Formal charge

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Formal charge

In chemistry

Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, a formal charge (FC) is a partial charge

Partial charge

A partial charge is a charge with an absolute value of less than one electric charge unit ....
on an atom

Atom

|-! bgcolor=gray | Properties|-||}The atom is a basic unit of matter consisting of a dense, central atomic nucleus surrounded by a electron cloud of electric charge electrons....
in a molecule

Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
assigned by assuming that electron

Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
s in a chemical bond

Chemical bond

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....
are shared equally between atoms, regardless of relative electronegativity

Electronegativity

Electronegativity, symbol χ, is a chemical property that describes the ability of an atom to attract electrons towards itself in a covalent bond....
or in another definition the charge remaining on an atom when all ligands are removed homolytically

Homolysis

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....
.

The formal charge of any atom in a molecule can be calculated by the following equation: FC = number of valence electron

Valence electron

In science, valence electrons are the electrons contained in the outermost, or valence, electron shell of an atom. Valence electrons are important in determining how an chemical element reacts chemically with other elements: The fewer valence electrons an atom holds, the less reactivity it becomes and the more likely it is to chemical rea...
s of the atom in isolation - lone pair

Lone pair

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....
electrons on this atom in the molecule - half the total number of electrons participating in covalent bond

Covalent bond

A covalent bond 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....
s with this atom in the molecule.

When determining the correct Lewis structure

Lewis structure

Oxidation number

The oxidation number of a central atom in a complex is the charge that it would have if all the ligands were removed along with the electron pairs that were shared with the central atom....
.

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Encyclopedia

In chemistry

Chemistry

Partial charge

A partial charge is a charge with an absolute value of less than one electric charge unit ....
on an atom

Atom

|-! bgcolor=gray | Properties|-||}The atom is a basic unit of matter consisting of a dense, central atomic nucleus surrounded by a electron cloud of electric charge electrons....
in a molecule

Molecule

Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
s in a chemical bond

Chemical bond

Electronegativity

Homolysis

Valence electron

Lone pair

Covalent bond

Lewis structure

carbon in methane
Methane

Methane is a chemical compound with the molecular formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees....
: FC = 4 - 0 - 8/2 = 0
Nitrogen in the nitro
Nitro

Nitro may refer to:...
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

Oxidation number

The oxidation number of a central atom in a complex is the charge that it would have if all the ligands were removed along with the electron pairs that were shared with the central atom....
. Other related concepts are valence

Valence (chemistry)

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 chemical element....
which counts number of electrons that an atom uses in bonding and coordination number

Coordination number

The coordination number of an atom in a molecule or a crystal is the integer number of its nearest Neighbourhood . This number is determined somewhat differently for molecules and for crystals....
, the number of atoms bonded to the atom of interest.

Examples

Ammonium

The ammonium cation is a positively electric charge polyatomic ion of the chemical formula NH4+. It has a formula weight of 18.05 and is formed by protonation of ammonia ....
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 × 4 + 5 - 1). Drawing the Lewis structure

Lewis structure

Lewis structures, also called Lewis-dot diagrams, Electron-dot diagrams or Electron-dot structures, are diagrams that show the chemical bonding between atoms of a molecule, and the lone pairs of electrons that may exist in the molecule....
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 + ½ × 2)) and nitrogen has a formal charge of +1 (5-(0 + ½ × 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 electron
Valence electron

In science, valence electrons are the electrons contained in the outermost, or valence, electron shell of an atom. Valence electrons are important in determining how an chemical element reacts chemically with other elements: The fewer valence electrons an atom holds, the less reactivity it becomes and the more likely it is to chemical rea...
s. 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, oxygen_double = 0, oxygen_single = -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 vs. Oxidation State

The concept of oxidation states constitutes a competing method to assess the distribution of electrons in molecules. If the formal charges and oxidation states of the atoms in carbon dioxide are compared, the following values are arrived at:

The reason for the difference between these values is that formal charges and oxidation states represent fundamentally different ways of looking at the distribution of electrons amongst the atoms in the molecule. With formal charge, the electrons in each covalent bond are assumed to be split exactly evenly between the two atoms in the bond (hence the dividing by two in the method described above). The formal charge view of the CO₂ molecule is essentially shown below:

The covalent (sharing) aspect of the bonding is overemphasized in the use of formal charges, since in reality there is a higher electron density around the oxygen atoms due to their higher electronegativity compared to the carbon atom. This can be most effectively visualized in an .

With the oxidation state formalism, the electrons in the bonds are "awarded" to the atom with the greater electronegativity

Electronegativity

Electronegativity, symbol χ, is a chemical property that describes the ability of an atom to attract electrons towards itself in a covalent bond....
. The oxidation state view of the CO₂ molecule is shown below:

Oxidation states overemphasize the ionic nature of the bonding; most chemists agree that the difference in electronegativity between carbon in oxygen is insufficient to regard the bonds as being ionic in nature.

In reality, the distribution of electrons in the molecule lies somewhere between these two extremes. The inadequacy of the simple Lewis structure view of molecules led to the development of the more generally applicable and accurate valence bond theory

Valence bond theory

In chemistry, valence bond theory is one of two basic theories, along with molecular orbital theory, that developed to use the methods of quantum mechanics to explain chemical bond....
of , Pauling, et al., and thenceforth the molecular orbital theory

Molecular orbital theory

In chemistry, molecular orbital theory is a method for determining molecular structure in which electrons are not assigned to individual chemical bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule....
developed by and Hund

Hund

Hund is a small village situated on the right bank of the Indus River about 15 km upstream of Attock Fort and at a distance of about 80 km to the east of Peshawar in the North-West Frontier Province, of Pakistan....
.

External links

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