What is the shape of BF 3 ? You do not need to take notes, this is just a review on what we did before our break --- ju

1. Valence Shell Electron Pair Repulsion Theory What is the shape of BF3?You do not need to take notes, this is just a review on what we did before our break --- just listen and watch.

2. Predicting molecular structures: The Strategy • Draw the correct Lewis dot structure. • Identify the central atom. • Designate the bonding pairs and lone pairs of electrons on central atom. • Count the regions of “high electron density” (HED)on the central atom. Each LONE PAIR on the central atom is one HED. Each ATOM bonded to the central atom is one HED.

3. Determine the arrangements of LONE PAIRS and ATOMS around the central atom. • VSEPR is a guide to the arrangement. • Determine the molecular shape around the central atom. • Ignore the lone pairs of electrons. • Adjust molecular shape for the effect of any lone pairs (repulsions).

4. Counting regions of HIGH ELECTRON DENSITY

5. VSEPR Theory: Geometries/Shapes • Main guiding principle: Regions of high electron density around the central atom are arranged as far apart as possible to minimize repulsions. • There are six basic electronic arrangements based on the number of regions of high electron density around the central atom.

6. VSEPR Theory Two regions of high electron density around the central atom. LINEAR

7. VSEPR Theory Three regions of high electron density around the central atom. TRIGONAL PLANAR

8. VSEPR Theory Four regions of high electron density around the central atom. TETRAHEDRAL

9. VSEPR Theory Five regions of high electron density around the central atom (Note: This has to be an EXCEPTION because it means 10 valence electrons instead of 8.) Trigonal bypramidal

10. VSEPR Theory Six regions of high electron density around the central atom. (Note: This has to be an EXCEPTION because it means 12 valence electrons instead of 8.) OCTAHEDRAL

11. VSEPR Theory Molecular shapedetermined by the arrangement of atoms around the central atom(s). Electron pairs are not used in the molecular shape determination, only the positions of the atoms are used. NOW, Take out your Green “VSEPR” Sheet.

12. Example Shape of molecule with only ATOMS and WITHOUT the unshared pairs Shape if you were considering the atoms AND the unshared pairs of electrons HED: 4 4 Bonded atoms: 4 2 Electronic arrangement: tetrahedral tetrahedral (this is the column marked “Geometry of Electron Pairs” on your green “VSEPR” sheet.) Molecular shape: tetrahedral bent (this is the column marked “Geometry of Atoms” on your green “VSEPR” sheet.)

13. Both have “tetrahedral” arrangements of both atoms and electrons because HED = 4 Bent Molecular shapes Tetrahedral

14. VSEPR Theory and Shape • Lone pairs of electrons (unshared pairs) require more volume than shared pairs. Repulsion of other atoms

15. CH4 vs. NH3 Note: Both CH4 and NH3 have four regions of HED and have tetrahedral electronic arrangements. Note how the angles change (but just a little)!

16. Molecular Shapes and Bonding • In the next sections we will use the following terminology: A = central atom B = bonding pairs around central atom E = lone pairs around central atom • For example: AB3E designates that there are 3 bonding pairs and 1 lone pair around the central atom.

17. Linear Molecules:AB2 Species (No Lone Pairs of Electrons on A) • Some examples of molecules with this arrangement are: BeCl2, BeBr2,BeI2, HgCl2, CdCl2 • All of these examples are linear, molecules.

18. Linear Molecules:AB2 Species Molecular shape

19. Linear Molecules:AB3 Species (no lone pairs) • Some examples of molecules with this arrangement are: BF3, BI3 BCl3 BBr3 • All of these examples are planar molecular

20. BCl3 Molecular shape

21. So Which on is the shape of BF3 according to VSEPR Theory? A or B

22. Answer: A • Because in the shape of “B”, the electrons are not spaced evenly apart from each other. • A B

23. Review of Strategy: Lewis dot structure # Regions high electron density Show where the Atoms and Lone Pairs are on around the Central Atom Lone pairs on central atom? Molecular shape

24. Tetrahedral Molecules AB4: (Four regions of high electron density) • Some examples of molecules with this arrangement are: CH4, CF4, CCl4, SiH4, SiF4 • All of these examples are tetrahedra

25. CH4 Dot Formula # HED = 4

26. What is the shape of CH4?

27. CH4 Molecular shape

28. Tetrahedral Molecular Shape:CH4 The shape looks like a tetrahedron!

29. Other Examples of Tetrahedral Shapes

30. Moving ahead…. • Trigonal bipyramidal and Octahedral • Molecules that have unshared (lone pair) electrons on the central atom

31. “Bent” Molecules: AB2E Species Recall BCl3

32. “Bent” Molecules: AB2E Species (Remember that A is the central atom, X =bonded atoms and E = unshared pair of electrons) • Consider SO2 .. .. :O::S :O: .. .. .. # HED = 3

33. Both have the same arrangement (trigonal planar) when you consider lone pairs and atoms but DIFFERENT molecular shapes: Trigonal planar Angular BF3 SO2

34. Trigonal Pyramidal Molecules: AB3E Species (One Lone Pair of Electrons on A) NH3, NF3, PH3, PCl3, AsH3 • All have one lone pair of electrons on the central atom. • NH3 and NF3 are trigonal pyramidal

35. Bent Molecules: AB2E2 Species (Two Lone Pairs of Electrons on A) • Some examples of molecules with this arrangement are: H2O, OF2, H2S • These molecules are our first examples of central atoms with two lone pairs of electrons.

36. H2O

37. H2O Molecular shape (You can’t see the lone pairs, so the molecule looks like an inverted vee.)

38. Trigonal Bipyramidal Molecular Shape • Some examples of molecules with this arrangement are: PF5, AsF5, PCl5, etc. • These molecules are examples of central atoms with five regions of high electron density: expanded octet! This means they have 10 electrons instead of 8.

39. Molecular shape (This is the best way to arrange five atoms around a center atom)

40. Trigonal Bipyramid

41. Trigonal Bipyramidal Electronic arrangement PF5

42. Trigonal Bipyramidal with Lone Pairs.AB5, AB4E, AB3E2, and AB2E3 • If lone pairs are incorporated into the trigonal bipyramidal structure, there are three possible new molecular shapes. • One lone pair - Seesaw shape • Two lone pairs - T-shape • Three lone pairs – linear • THESE ARE JUST INTERESTING, BUT I WON’T ASK YOU TO DO ANY OF THESE.

43. Example: SF4 # HED = 5 Arrangement with the Lone Pairs : trigonal bipyramidal Molecular shape: seesaw

44. Ex. ClF3 # HED = 5 Arrangement with the Lone Pairs: trigonal bipyramidal Molecular shape: t-shaped

45. Ex. XeF2 # HED = 5 Arrangement with the Lone Pairs: trigonal bipyramidal Molecular shape: linear

46. Octahedral Molecular Shapes with and without lone pairs: AB6, AB5E, and AB4E2 • Some examples of molecules with this arrangement are: SF6, SeF6, SCl6, etc. • These molecules have central atoms with six regions of high electron density. This means they have 12 electrons instead of 8! • You should be able to do only AB6

47. Octahedral Electronic arrangement: AB6, AB5E, and AB4E2 • If lone pairs are incorporated into the octahedral structure, there are two possible new molecular shapes. • One lone pair - square pyramidal • Two lone pairs - square planar • All octahedral shapes are exceptions because they need 6 bonds, which means 12 total valence electrons instead of 8.

48. e SeF6 or SF6 # HED = 6 Molecular shape: octahedral

49. IF5 # HED = 6. This is an exception because I has 10 total electrons!!! Molecular shape: square pyramidal

50. XeF4 # HED = 6, This molecule is an EXCEPTION Xe doesn’t normally make bonds because it is a Noble gas. But here it has 12 total electrons!! It would take a lot of Energy to do this. Molecular shape: square planar