Nitrogen is in group VA; thus it has 5 valence electrons. In NH 4 +, N has 4 bonds (i.e., 8
bonding electrons and no nonbonding electrons). So, V = 5; Nbonding = 8; Nnonbonding = 0
Thus, the formal charge on the N atom in NH 4 + is +1.RESONANCE STRUCTURES
For some molecules, two or more nonidentical Lewis structures can be drawn; these are called
resonance structures. The molecule doesn’t actually exist as either one of the resonance structures,
but is rather a composite, or hybrid, of the two. For example, SO 2 has three resonance structures.
Resonance structures are expressed with a double-headed arrow between them; thus,
represents the resonance structures of SO 2 . The actual molecule is a hybrid of these three structures
(the two S–O bonds are actually equivalent: No one bond is stronger than the other).
The last two resonance structures of sulfur dioxide shown above have equivalent energy or stability.
Often, nonequivalent resonance structures may be written for a molecule. In these cases, the more
stable the structure, the more that structure contributes to the character of the resonance hybrid.
Conversely, the less stable the resonance structure, the less that structure contributes to the
resonance hybrid. In the example above, it is the structure on the left of the diagram that is the most
stable. Formal charges are often useful for qualitatively assessing the stability of a particular
resonance structure. The following guidelines are used:
A Lewis structure with small or no formal charges is preferred over a Lewis structure with large