The octet rule says that many atoms become more stable when they are surrounded by 8 valence electrons, like the noble gases. This rule is very useful for drawing Lewis structures and predicting bonding in many common molecules. However, some real molecules are stable even when one or more atoms do not have 8 electrons.
These exceptions help students understand that the octet rule is a guideline, not a universal law.
The three major exception types are electron-deficient molecules, expanded octets, and odd-electron molecules. Electron-deficient molecules such as BF3 have a central atom with fewer than 8 electrons. Expanded octets such as SF6 place more than 8 electrons around a central atom, usually from period 3 or below.
Odd-electron molecules such as NO have an unpaired electron, so a perfect octet for every atom is impossible.
Key Facts
- Octet rule: atoms often form bonds to reach 8 valence electrons around each atom.
- Electron-deficient exception: the central atom has fewer than 8 electrons, such as B in BF3 with 6 electrons.
- Expanded octet exception: a central atom has more than 8 electrons, such as S in SF6 with 12 electrons.
- Odd-electron exception: the molecule has an odd total number of valence electrons, such as NO with 11 valence electrons.
- Total valence electrons = sum of valence electrons from all atoms, adjusted for charge.
- Formal charge = valence electrons - nonbonding electrons - 1/2 bonding electrons.
Vocabulary
- Octet rule
- The tendency of many atoms to form bonds until they are surrounded by 8 valence electrons.
- Lewis structure
- A diagram that shows atoms, bonds, lone pairs, and sometimes unpaired electrons using dots and lines.
- Electron-deficient molecule
- A molecule in which an atom, often boron or beryllium, is stable with fewer than 8 valence electrons.
- Expanded octet
- A bonding arrangement in which a central atom has more than 8 electrons around it.
- Radical
- A species that contains at least one unpaired electron and is often very reactive.
Common Mistakes to Avoid
- Forcing every atom to have 8 electrons, because some stable molecules like BF3, SF6, and NO are real octet rule exceptions.
- Putting an expanded octet on a second-period atom, because atoms such as C, N, O, and F cannot have more than 8 electrons in typical Lewis structures.
- Forgetting to count the total valence electrons first, because the electron count determines whether an odd-electron molecule or charged structure is needed.
- Ignoring formal charges, because the best Lewis structure usually minimizes formal charges while still using the correct number of electrons.
Practice Questions
- 1 BF3 has one boron atom and three fluorine atoms. Count the total valence electrons and state how many electrons surround boron in the Lewis structure.
- 2 SF6 has one sulfur atom and six fluorine atoms. Count the total valence electrons and determine how many electrons surround sulfur in the Lewis structure.
- 3 NO has 11 valence electrons. Explain why it cannot give both atoms a complete octet without changing the total electron count.