AP® Chemistry · Unit 2 · Topic 2.5
Lewis Diagrams
A Lewis diagram shows how atoms share and hold valence electrons as bonds and lone pairs, giving a map of a molecule's real electron distribution and shape.
Atoms bond by sharing or transferring valence electrons, and a Lewis diagram is just a careful accounting of where those electrons end up. Dots and lines are not decoration. A shared pair drawn as a line is a real concentration of negative charge between two nuclei, holding them together. An unshared pair drawn as dots is also real charge, sitting on one atom and pushing everything else away. If you get the count right, the diagram tells you how many bonds form, whether any atom still needs more electrons, and where the crowded, repulsive regions are.
To build one, count total valence electrons from every atom, adding one electron per negative charge or subtracting one per positive charge. Arrange atoms with the least electronegative one (usually) in the center, connect each outer atom to the center with a single bond, then distribute remaining electrons as lone pairs to satisfy octets, starting with outer atoms. If the central atom still lacks an octet after that, convert a lone pair on a neighboring atom into a second bond. Check formal charges afterward: the best structure minimizes formal charges and places any negative charge on the most electronegative atom.
This topic sets up everything that follows. Once the diagram is correct, you count electron domains around the central atom and use VSEPR to predict geometry, since both bonding pairs and lone pairs take up space and repel each other. Get the Lewis structure wrong and the predicted shape, polarity, and reactivity all go wrong with it.
Key terms
- Valence electron
- An electron in an atom's outermost shell, available for bonding.
- Lone pair
- A pair of valence electrons on one atom that is not shared with another atom.
- Formal charge
- The charge an atom would have if bonding electrons were split evenly between bonded atoms.
- Expanded octet
- A central atom holding more than eight valence electrons, possible for elements in period 3 or beyond.
- Electron domain
- A region around a central atom occupied by either a bonding pair or a lone pair, used to predict geometry.
Watch it explained
- How it worksWalk me through counting valence electrons and placing them to build a Lewis diagram.
- Worked exampleDraw the Lewis structure of the sulfate ion, SO4 2-, and account for every electron.
- Worked exampleDraw the Lewis structure of XeF4 and explain the expanded octet on xenon.
- Common mistakeWhy isn't a lone pair just an unused leftover with no effect on the molecule?
Check yourself
End of topic.