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VISIT THE PERIODIC TABLE LIVE AT THE CHEMED DL!
Knowing the formal charge on a particular atom in a structure is an important part of keeping track of the valence electrons (which typically are responsible for the bonds made in chemical reactions) and is important for establishing and predicting the reactivity of atoms in a molecule.
Although formal charge can be calculated via a formula, see below, it is also possible to do it "instinctively" based on comparing structures. The "instinctive method" is quicker, and is probably what your instructor uses, but it does require more skill and the experience of knowing common organic structures.
Formal charge equation is based on the comparing the number of electrons in the individual atom with that in the structure:
This method is based on comparing organic molecules with common, known neutral structures. To do this, you need to recognize these common bonding arrangements around each atom in a neutral molecule:
Carbon has 4 bonds, Nitrogen has 3 bonds and 1 lone pair, Oxygen has 2 bonds and 2 lone pairs, Hydrogen has 1 bond, and halogens like Flourine have 1 bond and 3 lone pairs. Once mastered, this method is much quicker.
In the middle of the following diagram are the neutral bonding situations for C, N, O and a halogen, F. (Just think of each bond being to a hydrogen).
To the left, a bond has been lost but converted to a lone pair, so the central atom has gained a net electron from the neutral state and become negative.
To the right, there are 2 scenarios:
- C and F have lost the shared electrons of a bond, so losing one electron from the electron "count" to become positive.
- N and O have converted two unshared electrons into a shared pair of electrons in a bond, so losing one electron from the electron "count" to become positive.