VSEPR

1. VSEPR • Valence shell electron pair repulsion • Repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible.

2. Two atoms • Linear • Electron pairs spread out as far as possible to minimize repulsive forces • Draw picture of example

3. AB2 • Be is used, it is an exception to the octet rule. • No unshared pairs of electrons. • Linear shape

4. AB3 • Equilateral triangle with all atoms on one plane • Trigonal planar • Angles 120 degrees apart • Example -- GaF3 • No unshared pairs

5. AB4 • Tetrahedron shape • Tetrahedral shape • No unshared pairs • Example CH4

6. Now, with unshared electron pairs • An unshared pair of electrons is associated with the central atom. • It is like an electron cloud shaped like a pear with one end attached to the nucleus. • FYI (a shared pair moves between two nuclei and therefore forms a more slender, stretched pear shaped cloud.)

7. continued • the unshared pair does not literally occupy space. • The unshared pair shows greater repulsion than shared electron pairs. • For geometry purposes only, we think of the unshared electron pair as occupying “space” around the nucleus.

8. AB3E • E represents the unshared pair. • Trigonal Pyramidal • Base forms a triangle, the unshared electron pair forms the top of the pyramid.

9. AB2E2 • Water --H2O • Bent, angular shape • 105 degree between the H atoms

10. Table on page 186

11. Hybridization • Carbon • Only has 2 “free” p electrons, but bonds with 4 atoms. • Forms tetrahedral shaped molecules

12. sp3 • The s orbital merges with the p orbitals to make a sp3 orbital • Hybrid orbitals • All 4 are identical

13. Other types of hybridization Hybridization explains the geometry of many group 15 and 16 elements sp linear • Sp2 trigonal planar • Sp3 tetrahedral • Table 6.6 page 189

14. Polarity • In a covalent bond the more electronegative atom will pull on the electron more. • So, the electron will spend more time around the more electronegative atom’s nuclei. • With geometry this creates a polar molecule.

15. Polar molecule • If you can draw a line through it making a partial negative and a partial positive side. • Polar molecule – uneven distribution of charge. • Polar molecules cause dipoles.

16. Intermolecular forces • Force of attraction between two molecules types: dipole - dipole induced dipole hydrogen bonding London dispersion

17. Intermolecular forces • Forces of attraction between two molecules. • Measured by boiling point- energy required for a molecule to break away from the other molecules. • Higher boiling point, stronger the attraction

18. Polar molecules • Strongest intermolecular forces • Form dipoles • Dipole • Arrow indicates the direction of the dipole • Positive to negative pole • Indicated on the bonds

19. Dipole-dipole forces • The forces of attraction between polar molecules • Short range, acts on nearby molecules • Larger the dipole dipole attraction, stronger the intermolecular forces

20. Induced dipoles • Short range • Weaker than dipole- dipole between polar molecules • Why some non-polar substances dissolve in polar water

21. Induced continued • A polar molecule comes into contact with a non-polar molecule.The partial charge either attracts or repels the electrons of the non-polar molecule. Thus creating a temporary dipole.

22. Hydrogen bonding • Some hydrogen containing compounds have unusually high boiling points. • Explained by a strong dipole dipole force called hydrogen bonding. • Represented by dashed lines---- • Intermolecular force

23. Hydrogen Bonding definition • The intermolecular forces in which a small hydrogen atom, that is bonded to a highly electronegative atom, is attracted to an unshared pair of electrons of an electronegative atom in a nearby molecule.

24. London dispersion forces • Electrons are in continuous motion. • At any given instant, the physical distribution of electrons could be uneven. • Momentary imbalance can cause a positive and negative pole. • This can then induce a dipole in a neighboring molecule!

25. London dispersion forces • The intermolecular attraction resulting from the constant motion of electrons and the creation of instantaneous dipoles.

26. Weak • Intermolecular • More electrons more possibility – stronger they are. • More electrons generally means more mass.