VSEPR Theory Flashcards
Definition of VSEPR Theory
Valence Shell Electron Pair Repulsion Theory
A model used to predict the 3-dimensional geometry of molecules based on the repulsion of the bonding and non-bonding electrons in the molecule.
Assumptions about VSEPR theory
THESE ARE ALL INCORRECT!!!!
Atoms in a molecule are bound together by pairs of electrons 🡪 bonding pairs
Lone pairs – electrons not involved in bonding
Pairs of electrons around any particular atom position themselves as far apart from each other as possible (as to minimize the repulsion of their negative charges)
Lone pairs occupy slightly more space than bonding electron pairs
Single, double, and triple bonds are all treated equally
What is VSEPR Theory???
The shape is determined by the number of bonding and non-bonding electrons in the molecule.
In order to determine the shape, the Lewis diagram must be drawn first.
When determining the shape of a molecule with double or triple bonds, treat them as ONE (1) bonding pair
Molecules with the central atom surrounded by four bonding pairs (i.e. four atoms)
If the central atom is placed at the center of a sphere, then each of the four pairs of electrons will occupy a position to be as far apart as possible
This will result in a TETRAHEDRAL SHAPE
Assuming all single bonds, the central atom will be sp3 hybridized since there are four orbitals
The angle between each bonding pair will be 109.5°
Molecules with the central atom surrounded by 3 bonding pairs and 1 non-bonding pair
Tetrahedral electron geometry due to 4 electron groups around the central atom
a non-bonding electron pair requires more space than a bonding pair, thus due to repulsion creating a TRIGONAL PYRAMIDAL shape
Angles in these molecules are 107°not 109.5°as in the tetrahedral molecules
Molecules with the central atom surrounded by 2 bonding pairs and 2 non-bonding pairs
Tetrahedral electron geometry due to 4 electron groups around the central atom
Two non-bonding electron pairs now require mote space and will push the bonding pairs into a BENT/V-SHAPE/ANGULAR
bond angle of 104.5°
For each pair of non-bonding electrons, the bond angle decreases by 2.5° (from tetrahedral 109.5o)
Molecules with the central atom surrounded by 5 pairs of bonding pairs
To minimize repulsion, the atoms will arrange themselves in a TRIGONAL BIPYRAMIDAL SHAPE which consists of 2 pyramids sharing the same base.
The 3 atoms making the base will lie in the same plane with the central atom in the middle of it. The other atoms will be positioned above and below this plane.
The bond angles within the base will 120°and the bond between the other atoms and the base will be 90°.
Molecules with the central atom surrounded by 6 pairs of bonding pairs
To minimize repulsion, the atoms will arrange themselves in an octahedral shape
The central atom is at the center of a square plane made up of 4 atoms and the other 2 atoms will be placed above and below the plane.
All bond angles will be 90°
Hybridization of Expanded Octets
Since sp3 hybrid orbitals allow for only four bonds, additional hybridized orbitals must come from the d-shell 🡪 results in an expanded octet
5 pairs of electrons = sp3d hybridized
6 pairs of electrons = sp3d2 hybridized
Molecules with the central atom with an incomplete/reduced octet
Molecules that only have 2 bonding pairs on the central atom will have a LINEAR SHAPE with a bond angle of 180°
sp hybridization is used, seeing as the atom only has two orbitals
Molecules that only have 3 bonding pairs on the central atom will have a TRIGONAL PLANAR SHAPE with bond angles of 120°
sp2 hybridization is used, since there are three orbitals
Things to Remember about VSEPR Theory
Draw a Lewis Structure of the molecule
Focus on the central atom in the molecule (A)
Determine the number of bonded atoms (X)
Determine the number of lone pairs (E)
State the VSEPR formula (AXaEb )
Use the VSEPR Table to find the geometry of the molecule
Draw the molecule.
(the general shapes must be memorized)
Electron Geometry versus Molecular Geometry
The geometric arrangement of electron groups is often not the shape of the molecule
The molecular geometry name is defined by the position of only the atoms in the molecule, not the non-bonding pairs
Within each electron geometry name, there might be more than one molecular geometry (ie. shape)
VSEPR Theory main idea
Electrons are not close to one another because they repel each other. As a result, the atoms in a molecule tend to separate as far as they can from each other so as to minimize repulsion between bonding and non-bonding pairs
