Guidelines for Drawing Lewis Structures Handout Chem 111 Fall 2024
Chem 111A Guidelines for Drawing Lewis Structures
1. Counting Valence Electrons
Identify the total number of valence electrons contributed by all atoms in the molecule.
2. Establishing Atomic Connectivity
Central Atom Selection:
The central atom usually has the lowest electronegativity.
Often written first in the chemical formula, but not always.
In molecules with three atoms, read left to right.
Arrange atoms symmetrically around the central atom.
For complex molecules, specific connectivity will be indicated.
3. Connecting Atoms
Use electron-pair bonds to connect adjacent atoms.
4. Completing Octets for Outer Atoms
Add lone pairs to ensure outer atoms achieve full octets.
5. Utilizing Remaining Electrons
Distribute leftover electrons by forming multiple bonds or lone pairs on the central atom.
6. Adjusting Electrons to Achieve Octets
“Push” electrons as necessary to fulfill octet requirements where possible.
7. Determining Formal Charges
Calculating Formal Charge:
Divide bond electrons evenly between bonded atoms.
Assign lone pairs to the atom they reside on.
Use the formula:
Formal charge = (Number of valence electrons for the atom) – (Number of electrons obtained in the division)
Check to ensure the total formal charges reflect the overall charge of the molecule.
Rules for Selecting the Preferred Lewis Structure
1. Obeying the Octet Rule:
Strive to satisfy the octet rule whenever feasible.
2. Minimizing Formal Charges:
Aim for the lowest possible sum of absolute values of formal charges while adhering to the octet rule.
3. Placing Formal Charges Appropriately:
Place positive formal charges on less electronegative (more electropositive) atoms, and negative formal charges on more electronegative atoms, whenever possible.