Formal charge

With a formal charge of atoms are provided in Valenzstrichformeln to specify the difference between the positive charges and one atom of formal ( drawing ) allocated valence electrons. The distribution of electrons in Valenzstrichformeln often oriented to the fulfillment of the noble gas configuration for each atom. Formal charges usually give not the actual charge distributions in a molecule again. Often they are mesomeric formulas of a molecule. A formal charge is given as a superscript plus or minus sign in a circle icon.

Nature of the formal charge

An electrically neutral atom has the same number of outwardly electrically negative electrons in its shell as it has positive protons in its nucleus. In a covalent bond between atoms that make up a molecule whose bonds by two electrons causes of gravity of the cargo between these atoms (where the exact geometry of the charge distribution of the type of binding depends ). In order to determine the formal charge state of the atoms in one molecule, all the bonding electrons are respectively the atoms involved allocated uniformly, so that each of a certain number of electron - negative charge carriers - is replaced. If we compare this for a particular atom - in accordance with its lone pairs and no - valence - with its positive nuclear charge, so, the difference of the formal charge of the atom.

The sum of the formal charge balances the overall charge of the molecule ( ion) p

Examples:

• The ozone molecule (O3 ) is electrically neutral and the total charge thus zero.
• The nitrate anion ( NO3- ), however, is a single negative charge, the sum of the formal charges is therefore -1.
• The calcium ion ( Ca2 ) in turn is doubly positively charged, the formal charge thus corresponds to the real charge and is 2.

Use

In most representations of covalent bonds, the number of assigned electrons and core protons balance each other, ie, the formal charge of the atoms involved is zero. However, for the interpretation of reactivity of chemical compounds bonds can be distributed differently, so in certain mesomeric formulas appear formal charges. Sometimes they will also be required to represent Lewis structures of some molecules compatible with energetically probable electron distributions ( octet rule ) can.

The number of formal charges in the representation of a molecule has an important impact on the proportion of mesomeric structure on the actual charge distribution. In general, from this point as few atoms a formal charge is allocated and the charges ( which are always integers assume ) should be as small as possible. In addition, the distribution of formal charges should take into account the electronegativities of the elements present. Resonance structures with adjacent formal charges of the same sign are thereby usually ignored.

For the consistent distinction of formal and actual charges of ions concerned should be represented with the symbols indicated in the figure. In the literature, however, this is often neglected and also waived the additional indication of the overall charge.