4.4 Lewis Structures and Electron Rules

Overview

This lecture explains how to use Lewis symbols and structures to represent valence electrons and bonding in atoms, ions, and molecules, including exceptions to the octet rule.

Lewis Symbols for Atoms and Ions

• Lewis symbols show valence electrons as dots around the elemental symbol.
• The number of dots equals the number of valence electrons.
• Cations (positive ions) are formed when atoms lose electrons, shown by fewer dots.
• Anions (negative ions) are formed when atoms gain electrons, shown by more dots.
• Total number of electrons remains constant during ion formation.

Lewis Structures for Molecules

• Lewis structures depict covalent bonding using lines (single, double, triple) for shared electron pairs and dots for lone pairs.
• Each covalent bond equals a shared pair of electrons.
• Atoms try to achieve a stable electron configuration, often matching noble gases.

The Octet Rule

• Most main group atoms form bonds to have eight valence electrons (octet rule).
• Hydrogen is an exception and achieves a duet (2 electrons).
• The number of covalent bonds usually equals the number of electrons needed to reach an octet.

Single, Double, and Triple Bonds

• Single bond: sharing one pair of electrons (e.g., Cl₂, H₂).
• Double bond: sharing two pairs of electrons (e.g., O₂, CH₂O).
• Triple bond: sharing three pairs of electrons (e.g., N₂, CO).

Steps for Drawing Lewis Structures

• Count total valence electrons (adjust for charges).
• Choose a central atom (usually least electronegative, never hydrogen).
• Draw a skeleton with single bonds.
• Complete octets on terminal atoms first, then place extra electrons on central atom.
• Create multiple bonds if needed for central atom octet.

Example Applications

• Examples: HCN, H₃CCH₃, HCCH, NH₃, CO, CO₂ drawn using the above steps.

Exceptions to the Octet Rule

• Odd-electron molecules (free radicals) have unpaired electrons (e.g., NO).
• Electron-deficient molecules have a central atom with less than eight electrons (e.g., BeH₂, BF₃).
• Hypervalent molecules have a central atom with more than eight electrons, common with elements in period 3 or higher (e.g., PF₅, SF₆, XeF₂, XeF₆).

Key Terms & Definitions

• Lewis Symbol — representation of an atom's valence electrons as dots around the atomic symbol.
• Lewis Structure — diagram showing bonding and lone pairs in molecules.
• Octet Rule — atoms tend to have eight electrons in their valence shell.
• Lone Pair — non-bonding pair of electrons on an atom.
• Single/Double/Triple Bond — one, two, or three shared electron pairs between two atoms.
• Free Radical — molecule with an odd number of electrons (unpaired electron).
• Electron-deficient — atom with fewer than eight electrons in its valence shell.
• Hypervalent — atom with more than eight electrons in its valence shell.

Action Items / Next Steps

• Practice drawing Lewis structures for given formulas, including exceptions to the octet rule.
• Review homework or exercises on Lewis symbols and structures as assigned.