Formal charge on carbon

An electron count of 5 is one more than that for a neutral carbon atom. The formal charge on carbon is -1, as is the net charge on this species. 

Because HCN has formal charges of zero on all the atoms, whereas HNC has a formal charge on nitrogen of +1 and a formal charge on carbon of — 1, we would expect HCN to be more stable than HNC. Indeed, this is the case. We will encounter many cases of constitutional isomers in which the difference in stability is much less than in this example. In such cases, much more subtle arguments must be used to predict which is more stable, if a prediction can be made at all without experimental measurements. 

Problem Example Lewis structures for carbon monoxide can be written in which the C-O bond is either double or triple. Calculate the formal charges on carbon and oxygen in the two alternative structures. 

B) Structure 1 is preferred because the formal charge on carbon is negative. 

Consider each of the following highly reactive carbon species. What is the formal charge on carbon in each of these structures ... 

A formal charge is a charge associated with an atom that does not exhibit the expected number of valence electrons. When calculating the formal charge on an atom, we first need to know the number of valence electrons the atom is supposed to have. We can get this number by inspecting the periodic table, since each column of the periodic table indicates the number of expected valence electrons (valence electrons are the electrons in the valence shell, or the outermost shell of electrons— you probably remember this from high school chemistry). For example, carbon is in Column 4A, and therefore has four valence electrons. Now you know how to determine how many electrons the atom is supposed to have. 

From all of the cases above (oxygen, nitrogen, carbon), you can see why you have to know how many lone pairs there are on an atom in order to figure out the formal charge on that atom. Similarly, you have to know the formal charge to figure out how many lone pairs there are on an atom. Take the case below with the nitrogen atom shown ... 

Notice that the zinc atom is associated with only four valence electrons. Although this is less than an octet, the adjacent carbon atoms have no lone pairs available to form multiple bonds. In addition, the formal charge on the zinc atom is zero. Thus, Zn has only four electrons in the optimal Lewis structure of dimethyizinc. This Lewis stmcture shows two pairs of bonding electrons and no lone pairs on the inner atom, so Zn has a steric number of 2. Two pairs of electrons are kept farthest apart when they are arranged along a line. Thus, the C—Zn—C bond angle is 180°, and linear geometry exists around the zinc atom. 

Both structures II and III have an arrangement of atoms that places a positive formal charge on atoms that are higher in electronegativity than carbon. Consequently, the most stable arrangement of atoms is as shown in structure I. Some compounds containing the ion having structure III (the fulminate ion) are known, but they are much less stable than the cyanates (structure I). In fact, mercury fulminate has been used as a detonator. 

Valency is the number of electrons lost, borrowed or shared in a chemical bond. Formal charges are indicated with Arabic numerals, so the formal charge on a copper cation is expressed as Cu2+, meaning each copper cation has a deficiency of two electrons. In this system of thought, the charge on the central carbon of methane is zero. 

With the assumption that the 4100 structures have equal weights, the carbon-carbon bonds in the ring are calculated to have bond number n = 1.173 and the nickel-carbon bonds to have n = 0.439, with 34.6 percent d character for the nickel bond orbitals. The number of unshared pairs on the nickel atom is 2.89. The formal charge on the nickel atom is —0.64 of this, the 4.39 Ni—C bonds, with 12 percent ionic character, would provide the opposite charge -f 0.53, leaving the nickel atom essentially neutral (charge —0.11). 

A formal positive charge on carbon changes the oxidation state by +1, and a formal negative charge by — 1 an odd electron on carbon leaves the oxidation state unchanged. 

FORMAL CHARGES ON THE ATOMS IN CARBON MONOXIDE THE NET CHARGE ON THE MOLECULE IS -1 +1 = 0. 

Draw a Lewis structure for carbon monoxide (CO). Calculate the formal charges on the atoms and comment on the stability of this compound. 

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