Charge: Three Related but Fundamentally Different Concepts Parkin, Gerard J. Chem. Educ. 2006, 83, 791. Abstract
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A partial charge is a charge with an absolute value of less than one elementary charge unit (that is, smaller than the charge of the electron).

## Partial atomic charges

Partial charges are created due to the asymmetric distribution of electrons in chemical bonds.
atom (Greek ἄτομος or Ã¡tomos meaning "indivisible") is the smallest particle still characterizing a chemical element.
molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.[1][2] In organic chemistry and biochemistry, the term molecule
Electron

Theoretical estimates of the electron density for the first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: First
A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds.
Electronegativity, symbol χ, is a chemical property which describes the power of an atom (or, more rarely, a functional group) to attract electrons towards itself.[1] First proposed by Linus Pauling in 1932 as a development of valence bond theory,[2]
In chemistry, homolysis or homolytic fission is chemical bond dissociation of a neutral molecule generating two free radicals. That is, two electrons that are involved in the bond are distributed one by one to the two species.
In chemistry, valence electrons are the electrons contained in the outermost, or valence, electron shell of an atom. Valence electrons are important in determining how an element reacts chemically with other elements: The fewer valence electrons an atom holds, the less
A lone pair is a (valence) electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons.
Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds.
Lewis structures, also called Lewis-dot diagrams, electron-dot structures or electron-dot diagrams, are diagrams that show the bonding between atoms of a molecule, and the lone pairs of electrons that may exist in the molecule [1] [2].
Resonance in chemistry is a tool used to represent and model certain types of non-classical molecular structures.
Methane is a chemical compound with the molecular formula CH4. It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.
Nitro may refer to:

### Chemicals

• Nitroglycerin, an extremely explosive chemical compound
• Nitrous oxide, "laughing gas", used in some dental procedures as an anaesthetic; also used in high-performance internal-combustion engines as an oxidizing additive

The oxidation number of an element in a molecule or complex is the charge that it would have if all the ligands (basically, atoms that donate electrons) were removed along with the electron pairs that were shared
In chemistry, valence, also known as valency or valency number, is a measure of the number of chemical bonds formed by the atoms of a given element. Over the last century, the concept of valence evolved into a range of approaches for describing the chemical bond,
In chemistry, coordination number (c.n.), as defined originally in 1893 by Alfred Werner, is the total number of neighbors of a central atom in a chemical compound.[1][2] In methane the coordination number for the carbon atom is 4.
Ammonium is also an old name for the Siwa Oasis in western Egypt.

The ammonium cation is a positively charged polyatomic cation of the chemical formula NH4+. It has a molecular mass of 18.