MOLECULAR POLARITY Ch 8

BOND VS. MOLECULAR POLARITY

Covalent Bonds: Covalent Bonds: Dogs of equal strength. Covalent bonds can be thought of as two or more dogs with equal attraction to the bones. Since the dogs (atoms) are identical (same electronegativities, then the dogs share the pairs of available bones evenly. Since one dog does not have more of the bone than the other dog, the charge is evenly distributed among both dogs. The molecule is not "polar" meaning one side does not have more charge than the other.

Electronegativity The ability of an atom to attract electrons when the atom is in a compound. See Table for Individual Element Values

Polar Covalent Bonds: Unevenly matched but willing to share. These bonds can be thought of as two or more dogs that have different desire for bones (different electronegativities). The bigger dog has more strength to possess a larger portion of the bones. Sharing still takes place but is an uneven sharing. In the case of the atoms, the electrons spend more time on the end of the molecule near the atom with the greater electronegativity (desire for the electron) making it seem more negative and the other end of the molecule seem more positive.

BOND POLARITY 1. Identical non-metallic atoms have the same 1. Identical non-metallic atoms have the same electronegativities and form non-polar bonds. electronegativities and form non-polar bonds. -electrons are shared equally between the two atoms -electrons are shared equally between the two atoms EX:F 2 = EX:F 2 = nonpolar covalent bond 2. When two non-metallic atoms have different electronegativities (< 1.67) they form polar electronegativities (< 1.67) they form polar covalent bonds. covalent bonds. -electrons are shared unequally between the two atoms EX:O-H = EX:O-H = polar covalent bond

BOND POLARITY Electronegativity difference (|x|) Type of bond Description nonpolar covalent bond Equal sharing of electrons between nonmetals polar covalent bond Unequal sharing of electrons between nonmetals >1.67Ionic Give and Take of electrons between metal and nonmetal

To determine polarity First – identify if bond is ionic or covalent Second- if there is a metal and nonmetal it is IONIC it is IONIC (even if subtraction says otherwise) (even if subtraction says otherwise) if its COVALENT (non metals) if its COVALENT (non metals) - determine if bond is - determine if bond is nonpolar or polar covalent (even if subtraction says otherwise) nonpolar or polar covalent (even if subtraction says otherwise) NOTE: REMEMBER TO TAKE ABSOLUTE VALUE

CALCULATIONS:  F 2 F – F –| 4.0 – 4.0| = 0Nonpolar Covalent (NPC)  CO 2 C – O –| 2.51 – 3.5| = 0.99Polar Covalent (PC)  NaClNa – Cl –| 0.9 – 3.0| = 2.1Ionic

DRAWINGS:  Delta (-)Indicates the more electronegative  Delta (+)Indicates the lower electronegative atom  O= 2.5 H = 2.1

1. Must have at least one polar bond 2. One end of molecule is (+) & the other end is (-) 3. Polar molecules are also known as dipoles 3. Polar molecules are also known as dipoles 4. Uneven distribution of electrons in the molecule 4. Uneven distribution of electrons in the molecule Nonpolar covalent polar covalent POLAR MOLECULES

2 Things Determine Whether a Molecule is Polar: 1. Polarity of a Molecule’s Bonds - Electronegativity differences of atoms - Use Table to determine if a bond is Nonpolar covalent, Polar covalent, or Ionic 2. Shape of a Molecule (Molecule Polarity) - If molecule is made of more than two atoms, the shape of the molecule determines the polarity

So, if bond is polar….. Then look at the molecules shape. EX: H 2 O O-H polar covalent bond O-H polar covalent bond Look at shape of water Bent, so molecule is also polar

BUT….. Sometimes you have polar bonds and a nonpolar molecule.

If polar bonds are distributed evenly throughout the molecule, then the molecule is: NONPOLAR COVALENT NONPOLAR COVALENT EX: CCl 4 - C-Cl bond is Polar covalent, but overall molecule is Nonpolar due to the shape of the molecule. Nonpolar due to the shape of the molecule. -Symmetrical Distribution cancels out the effects cancels out the effects of bond polarity. of bond polarity. POLAR BONDS BUT NONPOLAR MOLECULE

ONLY SYMMETRICAL MOLECULES CANCEL OUT BOND POLARITY Polar bonds= =.41 Nonpolar molecule= Due to the symmetrical shape of the molecule

Molecular Geometry Picture Linear Tetrahedral Pyramidal Bent ONLY SYMMETRICAL MOLECULES CANCEL BOND POLARITY

MOLECULAR POLARITY MOLECULAR POLARITY

Intermolecular Forces 1. Attraction between 1. Attraction between MOLECULES 2. These forces are weaker than either ionic or covalent bonds. ionic or covalent bonds.TYPES: A. Van der Waals Forces 1. Dipole interactions 2. Dispersion Forces B. Hydrogen Bonds

Van der Waals Forces the two weakest attractions between molecules the two weakest attractions between molecules 1. Dipole Interactions: -Occur when polar molecules are -Occur when polar molecules are attracted to one another attracted to one another Ex: H 2 O is attracted to another H 2 O 2. Dispersion Forces: -The weakest of all intermolecular interactions -The weakest of all intermolecular interactions -Occurs between nonpolar molecules -Occurs between nonpolar molecules -Caused by the motion of e-s -Caused by the motion of e-s Ex: F 2, Cl 2 (gases), Br 2 (liquid), I 2 (solid) Ex: F 2, Cl 2 (gases), Br 2 (liquid), I 2 (solid)

Dipole Interaction -- ++

Dispersion Forces Br δ - δ + More e- Br δ - δ + More e-

Hydrogen Bonds 1. When a hydrogen atom that is bonded to a very electronegative atom is attracted to an unshared pair of another electronegative atom. 2. When H is bonded directly to an O,N,F 3. Strongest of Intermolecular Forces

Hydrogen Bonding

CHAPTER 15 SECTION 1: WATER AND ITS PROPERTIES

Water and Its Properties  High surface tension and low vapor pressure result from hydrogen bonding.  Solid water has a lower density than liquid water. Review article on Removing Stains