Atoms formal charge

Given a set of donor atom positions around a central ion it is fairly straightforward to derive a CFP of the form of equation (2). If all but the main terms arising from the donor atom formal charges are omitted, the potential is of the general form30-33... 

In contrast to what occurs in most of the VB structures of Figures 1-13, atomic formal charges are not indicated in VB structures III-XXXIX. In Figures 1-13, the formal charges are derived on the assumption that bonding electrons are shared equally by pairs of adjacent atoms, cf.[4]. 

Molecule Atom Formal Charge Atom Formal Charge Expanded to ... 

A double bond is represented by two pairs of dots, etc. Dots representing nonbonded outer-shell electrons are placed adjacent to the atoms with which they are associated, but not between the atoms. Formal charges (e.g. +, -, 2+, etc.) are attached to atoms to indicate the difference between the positive nuclear charge (atomic number) and the total number of electrons (including those in the inner shells) on the formal basis that bonding electrons are shared equally between atoms they join. (Bonding pairs of electrons are usually denoted by lines, representing covalent bonds, as in line FORMULAe.)... 

The structures of covalently bonded molecules can be represented by leivis structures, which are extensions of Lewis symbols for atoms. Formal charges can be assigned to atoms in molecules, by following simple rules. 

Formal charge. For formal charge we assume all electrons are equally shared regardless of bond polarity, and we assign one of a pair of shared electrons to each atom. Formal charge is then defined as... 

C = 0 and C = 0 as well as C = 0 have appreciable weights. Presumably because they do not carry atomic formal charges, these CO double-bond structures are important, whereas such charges are present in c = O . The electroneutrality principle requires that the formal charges of atoms in nentral molecules have small magnitudes, and the contributions to resonance from the double-bond stmctures will assist this requirement. 

Using CIO2 as the example, the Addendum 2014 provides an alternative, non d-orbital approach that can be used to reduce the magnitudes of the atomic formal charges of valence-bond structures (30), (31), (41) and (42) for CIO2, CIO3 and SOi. 

As indicated already in Chapter 2, unless stated otherwise (see for example Chapter 23), the equivalent Lewis structure resonance theory assumes that electron-pair bond wavefunctions are of the Heitler-London atomic orbital type - for example y(l)a(2) + a(l)y(2) a.nAy l)b(2) + b )a 2) for structures (6) and (7). Atomic formal charges are not indicated in the generalized valence bond structures that involve the Y, A, B, C and D atoms. 

Figure 13-7 S2N2 polymerization and electron conduction in the (SN)x polymer (with atomic formal charges omitted). Reproduced from Ref 49 with permission from Wiley-VCH.

By delocalizing lone-pair electrons from the Fe and 0 of structures (35) and (36) into vacant bonding Fe-O or 0-0 orbitals, we reduce the magnitude of the atomic formal charges, and obtain increased-valence structures (37) arrd (38)16-20... 

The electronic reorganization that occurs in a base displacement reaction is usually formulated according to Eqn. (1) (with atomic formal charges omitted here) ... 

Although 3-electron 3-centre bonding nnits with three overlapping atomic orbitals are not electron-rich, for them increased-valence structures of the general types X R Y and X R Y (with atomic formal charges omitted) can be constructed. With one atomic orbital (x, r and y) per atomic centre, their electrons occupy the orbitals of Eqs. (2) and (3), respectively. 

The mechanism for the radical transfer reaction X + R Y->X R + Y has been formulated (without atomic formal charges) as... 

The results of ST0-6G VB calculations for the 2p7i-electrons of CeHe and the six Is CT-electrons of He with Deh symmetry show that resonance between the Dewar-type increased-valence stmctme generates a lower energy than does resonance between the Kekule-type increased-valence stmctmes. This resnlt has been associated with the more favonrable distribntion of the atomic formal charges in stmctme IV relative to that for stmctme III. 

Formal charge The charge an atom would have if the bonding electrons in each bond were divided equally between the two atoms involved. The formal charge concept tends to exaggerate the covalent character of the bonding between atoms. Formal charges are used to assess which Lewis structure is the most satisfactory representation of the true structure. 

---