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# Partial charge

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. The resulting partial charges are a property only of zones within the distribution, and not the assemblage as a whole. For example, chemists often choose to look at a small space surrounding the nucleus of an atom: When an electrically neutral atom bonds chemically to another neutral atom that is more electronegative, its electrons are partially drawn away. This leaves the region about that atom's nucleus with a partial positive charge, and it creates a partial negative charge on the atom to which it is bonded.

In such a situation, the distributed charges taken as a group always carries a whole number of elementary charge units. Yet one can point to zones within the assemblage where less than a full charge resides, such as the area around an atom's nucleus. This is possible in part because particles are not like mathematical points--which must be either inside a zone or outside it--but are smeared out by the uncertainty principle of quantum mechanics. Because of this smearing effect, if one defines a sufficiently small zone, a fundamental particle may be both partly inside and partly outside it.

### Uses

Partial atomic charges are used in molecular mechanics force fields to compute the electrostatic interaction energy using Coulomb's law. They are also often used for a qualitative understanding of the structure and reactivity of molecules.

### Methods of determining partial atomic charges

Despite its usefulness, the concept of a partial atomic charge is somewhat arbitrary, because it depends on the method used to delimit between one atom and the next (in reality, atoms have no clear boundaries). As a consequence, there are many methods for estimating the partial charges. The following list is taken from Meister and Schwarz, 1994 (see the article for details and references about each method).

• Partitioning of electron density distributions
• Hirshfield charges
• Density fitted atomic charges
• Bader charges (obtained from an atoms in molecules analysis)
• Politzer's charges
• Charges derived from density-dependent properties
• Partial derived charges
• Dipole charges
• Dipole derivative charges
• Charges derived from spectroscopic data

## Fundamental particles with non-integer charge

An "up-type quark" has an intrinsic charge of +2/3 of a unit and a "down-type quark" has an intrinsic charge –1/3 of a unit.

## References

J. Meister, W. H. E. Schwarz. Principal Components of Ionicity. J. Phys. Chem. 1994, 98, 8245-8252.