Molecular Geomoetry

How molecules bond and interact

Lewis Structures

Respresent distribution of valence electrons

Drawing Lewis Structures

• Draw atoms using chemical symbols
• Each side of a chemical symbol represents the orbitals for valence electrons.
  • Most atoms have 1 s-orbital and 3 p-orbitals = 4 total.
• Place valence electrons one electron in each orbital first before filling in second electron.

Lewis Structures for Molecules

• Select central atom
  • Atom with highest bonding capacity
  • If multiple have same, choose largest (by radius).

  • H20 = Oxygen

• Count total valence electrons in entire molecule

  • H20 = 8

• Connect all other atoms to central atom using 1 bond.
  • Subtract 2 valence electrons for every single bond.
• Start with outer atoms when drawing in remaining electrons.

Higher Bond Order

• If electrons shells aren't a full octet, then shift two electrons to make a bond.
• Shift electrons to fill all valence shells.

Lewis Structures

Charge determines best lewis structure.

• Best will have minimized formal charge.

  • Any negative charge will be on most electronegative atom.

  Formal Charge

  Number of valence electrons in free atom - # bonds - # non-bonding electrons.

  # bonds

  count 1 per bond.

  # non-binding electrons

  count individual dots.

  • Formal charges are calculated per atom in molecules
    • charge written as superscript on element symbol.

Electronegativity Trend

• Ability for an atom to attract shared electrons in a covalent bond.
• Increases down and to the right.

Resonance Structure

• Multiple ways to draw lewis structures, if bonds and elements are symmetric, bonds may be either/or.

Resonance Hybrid

• Bond length is averaged.
• O-O and O=O have different bond lengths.

Molecular Ions

• Account for charge in valence electron count.

• #valence electrons -> minus charge of ion.

H3O+ -> (3 * 1) + (6) - (1) (minus 1 from positive charge)

• Put molecules in brackets with total charge as superscript.

Radicals

Molecule with odd number of valence electrons.

One of your non-bonding electrons will be unpaired.

Phosphorous has exceptions to the octet rule and can form bonds up to bonding capacity (5 & 6 respectively).

• Goes down due to octet rule, going up only gives you 4 bonding capacity.
• N maxes out 4 bonds due to octet rule.