Formal charge
Atoms are given a formal charge in valence bar formulas in order to indicate the difference between the positive nuclear charges and the valence electrons that are formally (graphically) assigned to an atom. The distribution of electrons in valence bar formulas is often based on the fulfillment of the noble gas configuration for the individual atoms. Formal charges usually do not reflect the actual charge distributions in a molecule. They are often the mesomeric limit formulas of a molecule. A formal charge is indicated as a superscript plus or minus sign in a circle symbol.
Essence of formal charge
An externally electrically neutral atom has as many electrically negative electrons in its shell as it has positive protons in its atomic nucleus . In a covalent bond between atoms that form a molecule , their cohesion is brought about by two electrons with a charge center between these atoms (the exact geometry of the charge distribution depends on the type of bond). In order to determine the formal charge state of the atoms in a molecule, all binding electrons are distributed equally to the atoms involved so that each receives a certain number of electrons - negative charge carriers. If one compares this for a certain atom - taking into account its free electron pairs and not - valence electrons - with its positive nuclear charge, the difference corresponds to the formal charge of the atom under consideration.
The sum of all formal charges equals the total charge of the molecule ( ion ).
Examples :
- The ozone molecule (O 3 ) is electrically neutral and the total charge is therefore zero .
- The nitrate - anion (NO 3 - ), however, is simply negatively charged, the sum of the formal charges is therefore -1 .
- The calcium - cation (Ca 2+ ), in turn, is doubly positively charged, thus, the formal charge corresponding to the true charge and is +2 .
use
In most representations of covalent bonds, the number of allocated electrons and nuclear protons equalize each other; i.e. the formal charge of the atoms involved is zero. In order to interpret the reactivities of chemical compounds, however, bonds can be divided up differently, so that formal charges appear in certain mesomeric limit formulas. Sometimes they are also needed in order to be able to represent Lewis structures of some molecules compatible with energetically probable electron distributions ( octet rule ).
The number of formal charges in the representation of a molecule has a major influence on the proportion of the mesomeric boundary structure in the actual charge distribution. In general, from this point of view, a formal charge is assigned to as few atoms as possible and these charges (which always assume whole numbers) should be as small as possible. In addition, the distribution of formal charges should take into account the electronegativities of the elements present. Limit formulas with neighboring formal charges of the same sign are usually not taken into account.
For the consistent differentiation of formal and actual charges of ions, the former should be represented with the symbols shown in the figure. In the literature, however, this is often neglected and the additional specification of the total charge is dispensed with.
See also
literature
CE Mortimer, U. Müller: Chemie , 8th edition, 114ff., Thieme, Stuttgart, 2003