lewis diagrams and molecular shape
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Lewis diagrams and Molecular Shape. There is a direct link between Lewis diagrams and molecular shape. The V alence S hell E lectron P air R epulsion theory states a molecule’s shape is determined by the electron pairs that surround central atoms. - PowerPoint PPT PresentationTRANSCRIPT
Lewis diagrams and Molecular Shape
Four electron pairs define the tetrahedral shape family, as in SiCl4
There is a direct link between Lewis diagrams and molecular shape
The Valence Shell Electron Pair Repulsion theory states a molecule’s shape is determined by the electron pairs that surround central atoms
These electron pairs (EP) include both bond pairs (BP) and lone pairs (LP)
We define five shape families based on the need to accommodate mutually repelling electron pairs around a central atom
The balloons illustrate these “natural” shapes
The 5 Shape Families
The five shape families that electron pairs develop are illustrated above
Linear – Trigonal-planar – Tetrahedral – Trigonal-bipyramidal – Octahedral
In this case each molecule has all its electron pairs as bond pairs around the central atom
Not all the shape-determining electrons pairs need be bond pairs
General Shape Family Scheme (1)
0 LP 2 BP
1 LP 1 BP X X..
X....2 EP’s 3 EP’s
0 LP 3 BP 1 LP 2 BP 2 LP 1 BP X
..X
Lone pairs and shape: 4 Electron Pairs
Linear
Hydrogen Fluoride, HF1 bond pair3 lone pairs
When there are four electron pairs, the shape family is tetrahedral
Four possibilities exist: with 0, 1, 2 or 3 lone pairs
0 LP – tetrahedral shape
1 LP – trigonal-pyramidal shape
2 LP – bent shape
3 LP – linear shape
General Shape Family Scheme
X X: X:......X:
4 BP 0 LP 3 BP 1 LP 2 BP 2 LP 1 BP 3 LP
General Shape Family Scheme
5 BP 0 LP 3 BP 2 LP 2 BP 3 LP 1 BP 4 LP
X X..
4 BP 1 LP
X.. .. ..
X .... X
.. ..
....
Special features of 5 EP shapesIn the trigonal bipyramidal geometry, alone among the 5 shape families, the sites are not identical
There are three equivalent sites which are identical to those in trigonal-planar, with bond angles of 120 . These are called the equatorial sites
Axial Sites - above and below the plane in a mutually linear relationship.
Note - the angles are different
Note - if there are LP’s, these will always first occupy equatorial sites
General Shape Family Scheme
X..X X
.... ..
X.. .. ..
X
........X
......
..
6 BP 0 LP 5 BP 1 LP 4 BP 2 LP 3 BP 3 LP 2 BP 4 LP 1 BP 5 LP
Some special features of VSEPRmultiple bonds - occupy almost the same volume of space
- considered as single BP of electrons
Ex) NO2
LP are considered to be larger than BP because constrained by only a single positively charged nucleus as opposed to two for a BP
For 5 EP since neighboring LP’s at 120 experience much less repulsion than at 90o they tend to cluster on the equatorial sites of the trigonal-bipyramidal geometry
Bent shape
For 6 EP case LP prefer to be opposite since otherwise at 90o they would experience strong repulsive forces
N has 4 EP’s
1 LP & 3 BP’sOne double bond
Exercise Predict the shape of NO3-
4 EP’s 4 BP’s
3 Bonds
Triangular planar
SO42-
S
O•• ••
• •O• •
••O ••
••
• •O•• ••
-1
-1
6 EP’s 6 BP’s
4 Bonds
Tetrahedral
NO
OO
-1
SO
OO
O
-1-1
N
O•• ••
• •O• •
• •O• •
•• ]-••
POF2-
P
O•• ••
• •F• •
• •
•• ••
• •F• •
-15 EP’s
4 BP’s
1 LP
Trigonal pyramid
PF
OF
• •-1
Geometry of large moleculesVSEPR method extends to larger molecules by treating them as a chains central atoms
The geometry at the S, O and N are similar to those in water and ammonia, i.e. bent and trigonal pyramidal
The geometry at the C1 and C2 atoms are tetrahedral, while at C3 it is trigonal planar
The complete molecule simply moves parts around by twisting around any single bond to relieve congestion.
Shape and polarityRecall that most covalent bonds are polar
Symmetry in a molecule can cause bond dipole vectors to cancel each other making the molecule non-polar even though its individual bonds are very polar
Therefore molecules must be polar Not always!!!
Permanent Dipole MomentsMolecules that have permanent dipole moment are called polar
The unit of dipole moment is the Debye
Note that for molecules like H2 and CCl4, there is no permanent dipole moment!Such molecules are non-polar.