Write correct Lewis structures and assign a formal charge to...

Write correct Lewis structures and assign a formal charge to each atom.
(a) $\mathrm{SO}_{3}$
(b) $\mathrm{C}_{2} \mathrm{~N}_{2}$ (atoms bonded in the order $\mathrm{NCCN}$ )
(c) $\mathrm{NO}_{2}^{-}$

Key Concepts

Lewis Structures

Lewis structures are diagrams that show the arrangement of electrons among atoms in a molecule. They illustrate both bonding pairs (shared electrons in bonds) and nonbonding pairs (lone pairs) of electrons, providing a visual representation of molecular connectivity and electron distribution which is essential for predicting reactivity and physical properties.

Octet Rule

The octet rule suggests that atoms tend to form bonds until they are surrounded by eight electrons, achieving a stable electron configuration similar to that of noble gases. This principle guides the placement of electrons in a Lewis structure, although there are notable exceptions such as molecules with an odd number of electrons or those involving elements that can have expanded valence shells.

Formal Charge

Formal charge is a calculated charge on an atom within a molecule, determined by subtracting the number of electrons assigned to the atom in the Lewis structure (including half the bonding electrons) from its number of valence electrons in the free atom. This concept helps in identifying the most stable electron arrangement among possible Lewis structures by minimizing charge separation.

Electron Counting

Electron counting is the process of summing the valence electrons from all atoms in a molecule to ensure that the total number of electrons used in constructing the Lewis structure is correct. This is fundamental in correctly assigning bonds and lone pairs, which in turn affects the placement of formal charges and the overall stability of the molecule.

Resonance Structures

Resonance structures are different possible Lewis representations for a molecule that cannot be accurately described by a single structure. They depict the delocalization of electrons within the molecule and are considered together to represent the true electron distribution, enhancing the understanding of the molecule’s reactivity and stability.

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