Overview
This lecture covers drawing Lewis structures for covalent molecules with only single bonds, emphasizing the reasoning behind structure and the step-by-step approach.
Steps for Drawing Lewis Structures (Single Bonds)
- Step 1: Count and sum the total number of valence electrons in the molecule.
- Step 2: Place the atoms; hydrogen always goes on the outside, the least electronegative atom in the center.
- Step 3: Bond all atoms with single bonds; assume simplicity unless necessary to change.
- Step 4: Give all atoms a full shell (typically 8 electrons, 2 for hydrogen).
- Step 5: Recount electrons to ensure all are used correctly.
- Step 6: Fix the structure if needed (using double/triple bonds if electrons don't add up); this is covered later.
Guidelines for Atom Placement
- Hydrogen is always placed on the outside as it forms only one bond.
- The least electronegative atom is usually placed in the center.
- Symmetry in the structure is preferred when possible.
Example Structures
- Hydrogen (H₂): Each hydrogen shares an electron for a stable pair; represented with either dots or a single line (bond).
- Water (H₂O): Oxygen in the center, hydrogens on the outside; oxygen has two lone pairs, hydrogens are satisfied with two electrons each.
- Carbon Tetrafluoride (CF₄): Carbon in the center, surrounded symmetrically by four fluorines; all atoms achieve full shells, fluorines each have three lone pairs.
- Ammonia (NH₃): Nitrogen in the center with three hydrogens; nitrogen has one lone pair that affects molecular shape.
- Methanol (CH₃OH): Recognize chunked formulas (CH₃ and OH groups); hydrogens on the outside, connect the groups, lone pairs go on the oxygen.
Key Terms & Definitions
- Covalent bond — a bond where atoms share electrons.
- Valence electrons — electrons in the outermost shell available for bonding.
- Lone pair — a pair of valence electrons not involved in bonding.
- Electronegativity — the tendency of an atom to attract shared electrons.
- Lewis structure — a diagram showing the arrangement of electrons among atoms in a molecule.
Action Items / Next Steps
- Practice drawing Lewis structures for molecules with only single bonds.
- Ensure diagrams are neat and dots are visible.
- Prepare for the next lesson on double and triple bonds.