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Lewis diagrams and Molecular Shape Four electron pairs define the tetrahedral shape family, as in SiCl 4 There is a direct link between Lewis diagrams.

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Presentation on theme: "Lewis diagrams and Molecular Shape Four electron pairs define the tetrahedral shape family, as in SiCl 4 There is a direct link between Lewis diagrams."— Presentation transcript:

1 Lewis diagrams and Molecular Shape Four electron pairs define the tetrahedral shape family, as in SiCl 4 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

2 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

3 General Shape Family Scheme (1) 0 LP 2 BP 1 LP 1 BP X X.. X 2 EP’s3 EP’s 0 LP 3 BP1 LP 2 BP2 LP 1 BP X.. X

4 Lone pairs and shape: 4 Electron Pairs Linear Hydrogen Fluoride, HF 1 bond pair 3 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

5 General Shape Family Scheme XX : X :.. X : 4 BP 0 LP3 BP 1 LP2 BP 2 LP1 BP 3 LP

6 General Shape Family Scheme 5 BP 0 LP3 BP 2 LP2 BP 3 LP1 BP 4 LP X X.. 4 BP 1 LP X.. X X

7 Special features of 5 EP shapes In 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

8 General Shape Family Scheme X.. XX X X X 6 BP 0 LP5 BP 1 LP4 BP 2 LP3 BP 3 LP2 BP 4 LP1 BP 5 LP

9 Some special features of VSEPR multiple bonds - occupy almost the same volume of space - considered as single BP of electrons Ex) NO 2 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 90 o 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 90 o they would experience strong repulsive forces N has 4 EP’s 1 LP & 3 BP’s One double bond

10 Exercise Predict the shape of NO EP’s 4 BP’s 3 Bonds Triangular planar SO 4 2- S O O O O 6 EP’s 6 BP’s 4 Bonds Tetrahedral N O O O S O O O O N O O O ]-]-

11 POF 2 - P O F F 5 EP’s 4 BP’s 1 LP Trigonal pyramid P F O F

12 Geometry of large molecules VSEPR 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 C 1 and C 2 atoms are tetrahedral, while at C 3 it is trigonal planar The complete molecule simply moves parts around by twisting around any single bond to relieve congestion.

13 Shape and polarity Recall 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 polarNot always!!!

14 Permanent Dipole Moments Molecules that have permanent dipole moment are called polar The unit of dipole moment is the Debye Note that for molecules like H 2 and CCl 4, there is no permanent dipole moment! Such molecules are non-polar.


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