Download VSEPR Theory: Predicting Molecular Structures and Shapes Based on Electron Pair Repulsion and more Exercises Molecular Structure in PDF only on Docsity!
VSEPR , Valence Shell Electron Pair Repulsion theory, allows one to
predict molecular structure. If molecular structure can be
predicted then molecular properties, like polarity, can be predicted.
The basic theory is that pairs of electrons repell each other; so, to
minimize repulsion electron pairs try to be as far apart as possible.
1. First the number of electron pairs around the atoms must be
counted.
Multiple bonds are treated as though they are one pair, because
the two bond must point in the same direction.
2. Then the pairs must be distributed in space to minimize pair
replusions. Lone pairs take up more space than bonding pairs.
Numbers of pairs and the shapes the electrons form.
1 pair—linear H — H
2 pair—linear, electrons are 180° apart
Cl ••••
•••• •••• Cl ••••
•••• Be
3 pair—trigonal planar, electrons are 120° apart
N
O
O
O
B
Cl
••••Cl
••••
••••
4 pair—tetrahedral, pairs are 109.5° apart
H
H
N
••
H
C
H
H
H
H
5 pair—trigonal bipyramid, triangular arrangement around the center, with a pair on top and bottom.
P
Cl
Cl
••••Cl
••••
••••
Cl
•••• ••••
••••
Cl
••••
••••
••••
6 pair—octahedral, a pair of electrons along the positive and negative direction of each axis (x,y,z).
Xe
••••
••••
F
F
• •^ F •
F
3. The positions of the atoms in the molecule are used to name the
shape of the molecule
Trigonal Pyramidal: (must be at least five atoms) one atom is at the center but is raised above the plane of the triangle, and the other three atoms are positioned on the vertices. A lone pair occupies the final vertex. Trigonal pyramidal is unlikely because the lone pair is 90° from the three bonding pair, and if the structure adopted a see-saw geometry the lone pair is 90° and 120° from the bonding pairs. Trigonal Bipyramidal: (must have at least 5 atoms) an atom at the center, triangular arrangement around the center, with an atom on top and bottom.
Common shapes that result from an Octahedral arrangement of electrons Linear: Three atoms in a linear arrangement. Four empty verticies around the center are occupied by lone pairs. T-shaped: (four atoms) a center atom and three other atoms arranged to form a T. This arrangement would be more stable than forming a pyramid with the atoms. Square Planar: (must have at least 5 atoms) one atom at the center four others on the corners of the square. A piar of electrons occupies the top and bottom verticies. While a see-saw is possible, electron pair repulsions would be higher than the repulsions in the sqaure planar arrangment. Square pyramidal: (must have atleast 6 atoms) one atom at the center of a square formed by four atoms, the sixth atom in a vertex directly above the center atom, and the bottom vertext occupied by a lone pair. Octahedral: all six veticies, and the center position occupied by atoms.
