10.3
Predicting Molecular Geometry
VSEPR Theory for Determination of Electron Pair Geometries
The following procedure uses VSEPR theory to determine the electron pair geometries and the…
VSEPR theory helps to determine electron-pair geometries and molecular geometries.
A series of steps is used to predict the geometry and bond angles of molecules, such as phosphorus trichloride.
The first step is to draw the Lewis structure of the molecule.
Next, count the total number of electron groups on the central atom. Around phosphorus, there are four electron groups: three bonding pairs and one lone pair.
Now determine the electron-pair geometry. The electron pair geometry is tetrahedral. However, because of the lone pair, the molecular geometry is trigonal pyramidal. The lone pair reduces the bond angle to less than 109.5°.
The same protocol is used to predict the electron-pair geometry and molecular structure for carbon dioxide.
The Lewis structure of carbon dioxide shows the two-electron groups around the carbon atom—as each double bond counts as one electron group. The two-electron groups orient themselves on opposite sides of the central carbon atom with a bond angle of 180°. The electron-pair and molecular geometries are identical because there are no lone pairs on the central atom, and carbon dioxide molecules are linear.
The Lewis structure of tellurium tetrachloride has five electron groups around the tellurium atom: four bonding pairs and one lone pair. The electron groups have a trigonal bipyramidal geometry. The lone pair occupies one of the equatorial positions, and the molecule is seesaw-shaped.
These steps can again be used to determine the electron-pair geometry and molecular structure of the iodine tetrachloride anion.
The Lewis structure has six electron groups around the iodine atom: four bonding pairs and two lone pairs. The electron groups have an octahedral arrangement. The bonding pairs stay in one plane and the lone pairs are placed on either side of this plane, minimizing the repulsion. The molecular geometry is square planar.
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Q1: What are the first steps to predict molecular geometry using VSEPR theory?
Start by drawing the Lewis structure of the molecule to visualize bonding and lone pairs. Then count the total number of electron groups on the central atom, where single, double, or triple bonds each count as one electron group. These steps establish the foundation for determining both electron-pair geometry and molecular structure.
Q2: How do lone pairs affect molecular geometry and bond angles?
Lone pairs occupy more space than bonding pairs, causing greater repulsion and reducing bond angles below their ideal values. In phosphorus trichloride, one lone pair on the central phosphorus atom reduces the bond angle to less than 109.5 degrees, changing the molecular geometry from tetrahedral to trigonal pyramidal despite the tetrahedral electron-pair geometry.
Q3: Why is carbon dioxide linear while phosphorus trichloride is pyramidal?
Carbon dioxide has two electron groups with no lone pairs on the central carbon atom, so its electron-pair geometry and molecular geometry are both linear with a 180-degree bond angle. Phosphorus trichloride has four electron groups including one lone pair, resulting in a trigonal pyramidal molecular geometry despite tetrahedral electron-pair geometry.
Q4: What is the molecular geometry of tellurium tetrachloride?
Tellurium tetrachloride has five electron groups around the central tellurium atom: four bonding pairs and one lone pair. The electron-pair geometry is trigonal bipyramidal, but the lone pair occupies an equatorial position to minimize repulsion, resulting in a seesaw-shaped molecular geometry.
Q5: How are lone pairs positioned in octahedral electron-pair geometries?
In octahedral arrangements with two lone pairs, repulsion is minimized when the lone pairs occupy opposite sides of the central atom, perpendicular to the plane containing the bonding pairs. This arrangement in iodine tetrachloride anion produces a square planar molecular geometry with four bonding pairs in one plane.
Q6: When are electron-pair geometry and molecular geometry identical?
Electron-pair geometry and molecular geometry are identical only when there are no lone pairs on the central atom. In carbon dioxide, both geometries are linear because all electron groups are bonding pairs with no lone pairs present to distort the arrangement.
Q7: How does VSEPR theory minimize electron pair repulsion?
VSEPR theory positions electron groups as far apart as possible to minimize repulsion. In trigonal bipyramidal arrangements, lone pairs occupy equatorial positions rather than axial positions. In octahedral geometries, two lone pairs are placed on opposite sides of the central atom to maximize distance and reduce repulsive forces.