Intermolecular Forces: Review
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Molecular Geometry
Many of the physical and chemical properties of a molecule or ion are determined by its three-dimensional shape (or molecular geometry). Lewis structures are very useful in predicting the geometry of a molecule or ion.
The valence shell electron-pair repulsion theory (abbreviated VSEPR) is commonly used to predict molecular geometry. The theory says that repulsion among the pairs of electrons on a central atom (whether bonding or non-bonding electron pairs) will control the geometry of the molecule. In other words, the electrons will try to be as far apart as possible while still bonded to the central atom.
Molecular geometry can be predicted using VSEPR by following a series of steps:
Step 1: Count the number of lone electron pairs on the central atom.
How many lone electron pairs are on the central atom in each of the following Lewis structures?
Good!
A lone electron pair is represented as a pair of dots in a Lewis structure. Try again.
The correct answers have been entered for you. Make sure you understand why they are correct.
Step 2: Count the number of atoms bonded to the central atom.
How many atoms are bonded to the central atom in each of the following structures?
Good!
Remember to count the number of atoms bonded to the central atom. If an atom is bonded to the central atom by a double bond, it is still counted as one atom. Try again.
The correct answers have been entered for you. Make sure you understand why they are correct.
Step 3: Add these two numbers together to get the regions of electron density around the central atom. Use this number to determine the electron pair geometry.
Each bond (whether it be a single, double or triple bond) and each lone electron pair is a region of electron density around the central atom. The regions of electron density will arrange themselves around the central atom so that they are as far apart from each other as possible.
|
Regions of
Electron Pair Density |
|||||
| linear | triangular planar | tetrahedral | trigonal bipyramid | octahedral |
The table below shows the electron pair geometries for the structures we've been looking at:
|
Lewis Structure
of Molecule or Ion |
Regions of
Electron Pair Density |
Electron Pair Geometry * |
|
4 |
tetrahedral |
|
4 |
tetrahedral |
|
3 |
triangular planar |
|
* Lone electron pairs are represented by a line without an atom attached. |
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|
Step 4: The molecular geometry describes the position only of atomic nuclei (not lone electron pairs) of a molecule (or ion). If there are no lone electron pairs on the central atom, the electron pair and molecular geometries are the same. |
|
Click here to see the various molecular geometries. |
Choose the correct molecular geometries for the following molecules or ions below.
Review the various molecular geometries by clicking on the test tube above and then try again.
Good! The geometry for these three molecules and ions is summarized in the table below.
| Molecule or Ion |
Regions of
Electron Pair Density |
# of Lone Electron Pairs
on Central Atom |
Electron Pair Geometry | Molecular Geometry |
|
|
4 | 1 |
tetrahedral |
triangular pyramidal |
|
|
4 | 0 |
tetrahedral |
tetrahedral |
|
|
3 | 0 |
triangular planar |
triangular planar |
Notice when there are no lone electron pairs on the central atom, the electron pair and molecular geometries are the same.
| Lewis Structures | Molecular Geometry | Electronegativity | Molecular Polarity |