Molecular Geometry Exceptions and Intermolecular Forces in Chemistry

1. Section 6-5 Molecular Geometry

2. Exceptions to the Octet Rule • Hydrogen forms bonds in which it is surrounded by only two electrons. • Boron tends to form bonds in which it is surrounded by only six electrons. Ex: BF3 • Other elements can be surrounded by more than eight electrons when they combine with the highly electronegative elements fluorine, oxygen, and chlorine.

3. VSEPR Theory • Repulsion between electrons causes these sets of electrons to be as far apart as possible.

4. 2. Trigonal Planar • 3 Atoms bonded to central atom • No lone pairs of electrons • Ex: BF3

5. 3. Tetrahedral • 4 Atoms bonded to central atom • 0 Lone pairs of electrons • Ex: CH4

6. 4. Trigonal pyramidal • 3 atoms bonded to central atom • 1 lone pair of electrons • Ex: NH3

7. 5. Trigonal bipyramidal • 5 atoms bonded to central atom • 0 Lone pairs of electrons • Ex: PCl5

8. 6. Bent or Angular • 2 Atoms bonded to central atom • 1 or 2 lone pairs of electrons • Ex: H2O

9. 7. Octahedral • 6 Atoms bonded to central atom • 0 Lone pairs of electrons • Ex: SF6

10. Intermolecular Forces • Forces of attraction between molecules • Ionic bonds have stronger intermolecular forces than covalent bonds. • This contributes to a higher melting and boiling point.

11. Dipole-Dipole Forces • Polar molecules act as tiny dipoles because of their uneven charge distribution. • The negative region in one polar molecule attracts the positive region in adjacent molecules.

12. Hydrogen Bonding • Type of Dipole-Dipole force. • Occurs when hydrogen is bonded to a highly electronegative atom giving it a slightly positive charge. • Hydrogen (slightly positive) is attracted to the unshared pair of electrons on the electronegative element. • Ex: HF, H2O, NH3 • Causes these compounds to have a high boiling point

13. Hydrogen Bonding