MOLECULAR GEOMETRY
 Elmhurst College
Lewis Diagrams Intro. to Molecular Geometry  Organic Molecular Geometry  Chemistry Department
Electron Pair Geometry Molecular Geometry Types  Virtual ChemBook

  
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  Introduction to Molecular Geometry

Introduction:

The specific three dimensional arrangement of atoms in molecules is referred to as molecular geometry.

Molecular geometry is associated with the specific orientation of atoms as a result of bonding and non bonding electrons about the central atom. A careful analysis of electron pairs will usually result in correct molecular geometry determinations.

In addition, the simple writing of Lewis diagrams which show the electron arrangements can also provide important clues for the determination of molecular geometry.

Molecules with no lone electron pairs:
Molecular geometry has its basis in the electron pair geometry of a molecule.

If the molecule has all electron pairs bonded to atoms, then the the molecular geometry is identical with the electron pair geometry. This is a common occurrence. This is the case for methane shown on the upper left.

  
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 Molecules with lone electron pairs:
If a molecule has lone electron pairs, these provide the basis to set the electron pair geometry, but become invisible when determining the molecular geometry. The molecular geometry describes only the orientation of the atoms, and does not included the lone electron pairs.

See what happens for ammonia shown on the middle left. Ammonia has three bonded H atoms and one lone electron pair around the N atom.
The electron pair geometry is a tetrahedral.
The molecular geometry is a trigonal pyramid.

   
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Compare Triatomic Molecules:
In the graphic on the left, three similar molecules containing three atoms are compared.
Carbon dioxide - no lone pairs has both linear electron pair and molecular geometry since there are two carbon-oxygen double bonds (each counting as one pair and no lone pairs about the central C atom.
For nitrogen dioxide, the lone non bonded electron occupies a position equivalent to a pair of electrons in the trigonal planar electron pair geometry. This gives rise to a bent or V-shaped molecular geometry. The single non bonded electron exerts less repulsion than a bond or a pair of non bonded electrons would, allowing the bond angle to be greater than 120 degrees predicted by the trigonal planar electron pair geometry. The O-N-O bond angle in nitrogen dioxide is 134.1 degrees.
Water has two bonded H atoms and two lone electron pairs around the O atom. Its electron pair geometry is tetrahedral. The molecular geometry is bent or V-shape.