To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:
For instance, consider SiH4, CHO2–, NO+, and OF2 as examples for which this general guideline can be applied to determine their Lewis structures.
For a molecule like SiH4, the number of valence electrons on each atom in the molecule is added:
= [4 valence e−/Si atom × 1 Si atom] + [ 1 valence e−/H atom × 4 H atoms] = 8 valence e−
For a negative ion, such as CHO2–, the numbers of valence electrons on the atoms are added to the number of negative charges on the ion (one electron is gained for each single negative charge):
= [4 valence e−/C atom × 1 C atom] + [1 valence e−/H atom × 1 H atom] + [6 valence e−/O atom × 2 O atoms] + [1 additional e−] = 18 valence e−
For a positive ion, such as NO+, the numbers of valence electrons on the atoms in the ion are added, followed by subtraction of the number of positive charges on the ion (one electron is lost for every single positive charge) from the total number of valence electrons:
= [5 valence e−/N atom × 1 N atom] + [6 valence e−/O atom × 1 O atom] + [−1 e−] = 10 valence e−
OF2 being a neutral molecule, the number of valence electrons are simply added:
= [6 valence e−/O atom × 1 O atom] + [7 valence e−/F atom × 2 F atoms] = 20 valence e−
This text is adapted from Openstax, Chemistry 2e, Section 7.3: Lewis Symbols and Structures.
From Chapter 9:
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