Overview
This lecture explains how to draw Lewis structures (electron dot formulas) for molecules, focusing on bonding patterns, lone pairs, and the identification of central atoms.
Basics of Molecular Bonding
- Molecules are covalent compounds made of non-metals.
- Each atom's group number in the periodic table indicates its number of valence electrons.
- Hydrogen has one valence electron; fluorine has seven.
- Shared electrons between atoms form covalent bonds (represented as a line or two dots).
- Lone pairs are non-bonding electron pairs that remain on the atom.
Determining Bonds and Lone Pairs
- The number of single dots (unpaired electrons) on an atom shows the number of bonds it can form.
- Hydrogen forms one bond, beryllium two bonds, boron three bonds, nitrogen three bonds (plus one lone pair), oxygen two bonds (plus two lone pairs).
Steps to Drawing Lewis Structures
- Identify the central atom (usually the single atom in the formula).
- Place the central atom in the center, surround with outer atoms.
- Use the atom's valence electrons to form the correct number of bonds.
- Add lone pairs as needed to complete the octet (or duet for hydrogen).
Examples
- Silicon tetrafluoride: silicon is central, forms four bonds to four fluorines; each fluorine has three lone pairs.
- Boron tribromide: boron is central, forms three bonds to bromines; each bromine has three lone pairs.
- Phosphorus triiodide: phosphorus is central, forms three bonds to iodines, and has one lone pair.
- Dihydrogen monosulfide: sulfur is central, forms two bonds to hydrogens, and has two lone pairs.
- Carbon dioxide: carbon is central, forms two double bonds to oxygens; each oxygen has two lone pairs.
- HCN (cyanic acid): carbon is central, triple-bonded to nitrogen (with one lone pair), single-bonded to hydrogen.
Key Terms & Definitions
- Valence electron — Electron in the outermost shell, involved in bonding.
- Lewis structure — Diagram showing bonds and lone pairs of electrons in a molecule.
- Covalent bond — Bond formed by sharing electron pairs between atoms.
- Lone pair — Pair of valence electrons not involved in bonding.
- Central atom — The atom in a molecule that is bonded to multiple other atoms.
- Double bond — Bond involving two shared electron pairs.
- Triple bond — Bond involving three shared electron pairs.
Action Items / Next Steps
- Practice drawing Lewis structures for assigned molecules.
- Review the periodic table for valence electron counts of non-metals.