Predicting electron-pair geometry and molecular structure: xef
4
Of all the noble gases, xenon is the most reactive, frequently reacting with elements such as oxygen and fluorine. Predict the electron-pair geometry and molecular structure of the XeF
4 molecule.
Solution
The Lewis structure of XeF
4 indicates six regions of high electron density around the xenon atom: two lone pairs and four bonds:
These six regions adopt an octahedral arrangement (
[link] ), which is the electron-pair geometry. To minimize repulsions, the lone pairs should be on opposite sides of the central atom (
[link] ). The five atoms are all in the same plane and have a square planar molecular structure.
(a) XeF
4 adopts an octahedral arrangement with two lone pairs (red lines) and four bonds in the electron-pair geometry. (b) The molecular structure is square planar with the lone pairs directly across from one another.
Check your learning
In a certain molecule, the central atom has three lone pairs and two bonds. What will the electron pair geometry and molecular structure be?
Answer:
electron pair geometry: trigonal bipyramidal; molecular structure: linear
When a molecule or polyatomic ion has only one central atom, the molecular structure completely describes the shape of the molecule. Larger molecules do not have a single central atom, but are connected by a chain of interior atoms that each possess a “local” geometry. The way these local structures are oriented with respect to each other also influences the molecular shape, but such considerations are largely beyond the scope of this introductory discussion. For our purposes, we will only focus on determining the local structures.
Predicting structure in multicenter molecules
The Lewis structure for the simplest amino acid, glycine, H
2 NCH
2 CO
2 H, is shown here. Predict the local geometry for the nitrogen atom, the two carbon atoms, and the oxygen atom with a hydrogen atom attached:
Solution
Consider each central atom independently. The electron-pair geometries:
nitrogen––four regions of electron density; tetrahedral
carbon (
C H
2 )––four regions of electron density; tetrahedral
carbon (
C O
2 )—three regions of electron density; trigonal planar
oxygen (
O H)—four regions of electron density; tetrahedral
The local structures:
nitrogen––three bonds, one lone pair; trigonal pyramidal
carbon (
C H
2 )—four bonds, no lone pairs; tetrahedral
carbon (
C O
2 )—three bonds (double bond counts as one bond), no lone pairs; trigonal planar
oxygen (
O H)—two bonds, two lone pairs; bent (109°)
Check your learning
Another amino acid is alanine, which has the Lewis structure shown here. Predict the electron-pair geometry and local structure of the nitrogen atom, the three carbon atoms, and the oxygen atom with hydrogen attached:
Answer:
electron-pair geometries: nitrogen––tetrahedral; carbon (
C H)—tetrahedral; carbon (
C H
3 )—tetrahedral; carbon (
C O
2 )—trigonal planar; oxygen (
O H)—tetrahedral; local structures: nitrogen—trigonal pyramidal; carbon (
C H)—tetrahedral; carbon (
C H
3 )—tetrahedral; carbon (
C O
2 )—trigonal planar; oxygen (
O H)—bent (109°)
