Chapter 10 · Chemical Bonding
10.5Writing Lewis Structures for Covalent Compounds
14 min · two checks
Predict
What is a reliable order for drawing a Lewis structure of a molecule?
The idea
- Draw Lewis structures for molecules and polyatomic ions.
- Recognize the common octet exceptions.
Count all valence electrons first. For an ion, add one electron for each negative charge and subtract one for each positive charge. Connect the skeleton with single bonds; hydrogen is always terminal, and the first element in many formulas is central. Complete octets on the terminal atoms. Place remaining electrons on the central atom. If the central atom still has fewer than eight, convert lone pairs from neighbors into multiple bonds. Check two things: the octet (or duet) and the original electron count.
Exceptions you should recognize: odd-electron molecules such as NO cannot give every atom an octet. Boron in BF₃ is often content with six electrons. Elements in period 3 and beyond, such as sulfur in SF₆ or phosphorus in PCl₅, can hold more than eight because they have d orbitals available in the older teaching picture — or, more carefully, because the octet is a period-2 rule that those atoms are not bound by. Do not expand the octet of carbon, nitrogen, or oxygen.
Keep these
- Count electrons, skeleton, outside octets, center, then multiple bonds.
- Ions: add electrons for negative charge, remove for positive.
- Odd-electron species, electron-deficient boron, and expanded octets past period 2 are the exceptions.
Worked path
Draw the electron count and bond pattern for CO₂.
Observe
C has 4 valence electrons, each O has 6. Total 16. Carbon is central.