1. 1 3D Shapes of Covalent Compounds Electron pairs around an atom repel each other. Bonding and unshared pairs of e - spread out in space to move as far apart as possible. Any bonded atoms move with the electron pairs.

2. 2 So, to ask “What is the shape of a compound?” means we first must ask (and answer): What is the arrangement of the electron pairs around an atom? “What is the arrangement of the electron pairs around an atom?” two electron pairs two electron pairs Z 180° linear arrangement maximum separation = 180° = linear arrangement

3. 3 three e - pairs Z 120° trigonal planar maximum separation = 120° = trigonal planar arrangement

4. 4 four e - pairs Z maximum separation in 3D = 109° tetrahedral 109° = tetrahedral arrangement.

5. 5 ifnumber of groups = number of electron pairs Result & Conclusion... if the number of groups connected to an atom = number of electron pairs on the atom, thengeometry of the molecule = geometry of the electron pairs then the geometry of the molecule around that atom = geometry of the electron pairs around the atom.

6. 6 Examples H-Be-H H-Be-H 2e - pairs = linear; 2 bonded atoms = linear molecule. BCl 3 3e - pairs = trigonal planar; 3 bonded atoms = trig planar molecule

7. 7 CHHH H 4e - pairs around C = tetrahedral, 109°; 4 bonded atoms around C = tetrahedral molecule, 109° bond angles

8. 8 Perspective Rendering CH 4 : CH H H H = bond in plane of page = bond back plane of page = bond projects out at viewer

9. 9 What if #e - pairs doesn’t = # bonded atoms?? N H H H 4 e - pairs around N; max separation at 109° = tetrahedral There are only 3 H atoms connected to the e - pairs: shape of molecule describes mass points in space. So...

10. 10 three H atoms lie in one plane with the N atom above trigonal pyramidal What we see in space are the heavy mass points-- the nuclei of the atoms arranged around the N. The three H atoms lie in one plane (making a triangle), with the N atom above, like the apex of a pyramide. The molecular shape is said to be trigonal pyramidal.

11. 11 Consider H 2 O H O H Around Oxygen: 4e- pairs, arranged tetrahedrally. bentV- shaped # of Atoms connected to O = 2. Molecular shape appears bent or V- shaped: O about 109° H H

12. 12 Shapes of Compounds with multiple bonds Formaldehyde, HCHO is H - C = O H - C = O 4e - pairs on C => H BUT they must point in the same general direction in space H BUT 2 pairs are in a double bond ===> they must point in the same general direction in space...

13. 13 the C=O e - pairs are considered as 1 pair for geometry purposes. Thus: the C=O e - pairs are considered as 1 pair for geometry purposes. it seems there are only 3 e - pairs around C, and no unshared pairs. So: it seems there are only 3 e - pairs around C, and no unshared pairs. shape of molecule around O is. Result: shape of molecule around O is trigonal planar.

14. 14 Make predictions for... * Ethylene, C 2 H 4, or H - C = C - H H H * Acetylene, C 2 H 2, or H - C C - H

15. 15 New subject... Covalent Bonds - all sharing isn’t equal Some nuclei have a stronger pull for electrons than others. Electronegativity Electronegativity describes the relative strength of attraction an atom has for e -.

16. 16 Metalslow electronegativity Metals have very low electronegativity values -- they give up e - easily. Non-metalslarger electronegativity Non-metals have larger electronegativity values -- they attract e -. across a periodgoing up a Group or column. Electronegativity increases: across a period, & going up a Group or column.

17. 17 Most electronegative element = Fluorine (F). Least Electronegative = Cesium (Cs)/Francium (Fr). Pauling electronegativity scale uses 0 - 4 rating. Most electronegative = 4 Not electronegative = 0

18. 18 When two non-metals form a covalent bond, the e - pair may not be evenly shared between the two nuclei. If one element is more electronegative than the other, the e - pair is pulled closer to that nucleus.

19. 19 Example Cl is more EN than H: H - ClCl is more EN than H: the electron pair in the bond is pulled closer to Cl. The H-Cl bond is a polar covalent bond, shown as: The H-Cl bond is a polar covalent bond, shown as:  + H-Cl   The symbol  + or  - means “partial - charge.”

20. 20 A polar covalent bond has a “dipole.” (+ and - ends) Molecules may have more than one polar covalent bond. Water, H 2 O has two O-H bonds each having a dipole as  + H - O  -. The entire molecule may be polar...

21. 21 if the molecular geometry creates an over-all net dipole moment: O  - net  + H H  + Water can solvate ions because it has polarity. (More on this later.)

22. 22 A molecule with polar bonds may be non-polar overall if the individual dipoles cancel: Cl CCl 4 is Cl - C - Cl Cl Each C-Cl bond is polar as  + C-Cl  -.

23. 23 But each C-Cl bond points away in space by 109° in opposite directions: there is no net dipole moment for CCl 4. We say it is a non-polar molecule.