3.3 Polar Covalent Bonds Ch. 3 Test: Monday, Sep. 23 & Polar Molecules

Homework Reading: Pg. 85-93 Practice: Pg. 86 # 11-13 Sec. Review: Pg. 94 # 1-8

Polar Covalent Bonds A covalent bond in which the electrons are not shared equally. Like a tug of war between a wrestle and a chemistry teacher. The difference in electronegativities is between 0.5 and 1.7 (<0.5 is nonpolar, >1.7 is ionic) (see p 73) Fluorine can NOT form a nonpolar bond!!

resulting in partial charge. Cl 3.16 2.20 Two atoms with a small difference in electronegativity will share unequally, resulting in partial charge.

resulting in partial charge. d – d + H Cl Two atoms with a small difference in electronegativity will share unequally, resulting in partial charge.

The bonding pair is, on average, Cl This is a polar bond: The bonding pair is, on average, closer to one atom.

Bonds can be polar and so can molecules!! How polarity is measured by the dipole moment.

Dipole Moment A substance possesses a dipole moment (is polar) if its centers of positive and negative charge do not coincide or if it has a (+) and a (-) end. + — not polar 17

Dipole Moment A substance possesses a dipole moment (is polar) if its centers of positive and negative charge do not coincide or if it has a (+) and a (-) end. — + polar 17

Molecular Dipole Moments Is it polar? To be polar a molecule must have polar bonds They are necessary, but not sufficient!! We need to know molecular shape because individual bond dipoles can cancel! 18

Molecular Dipole Moments C-O is a polar bond (0.8) Carbon is more (+) & oxygen more (-) Carbon dioxide has no dipole moment because they cancel out. There is not a (+) end & a (-) end. 18

4 polar bonds cancel nonpolar Dichloromethane 2 polar bonds & 2 nonpolar bonds Overall molecule is polar. Chlorine is (-) end. Carbon tetrachloride 4 polar bonds cancel nonpolar 20

Resultant of these two bond dipoles is NONPOLAR Carbon tetrachloride has no dipole moment because all of the individual bond dipoles cancel. 20

Resultant of these two bond dipoles is POLAR The individual bond dipoles do not cancel in dichloromethane; it has a dipole moment. (The C-H bond is not polar, but C-Cl is 0.61) 20

The two hydrogens of water are not symmetrically positioned around the oxygen.

d + d – d + The O-H polarities do not cancel, and the molecule carries a partial charge.

d – d + d + d – Water is a liquid instead of a gas because the partial positives and negatives attract each other.

d + d + d – d – This attraction holds the molecules together, forming a liquid rather than the spread out molecules of a gas.

When will they cancel? If all outer atoms are the same, the following will be nonpolar: 3 atom linear (2 atom linear can’t cancel) Trigonal planar Tetrahedral Trigonal bipyramidal Octahedral

Let’s Try! Are the bonds polar? Is the molecule polar? CF4 Br2O SiO2 Bond: Y Molecule: N Bond: Y Molecule: Y Bond: Y Molecule: N

These are a little harder! Are the bonds polar? Is the molecule polar? CNBr NSF POCl Bonds: N Molecule: N Bond: 1 Y & 1N Molecule: Y Bond: 2 Y Molecule: Y