Formal charge defined

Some atoms, even in covalent compounds, carry a formal charge, defined as the number of valence electrons in the neutral atom minus the sum of the number of unshared electrons and half the number of shared electrons. Resonance occurs when we can write two or more structures for a molecule or ion with the same arrangement of atoms but different arrangements of the electrons. The correct structure of the molecule or ion is a resonance hybrid of the contributing structures, which are drawn with a double-headed arrow () between them. Organic chemists use a curved arrow (O) to show the movement of an electron pair. 

Oxidation state is a frequently used (and indeed misused) concept which apportions charges and electrons within complex molecules and ions. We stress that oxidation state is a formal concept, rather than an accurate statement of the charge distributions within compounds. The oxidation state of a metal is defined as the formal charge which would be placed upon that metal in a purely ionic description. For example, the metals in the gas phase ions Mn + and Cu are assigned oxidation states of +3 and +1 respectively. These are usually denoted by placing the formal oxidation state in Roman numerals in parentheses after the element name the ions Mn- " and Cu+ are examples of manganese(iii) and copper(i). 

Formal charge and oxidation number are two ways of defining atomic charge that are based on the two limiting models of the chemical bond, the covalent model and the ionic model, respectively. We expect the true charges on atoms forming polar bonds to be between these two extremes. 

The nature of the charge density parameters to be added to those of the structure refinement follows from the charge density formalisms discussed in chapter 3. For the atom-centered multipole formalism as defined in Eq. (3.35), they are the valence shell populations, PLval, and the populations PUmp of the multipolar density functions on each of the atoms, and the k expansion-contraction parameters for... 

Distribution of forma] charges in a contributing structure must be reasonable. Formal charge, which will be more fully explained in the next section, may be defined as the charge an atom in a molecule would have if all of the atoms had the same electronegativity. Canonical forms in which adjacent like charges appear will probably be unstable as a result of the electrostatic repulsion. A structure such as A-—B+—C+—D is therefore unlikely to play a major role in hybrid formation. 

In order to keep track of electron shifts in oxidation-reduction reactions, it is convenient to use the concept of oxidation number or oxidation state of various atoms involved in oxidation-reduction reactions. The oxidation number is defined as the formal charge which an atom appears to have when electrons are counted in accordance with the following rather arbitrary rules. 

Mixed valence" was defined by chemists as follows If ions (or molecules) of two different formal charges or valences occupy the same crystallographic site, then an intermediate "mixed-valent" state (an average of the two valences) is assigned to the ions of molecules at that site [38]. 

On the surface the charge excess of an ion is defined as the formal charge Q of the ion considered plus the sum of the Pauling valencies, computed with respect to the other ion ... 

To express the collective solvent reaction coordinate as in equation (6), it is necessary to define the specific diabatic potential surface for the reactant and product state. This, however, is not a simple task, and there is no unique way of defining such diabatic states. What is needed is a method that allows the preservation of the formal charges of the fragments of reactant and product resonance states. At the same time, solvent effects can be incorporated into electronic structure calculations in molecular dynamics and Monte Carlo simulations. Recently, we developed a block-localized wave function (BLW) method for studying resonance stabilization, hyperconjugation effects, and interaction energy decomposition of organic molecules.20-23 The BLW method can be formulated to specify the effective VB states.14... 

There is a well defined relationship between the chemical structure of a Zintl phase and its electronic structure. For the majority of these compounds, AX homopolar X-X contacts are present and can be explained as two-electron, two-center bonds. The octet rule [5] is fijlfiUed for the A and for the X atoms. This is provided by a formal charge transfer of the valence electrons fiom A to X leading to... 

If an atom in a molecule has the same number of valence electrons as it does in the free state, the positive and negative charges just balance, and the atom has a formal charge of 0. If an atom has one more valence electron in a molecule than it has a free atom, it has a formal charge of -1, and so on. Thus the formal charge on an atom in a molecule is defined as... 

Here h and G are the one and two-electron matrices used in vacuo, and VCT collects the one-electron integrals related to the potential generated by the apparent surface charges [Pg.36]

Table 2 summarizes orbital populations for both the actinide atoms and the donor oxygen atoms. The electron configurations for the ideal perfect ions which are defined for formal charges in these actinyl nitrates are [Rn] 5f6d for U(VI), [Rn] 5f 6d for Np(VI) and [Rn] 5f-6d for Pu(VI). Any variation in the orbital populations from those of the formal configurations means that to some extent there is some covalent bond formation. A significant changes in the orbital population is an increase in the number of An 5f, 6d electrons. 

On the surface the charge excess of an ion is defined as the formal charge Q of the... 

These structures are known as resonance structures. The true structure is some weighted average of these three. That is, each N-0 bond is between a single and double bond in length, as is the N-N bond. In order to rank the relative importance of each structure, we need to use formal charges. Formal charge, FC, is an arbitrary property. We define the formal charge on a specific atom by... 

A provisional Lewis structure may contain the correct bonding framework, but the distribution of the valence electrons may not be the one that gives the maximum stability. The correct stereochemistry is predicted by the valence shell configuration using VSEPR theory, as shown in Chapter 6. A concept called formal charge (FC) can be used to predict which structure of a number of alternative structures is the most reasonable for a Lewis structure. The formal charge (FC) on any atom in a Lewis structure can be defined as ... 

---