When drawing lewis structure\'s how do we know whether the shape should be a tetrahedral or a square planar? For example in questions where we have to draw the lewis structure for XEBr4 or XeF4? Solution Lewis structures, along with VB theory, can predict the structures of molecules based on the number of electron pairs around the central atom in the molecule. The following shapes are possible based on the number of electron pairs. Number of electron pairs around the central atom Shape of the molecule 2 Linear 3 Planer Trigonal 4 Tetrahedral 5 Trigonal bipyramidal 6 Octahedral 7 Pentagonal bipyramid Now count the number of electron pairs in XeBr 4 or XeF 4 . Number of electron pairs = ½*(8 + 4) = 6 The electronic geometry of the molecule is octahedral; however, the actual shape of the molecule is square planer. This is due to the fact that there are 4 bonds between the Xe atom and the halogen. The Xe atom has two non-bonded electron pairs in the molecules. Non-bonded electron pairs are subject to maximum repulsions and hence, occupy diagonally opposite positions. Due to repulsions, the Xe-X (X = Br, F) bonds are squeezed and the molecules assume square planer geometry with the two lone pairs far away from each other. .

1. When drawing lewis structure's how do we know whether the shape should be a tetrahedral or a
square planar? For example in questions where we have to draw the lewis structure for XEBr4 or
XeF4?
Solution
Lewis structures, along with VB theory, can predict the structures of molecules based on the
number of electron pairs around the central atom in the molecule. The following shapes are
possible based on the number of electron pairs.
Number of electron pairs around the central atom
Shape of the molecule
2
Linear
3
Planer Trigonal
4
Tetrahedral
5
Trigonal bipyramidal
6
Octahedral
7

2. Pentagonal bipyramid
Now count the number of electron pairs in XeBr 4 or XeF 4 .
Number of electron pairs = ½*(8 + 4) = 6
The electronic geometry of the molecule is octahedral; however, the actual shape of the molecule
is square planer. This is due to the fact that there are 4 bonds between the Xe atom and the
halogen. The Xe atom has two non-bonded electron pairs in the molecules. Non-bonded electron
pairs are subject to maximum repulsions and hence, occupy diagonally opposite positions. Due
to repulsions, the Xe-X (X = Br, F) bonds are squeezed and the molecules assume square planer
geometry with the two lone pairs far away from each other.
Number of electron pairs around the central atom Shape of the molecule
2 Linear
3 Planer Trigonal
4 Tetrahedral
5 Trigonal bipyramidal
6 Octahedral
7 Pentagonal bipyramid