BackLewis Dot Structures: Octet Rule, Exceptions, and Odd Electron Molecules
Study Guide - Smart Notes
Tailored notes based on your materials, expanded with key definitions, examples, and context.
Lewis Dot Structures and the Octet Rule
Introduction to the Octet Rule
The octet rule is a fundamental principle in chemistry stating that atoms tend to form bonds until they are surrounded by eight valence electrons, achieving a stable electron configuration similar to noble gases.
Octet: Refers to eight electrons in the valence shell.
Some elements can have 8 (octet) or 3-8 electrons around themselves for stability.
Recall: The non-octet number of electrons is 2x their group number.
Incomplete Octet: Group 2A (2x2=4) and Group 3A (3x2=6).
Expanded Octet: Group 5A (10), Group 6A (12), Group 7A (14), Group 8A (16).
Example: Lewis Dot Structure for Xenon Dibromide Molecule, XeBr2
Step 1: Count valence electrons: Xe (8) + 2 × Br (7 × 2 = 14)
Step 2: Total valence electrons: 8 + 14 = 22e-
Step 3: Draw the structure with Xe in the center and Br atoms bonded to it.
Practice: Draw the Lewis Dot Structure for SOCl2.
Count valence electrons: S (6) + O (6) + 2 × Cl (2 × 7 = 14)
Total: 6 + 6 + 14 = 26e-
Odd Electron Molecules
Free Radicals and Unpaired Electrons
Free radicals are molecules or ions with a single unpaired electron around an element. These species are highly reactive and play important roles in chemical reactions.
Radical compounds always have an odd number of total valence electrons.
To draw, place the electron on the element that is less electronegative (except for hydrogen atom).
Example: Lewis Dot Structure for the Radical of Nitrogen Dioxide, NO2
Count valence electrons: N (5) + 2 × O (2 × 6 = 12)
Total: 5 + 12 = 17e-
Draw the structure with N in the center, O atoms bonded, and one unpaired electron on N.
Lewis Dot Structures: Exceptions (Simplified)
Introduction to Exceptions
Some molecules and ions do not follow the octet rule due to incomplete or expanded octets, or the presence of odd electrons. These exceptions are important for understanding real chemical bonding.
Practice: Lewis Dot Structure for Hydroxide Radical, OH
Count valence electrons: O (6) + H (1) = 7e-
Draw the structure with O bonded to H and one unpaired electron on O.
Practice: Lewis Dot Structure for POCl3
Count valence electrons: P (5) + O (6) + 3 × Cl (3 × 7 = 21)
Total: 5 + 6 + 21 = 32e-
Draw the structure with P in the center, O and Cl atoms bonded.
Practice: Lewis Dot Structure for SiF62-
Count valence electrons: Si (4) + 6 × F (6 × 7 = 42) + 2 extra electrons (for 2- charge)
Total: 4 + 42 + 2 = 48e-
Draw the structure with Si in the center and six F atoms bonded, showing expanded octet.
Summary Table: Octet Rule and Exceptions
The following table summarizes the types of octet rule exceptions and their characteristics:
Type | Description | Examples |
|---|---|---|
Incomplete Octet | Atoms with fewer than 8 electrons | BeCl2, BF3 |
Expanded Octet | Atoms with more than 8 electrons | PCl5, SF6 |
Odd Electron Molecules | Molecules with an odd number of electrons | NO, NO2, OH |
Additional info: The notes infer the use of expanded octets for elements in period 3 and beyond, and the placement of unpaired electrons in radicals. These concepts are essential for GOB Chemistry students to understand molecular structure and reactivity.
