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Chemistry Notes Bonding and Molecular Shapes. Molecular Shapes  Recall: Properties of elements are consequences of their atoms  Properties of molecules.

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Presentation on theme: "Chemistry Notes Bonding and Molecular Shapes. Molecular Shapes  Recall: Properties of elements are consequences of their atoms  Properties of molecules."— Presentation transcript:

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:  ure.htm ure.htm 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 = 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 = 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 = 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 linear BeF trigonal planar BF trigonal planar bentNO tetrahedral CCl tetrahedral trigonal pyrami dal NH tetrahedralbentH2OH2O

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


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