1. Chemistry Notes Bonding and Molecular Shapes

2. Molecular Shapes  Recall: Properties of elements are consequences of their atoms  Properties of molecules are consequences of their structure (bonding arrangement)  Shapes of molecules give them certain properties

3. Molecular Shapes  Ex: NH 3, H 2 O and C 6 H 14  Different molecular Shapes:  http://dwb4.unl.edu/ChemAnime/mole_struct ure.htm http://dwb4.unl.edu/ChemAnime/mole_struct ure.htm http://dwb4.unl.edu/ChemAnime/mole_struct ure.htm

4. Molecular Shapes Two explanations for how shapes of molecules arise Two explanations for how shapes of molecules arise  VSEPR theory – deals with the repulsive forces of electrons on each other  Atomic orbital overlap (hybridization and bonding)

5. VSEPR Theory ( Valance Shell Electron Pair Repulsion theory)  Recall Lewis Structures. There are two types of electrons pairs to consider... Unshared pair of electrons.. Unshared pair of electrons H – N – H H – N – H Share pair of bonded electrons Share pair of bonded electrons H  Each pair of shared and unshared electrons forms a negatively charged clouds that repel each other.

6. VSEPR Theory ( Valance Shell Electron Pair Repulsion theory)  Two reasons for this repulsion:  Electrons have same charges (electrostatic forces)  Pauli Exclusion Principle: two electrons of the same spin can not be in the same orbital

7. VSEPR Theory ( Valance Shell Electron Pair Repulsion theory)  The repulsion between charge clouds in the outer level of an atom determines the arrangement of the orbital  Electron clouds spread out as far as possible to minimize repulsion forces

8. VSEPR Theory ( Valance Shell Electron Pair Repulsion theory)  Shared and Unshared pairs of electrons within a molecule determine the shape of the molecule.  Unshared electrons pairs are acted on by only one nucleus therefore occupy more space  Shared (bonded) electron pairs are acted on by two nuclei and therefore occupy less space  Because unshared electron pairs occupy more space they also create more of a repulsive force than shared pairs of electrons.

9. VSEPR Theory ( Valance Shell Electron Pair Repulsion theory)  Unshared electrons pairs are acted on by only one nucleus therefore occupy more space  Shared (bonded) electron pairs are acted on by two nuclei and therefore occupy less space

10. VSEPR Theory ( Valance Shell Electron Pair Repulsion theory) Repulsion between two unshared pairs of electrons Repulsion between a pair of unshared electrons and a pair of shared electrons Repulsion between two unshared pairs of electrons Comparison of electron pair repulsion > >

11. Examples of Molecular Shapes CH 4 All electron pairs are equal in repulsion All angles = 109.5 o CH 4 is tetrahedral

12. Examples of Molecular Shapes  NH 3  One unshared pair of electrons put a greater force on bond electrons  Bond angles = 107.3 o  NH 3 is trigonal planar

13. Examples of Molecular Shapes  Two unshared pair of electrons put a greater force on bond electrons  Bond angles = 104.5 o  H 2 O is bent

14. Other Types of Geometric Molecular Shapes  T – planer  Octahedral bi-pyramidal  Linear  Trigonal bi-pyramidal

15. Electron Pair and Molecular Geometry Total # of electro n pairs SharedUnshared Bond Angle Electron Pair Geome try Molecular Geome try Example 2 2 0180linear BeF 2 3 3 0120 trigonal planar BF 3 3 2 1120 trigonal planar bentNO 2 - 4 4 0109.5tetrahedral CCl 4 4 3 1109.5tetrahedral trigonal pyrami dal NH 3 4 2 2109.5tetrahedralbentH2OH2O

16. Bonding and Molecular Shape  Key Point: the differences in molecular shape are due to the unequal space occupied by unshared and shared electrons.