Chem-To-Go Lesson 18 Unit 4 COVALENT BONDING – POLARITY & INTERMOLECULAR FORCES

The Lewis dot structure of a compound gives you hints about some of the compound’s physical properties. PHYSICAL PROPERTIES

Melting: process by which the particles of a substance begin to move more quickly and further apart Boiling: process by which the particles of a substance begin to move EVEN more quickly and EVEN further apart

The first step in predicting simple physical properties like melting point and boiling point is to determine the polarity of a molecule. POLARITY OF THE MOLECULE Polar MoleculesNonpolar Molecules Have a lone pair of electrons on the central atom in the Lewis dot structure OR Have one terminal atom that differs from the other terminal atoms (asymmetrical) Have no lone pair of electrons on the central atom AND All terminal atoms are similar (symmetrical)

EXAMPLE #1: CH 4 Does the molecule have a lone pair of electrons on the central atom? OR Have one terminal atom that differs from the other terminal atoms? (asymmetrical)

EXAMPLE #2: NH 3 Does the molecule have a lone pair of electrons on the central atom? OR Have one terminal atom that differs from the other terminal atoms? (asymmetrical)

INTERMOLECULAR FORCES Water molecules Polar molecule due to a lone pair of electrons on the central O atom Since O has more electrons surrounding it, the O end of the bond is partially negative. This leaves the H end of the bond partially positive. Opposites attract: The partially positive H end of one molecule is attracted to the partially negative O end of another molecule. INTER = BETWEEN

1)London dispersion forces (LDF) – Weakest Temporary Between nonpolar molecules Result in low melting & boiling points (easy to melt & boil) 2)Dipole-dipole forces – Forces of attraction/repulsion that exist between POLAR molecules as a result of the partial charges Result in high melting & boiling points (difficult to melt & boil) 3)Hydrogen bonding – SPECIAL TYPE OF DIPOLE FORCE STRONGEST intermolecular force Occurs when H is bonded to FON creating REALLY strong partial charges Results in highest melting & boiling points (most difficult to melt & boil) TYPES OF INTERMOLECULAR FORCES

EXAMPLE #3: Br 2 Is the molecule polar? London dispersion forces Nonpolar only Dipole-dipole forces Polar only Hydrogen bonding Polar AND H bonded to F, O, or N

EXAMPLE #4: HCl Is the molecule polar? London dispersion forces Nonpolar only Dipole-dipole forces Polar only Hydrogen bonding Polar AND H bonded to F, O, or N

EXAMPLE #5: CO 2 Is the molecule polar? London dispersion forces Nonpolar only Dipole-dipole forces Polar only Hydrogen bonding Polar AND H bonded to F, O, or N

EXAMPLE #6: H 2 O Is the molecule polar? London dispersion forces Nonpolar only Dipole-dipole forces Polar only Hydrogen bonding Polar AND H bonded to F, O, or N