Drawing Lewis Dot Structures
Lewis structures can be drawn with a set of general rules that give us a pretty good idea of how valence electrons are distributed around a molecule. For the most part Lewis structures reflect what is found when we gather experimental data for molecules in the laboratory. It’s not perfect but it is a useful tool for scientists trying to understand how molecules will react and their physical properties.
As we get more advanced there are more exceptions to our general rules for drawing Lewis structures. There are exceptions to the octet rule, resonance structures, and new tools for understanding like formal charges. As you get even more advanced Molecular Orbital Theory is another powerful tool for understanding.
Basic Rules
- Find the total valence electrons for the molecule.
- Put the least electronegative atom in the center. Note: H always goes outside. [examples: CH3OH, CH2Cl2]
- Put two electrons between atoms to form chemical bonds.
- Complete octets for the outside atoms. Note: Hydrogen (H) only needs two valence electrons. [examples: CH4, NCl3]
- If you run out of valence electrons for the Lewis structure, move electron pairs on the outer atoms and share them with the central atom. [example: O2, N2, CO2]
Advanced
Exceptions
- H only needs two valence electrons for a full outer shell.
- Be and B don’t need 8 valence electrons.
- S and P sometimes have more than 8 valence electrons.
- Elements in Period Three, Four, etc (on the periodic table) can hold more than 8 valence electrons.
Resources
Basic Examples
Intermediate
Advanced Example