# Molecular shapes Balls and sticks. Learning objectives  Apply VSEPR to predict electronic geometry and shapes of simple molecules.

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Molecular shapes Balls and sticks

Learning objectives  Apply VSEPR to predict electronic geometry and shapes of simple molecules

Roadmap to polarity  Establish skeleton of molecule  Determine Lewis dot structure using S = N – A  Determine electronic geometry using VSEPR  Identify molecular geometry from molecular

Valence shell electron pair repulsion  Lewis dot structure provides 2D sketch of the distribution of the valence electrons among bonds between atoms and lone pairs; it provides no information about molecular shape  First approach to this problem is to consider repulsion between groups of electrons (charge clouds)

Electron groups (clouds) minimize potential energy  Valence shell electron pair repulsion (VSEPR)  Identify all groups of charge: non-bonding pairs or bonds (multiples count as one)  Bonded atoms – single, double or triple count as 1  Lone pairs count as 1  Distribute them about central atom to minimize potential energy (maximum separation)

Total number of groups dictates electronic geometry  Octet rule:  Two – linear  Three – trigonal planar  Four – tetrahedral  Additional possibilities (expand octet):  Five – trigonal bipyramidal  Six - octahedral

Stage 3: Molecular shape:  What you get from electronic geometry considering atoms only  Same tetrahedral electronic geometry – different molecular shape

Three groups: trigonal planar  Two possibilities for central atoms with complete octets:  Trigonal planar (H 2 CO)  Bent (SO 2 )  BCl 3 provides example of trigonal planar with three single bonds  B is satisfied with 6 electrons

Four groups: tetrahedral  Three possibilities:  No lone pairs (CH 4 ) - tetrahedral  One lone pair (NH 3 ) – trigonal pyramid  Two lone pairs (H 2 O) – bent  Note: H-N-H angle 107°H-N-H angle 107° H-O-H angle 104.5°H-O-H angle 104.5° Tetrahedral angle 109.5°Tetrahedral angle 109.5°

Representations of the tetrahedron

Groups of charge Lone electron pairs Electronic geometry Molecular shape 20Linear 30Trigonal planar 31 Bent 40Tetrahedral 41 Trigonal pyramid 42TetrahedralBent

Two bonds  Equal bonds oppose (linear)  Nonpolar (CO 2 )  Unequal bonds oppose (linear)  Polar (HCN)  Equal bonds do not oppose (bent)  Polar (H 2 O)

Three bonds  Equal bonds oppose in trigonal planar arrangement  Nonpolar  Unequal bonds in trigonal planar arrangement  Polar

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