Download Molecular Geometry and Forces of Attraction

Molecular Shapes
(geometry)
and
Forces of Attraction
VSEPR Theory
Valence Shell Electron Pair Repulsion theory
predicts molecular shapes based on the
idea that pairs of valence electrons
surrounding an atom repel each other
or
The shape of a molecule is determined by
the valence electrons surrounding the
central atom.
Unshared pairs of electrons (on the
central atom) will cause atoms to bend.
Five basic shapes: linear, bent,
tetrahedral, trigonal pyramidal, and
trigonal planar.
Molecular Shapes
(geometry)
1. LINEAR
example: CO2
 central atom
ALL shared pairs
NO unshared
pairs
 bond angles
180°
no bending!
2. BENT
example: H2O
 central atom
2 shared pairs
2 unshared pairs
 bond angles
104.5°
bends!
3. TETRAHEDRAL
example: CH4
 central atom
4 shared pairs
0 unshared pairs
 bond angles
109.5°
“tetra” = four!
4. TRIGONAL PYRAMIDAL
example: NH3
 central atom
3 shared pairs
1 unshared pair
 bond angles
107.3°
bends!
5. TRIGONAL PLANAR
example: BH3
 central atom
3 shared pairs
0 unshared pairs
 bond angles
120°
triangle-shaped!
remember…
 all covalent compounds are
either polar or nonpolar…and
that polar means oppositely
charged ends!
 dipole = a polar bond or molecule
Shape Affects Polarity.
1. POLAR Molecules
all bent, trigonal pyramidal molecules
any molecule that has unshared pairs
linear, tetrahedral, and trigonal planar ONLY IF:
dipoles do not cancel each other out
no symmetry within the molecule
2. NONPOLAR Molecules
linear, tetrahedral, and trigonal planar ONLY IF:
dipoles cancel out
there is symmetry within the molecule
#
# Shared
Unshare
Pairs
d Pairs
Molecular
Shape
Bond
Angle (°)
Linear
180
ALL
0
depends
HF, CO2
Bent
104.5
2
2
polar
SCl2, H2O
107
3
1
polar
NH3
120
3
0
depends
BF3
109.5
4
0
depends
CH4
Trigonal
Pyramidal
Trigonal
Planar
Tetrahedral
Polar or
Examples
Nonpolar
Forces of
Attraction
 Dispersion forces (one type of
van der Waals forces) – the weakest kind of
intermolecular attraction (caused by the
motion of electrons)
 Dipole interactions (a second
type of van der Waals forces) – a weak
intermolecular force resulting from the
attraction of oppositely charged regions of polar
molecules
 Hydrogen Bonds – the strongest
intermolecular force; hydrogen is bonded
to FON (fluorine, oxygen and nitrogen).