Molecular Geometry

Lewis Dot §

• predicts structural formula -> sequence of atoms bonded
• diagrams with like formal charges on adjacent atoms -> unstable
• opposite formal charges on adjacent atoms are good

1. draw atom with valence $e^{-}$
2. octet rule: main group surrounded by $8e^{-}$
   • 2 in s orbital, 6 in p orbital
   • exception is $H$, $He$ (surrounded by 2)
3. assign formal charge

Exceptions §

Ozone §

• unstable
• rekt by CFC
• bond length not predicted by lewis dot
• resonance hybrid
  • both bonds are 1.5 order

Other §

1. odd $e^{-}$ molecules (NO)
2. octet-deficient molecules ($B{F}_3$)
3. valence shell expansion (3rd period and down)

Valence Shell Electron-Pair Repulsion Theory (VSEPR) §

• predicts shape of molecule based on Lewis Dot structure
• electron pairs repel
• 3 aspects that effect reactivity
  • bond length
  • size/shape
  • bond angles
  • dihedral angle
• geometry based off of nuclear coordinates
  • steric number (SN): number of atoms bonded to central atom and lone pair

Molecule	Steric No	Predicted Geometry	Example
$A{X}_2$	2	Linear	$C{O}_2$
$A{X}_3$	3	Trigonal Planar	$B{F}_3$
$A{X}_4$	4	Tetrahedral	$C{F}_4$
$A{X}_5$	5	Trigonal Bipyramidal	$P{F}_5$
$A{X}_6$	6	Octahedra	$S{F}_6$

• lone pairs occupy more space than bonding pairs
• dissimilar atoms cause different repulsions
  • $Cl$ attracts $e^{-}$ away from $C$ reduces repuulsion
  • electropositive repels more than electronegative atoms

Dipole moment §

• unequal distribution of $e^{-}$, vector
• good example: $H_{2}O$
• bad example: $CC{l}_4$
  • nonpolar