Presentation Transcript

1. Molecular Geometry2-2 Ch. 6 – Molecular shape

2. Molecules are three-dimensional objects that occupy a three-dimensional world; In general, only the smallest molecules can be said to have a fixed geometrical shape; the icosahedral C60 “soccer ball” is a rare exception.

3. VSEPR Theory • Valence Shell Electron Pair Repulsion Theory - focuses on the bonding and nonbonding electron pairs present in the outermost (“valence”) shell of an atom to which are connected two or more other atoms. • Electron pairs orient themselves in order to minimize repulsive forces.

4. Lone pairs repel more strongly than bonding pairs!!! VSEPR Theory • Types of e- Pairs • Bonding pairs - form bonds • Lone pairs - nonbonding e-

5. Know the 5 common shapes ! Determining Molecular Shape • Draw the Lewis Diagram. • Shape is determined by the # of bonding pairs and lone pairs.

6. AB2 1. Common Molecular Shapes 2 bond 0 lone LINEAR 180°

7. Double bonds are treated like single bonds when shape is determined!!!!!! Linear molecules: AB2 • Ex: BeCl2 and CO2. • -If you write out the electron dot formula for carbon dioxide, you will see that the C-O bonds are double bonds.

8. Cl Be Cl 180° 270° Be Be 90° 180° BeCl2 TWO ELECTRON PAIRS AROUND BERYLLIUM ATOM The shape of BeCl2 is linear. THE MOLECULAR SHAPE IS BASED ON THE POSITION OF THE ATOMS!

9. OCO • CO2 2 bond 0 lone LINEAR 180°

10. AB3 2. Common Molecular Shapes 3 bond 0 lone TRIGONAL PLANAR 120°

11. Trigonal planar: AB3 • In the molecule BF3, there are three regions of electron density extending out from the central boron atom. The repulsion between these will be at a minimum when the angle between any two is 120°. This requires that all four atoms be in the same plane; the resulting shape is called trigonal planar.

12. F F B F 120° 120° B B 120° BF3 NO OCTET ON B ! THREE ELECTRON PAIRS AROUND THE BORON ATOM. MOLECULAR SHAPE ATTACH FLOURINES TO EPG. F F F THE SHAPE OF BF3 IS TRIGONAL PLANAR.

13. AB2E AB2E2 3. Common Molecular Shapes 2 bond 1 lone BENT <120°

14. Shape with lone pairs: AB2E22 bonding electrons and 2 lone pairs or AB2E , 2 bonding electrons and 1 lone pair • Bent: • The nonbonding electrons are also in orbitals that occupy space and repel the other orbitals.

15. O Se O Se O O SeO2 VSEPR treats double bonds like a single bond SeO2 IS V-SHAPED, OR BENT

16. Se Se O O O O RESONANCE!

17. H2O Common Molecular Shapes 2 bond 2 lone BENT 104.5°

18. AB4 4. Common Molecular Shapes 4 bond 0 lone TETRAHEDRAL 109.5°

19. Tetrahedral: AB4 • Methane, CH4, contains a carbon atom to which are connected four hydrogens. Consequently, the four equivalent bonds will point in four equivalent directions in three dimensions.

20. H H C H H 90° 90° C 90° 90° CH4 There are four electron pairs around the carbon atom. BUT……….

21. C There is a better arrangement for four electron pairs: TETRAHEDRAL 109.5° The angle is….. Put on the H-atoms…….

22. C There is a better arrangement for four electron pairs: H TETRAHEDRAL H H H tetrahedral EPG 4 electron pairs The shape of CH4 is tetrahedral.

23. AB3E 5. Common Molecular Shapes 3 bond 1 lone TRIGONAL PYRAMIDAL 107°

24. F P F F • PF3 3 bond 1 lone TRIGONAL PYRAMIDAL 107°

25. Polyatomic ions • The charge will dictate how many less or extra electrons will be added to the lewis dot diagrams. • NO3- • the negative charge tells you that you need to add one more electron to the diagram. 5 + 3(6) + 1 = 24 Polyatomic ions

26. Nitrate anion O N O O VSEPR treats multiple bonds as effective single electron pairs. Trigonal planar is the shape