Lewis Structures of Molecular Compounds and Polyatomic Ions

The first step is to calculate the total number of valence electrons in the molecule. For neutral molecules, add the valence electrons from all atoms. For ions, adjust by subtracting one electron per positive charge or adding one electron per negative charge. This total determines how many electrons you must distribute throughout the structure.

The central atom is typically the least electronegative element in the molecule, as less electronegative atoms are more likely to form multiple bonds. Hydrogen and fluorine are exceptions—hydrogen is almost never central, and fluorine, being the most electronegative element, also cannot occupy the center. The central atom must be capable of forming at least two bonds.

Octet completion is the process of distributing electrons so that each atom achieves a stable electron configuration. Terminal atoms receive lone pairs to complete an octet of eight electrons, while hydrogen atoms need only two electrons (a duet). After bonding pairs are placed, remaining electrons are distributed as lone pairs on terminal atoms first, then on the central atom.

Form multiple bonds when atoms lack an octet after single bonds and lone pair distribution. Move lone pairs from terminal atoms into the bonding region to create double or triple bonds. For example, in ethene, carbon atoms form a double bond because single bonds and lone pairs alone cannot satisfy the octet rule for both carbons.

For polyatomic ions, follow the same procedure but adjust the valence electron count based on charge. Subtract one electron for each positive charge on cations; add one electron for each negative charge on anions. Then draw the skeletal structure, complete octets, and form multiple bonds as needed. Finally, enclose the structure in brackets with the ionic charge displayed outside.

Resonance structures are multiple valid Lewis structures for the same molecule where electrons are distributed differently among atoms. They occur when lone pairs can be moved to form multiple bonds in different positions. For example, in the carbonate ion, oxygen atoms can donate electrons to the central carbon in different arrangements, creating equivalent resonance structures that represent electron delocalization.

The skeletal structure establishes the connectivity framework by showing which atoms bond to the central atom. This arrangement determines how bonding pairs are placed and influences where lone pairs can be distributed. Correct skeletal structure placement—with the least electronegative atom centrally and hydrogen or more electronegative atoms terminally—ensures the final Lewis structure accurately represents the molecule's bonding.