Some chemists believe that satisfaction of the octet rule...

Some chemists believe that satisfaction of the octet rule should be the top criterion for choosing the dominant Lewis structure of a molecule or ion. Other chemists believe that achieving the best formal charges should be the top criterion. Consider the dihydrogen phosphate ion, $\mathrm{H}_{2} \mathrm{PO}_{4}^{-},$ in which the $\mathrm{H}$ atoms are bonded to $\mathrm{O}$ atoms. (a) What would be the predicted dominant Lewis structure if satisfying the octet rule is the top criterion? (b) What would it be if achieving the best formal charges is the top criterion? (c) Is there another Lewis structure you can draw that satisfies neither of these criteria?

Some chemists believe that satisfaction of the octet rule
should be the top criterion for choosing the dominant Lewis structure of a molecule or ion. Other chemists believe that achieving the best formal charges should be the top criterion. Consider the dihydrogen phosphate ion, $\mathrm{H}_{2} \mathrm{PO}_{4}^{-},$ in which the $\mathrm{H}$ atoms are bonded to $\mathrm{O}$ atoms.
(a) What would be the predicted dominant Lewis structure if satisfying the octet rule is the top criterion? (b) What would it be if achieving the best formal charges is the top criterion? (c) Is there another Lewis structure you can draw that satisfies neither of these criteria?

Key Concepts

Resonance Structures

Resonance structures are alternative Lewis structures for a molecule that cannot be fully represented by a single structure. They depict different possible arrangements of electrons, particularly in cases where neither complete octet satisfaction nor optimal formal charges can be achieved by one structural form alone. The actual electronic structure is a hybrid of these various forms.

Dominant Lewis Structure Criteria

When several valid Lewis structures can be drawn for a molecule or ion, chemists use criteria such as full adherence to the octet rule and the minimization of formal charges to identify the dominant structure. This dominant structure is considered the best representation of the molecule’s actual electronic configuration.

Octet Rule

The octet rule is a guiding principle in chemical bonding which states that atoms tend to form bonds so as to have eight electrons in their valence shell, mimicking the electron configuration of a noble gas. This concept is used to evaluate whether a proposed Lewis structure sufficiently fills the electron shells of all atoms involved.

Formal Charge

Formal charge is a bookkeeping method used to assign charges to atoms in a molecule based on the difference between the number of valence electrons in a free atom and the electrons assigned in the Lewis structure. It helps determine the most stable electron configuration by minimizing the overall charge separation across the molecule.

Lewis Structures

Lewis structures are diagrams that represent the valence electrons of atoms within a molecule. These structures use dots and lines to indicate electrons and bonds, respectively, and are essential for visualizing how atoms satisfy the octet rule and how formal charges are distributed throughout a molecule.

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