Transcript Molecular Shapes

Molecular Shapes
Chapter 6 Section 3
Molecular Structure
• It mean the 3-D arrangement of atoms in a
molecule
• Lewis dot structures show how atoms are bonded
together (2-D), but they often do not illustrate
the true shape of a molecule (3-D).
• How to determine the shape of a molecule?
 Valence Shell Electron Pair Repulsion Theory
(VSEPR)
VSEPR Theory
Pairs of electrons around a central atom want to be as
far away from each other as possible.
Main idea: Bonding and Non-bonding e- pairs are
positioned as far as possible so as to minimize
repulsions between electron pairs.
For VSEPR, treat double and triple bonds like single
bonds.
e- pairs: Bonding and Non-bonding pairs (lone pairs)
around the central atom, don’t include central atom
itself
5 Molecular Shapes
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Linear
Trigonal Planar
Tetrahedral
Trigonal Pyramid
Bent (2 kinds)
Linear
If two atoms bond to a central atom, like in CO2,
the molecule will form in a straight line (Linear).
 2 e- pairs, 3 atoms, angle = 180 degrees
Trigonal Planar
If three atoms bond to a central atom, with no
lone pairs, like in BF3 , the molecule forms a
Trigonal Planar shape.
 3 e- pairs, 4 atoms, angle = 120 degrees
Tetrahedral
If four atoms bond to a central atom, like in CH4,
the molecule takes a tetrahedral shape.
 4 e- pairs, 5 atoms, angle = 109.5 degrees
Lone Pairs around Central Atoms
Lone pair: Valence e- pair that belongs to only 1
atom / is not involved in bonding
Linear, Trigonal Planar, and Tetrahedral have NO
lone pairs around the central atoms
 Just look at how many atoms around the central
atom
 That is how many e- pairs
Lone Pairs around Central Atoms
What about if there are lone pairs around the
central atom?
REMEMBER:
1) Lone pairs also take up space!!!
2) Lone pairs repel each other
3) Lone pairs repel bonded pairs
Trigonal Pyramidal
If one lone pair of electrons and three atoms bond to
a central atom, like in CH4, the molecule takes on a
Trigonal Pyramidal shape.
 4 e- pairs (1 lone pair + 3 bonded pairs), 4 atoms,
angle = 107.2 degrees
Remember, lone pairs take up space!
Bent
If two lone pairs of electrons
and two atoms bond to
central atom, like in H2S
4 e- pairs (2 lone pairs + 2
bonded pairs), 3 atoms,
angle = 105 degrees
If one lone pairs of electrons
and two atoms bond to
central atom, like in SO2
3 e- pairs (1 lone pair + 2
bonded pairs), 3 atoms,
angle = 119.2 – 120 degrees
Steps for Predicting Molecular
Structures using VSEPR Model
1. Draw the Lewis Structure for the molecule.
2. Count the electron pairs and arrange them in
the way that minimize repulsions (put the
pairs as far apart as possible).
3. Determine the positions of the atoms from
the way the electrons pairs are shared.
4. Determine the name of the molecular
structure from the positions of the atoms.