Section 5.4—Polarity of Molecules

Section 5.4—Polarity of Molecules • Objectives • examine the trend of electronegativity on the periodic table • use electronegativity to determine the polarity of covalent bonds

Electronegativity • Electronegativity is the attraction an atom has for the electrons it shares with another atom in a bond. • Electronegativity has a periodic and group trend

increases decreases Periodic Table with Electronegativies

Polar Bonds • Recall from Sec. 5.1, that covalent bonds (where electrons are shared between 2 atoms) can be nonpolar or polar. • In nonpolar covalent bond, electrons are shared equally but in a polar covalent bond, they are not shared equally. • The electronegativity difference between the 2 atoms in the bond determines the type of covalent bond that forms.

N N Nonpolar: Two atoms sharing equally Each nitrogen atom has an electronegativity of 3.0. There is no electronegativity difference. They pull evenly on the shared electrons so, the electrons are not closer to one or the other of the atoms. This results in a non-polar covalent bond.

H H C H H Nonpolar: Atoms sharing almost equally Electronegativities: H = 2.1 C = 2.5 Electronegativity difference of each bond: 0.4 The carbon pulls on the electrons slightly more (having a higher electronegativity), but the difference isn’t enough to create a polar bond. This is a non-polar covalent bond.

H C O H Polar: Two atoms share unequally Electronegativities: H = 2.1 C = 2.5 O = 3.5 The C-H difference is 0.4; the C-O difference is 1.0. The C-H bonds are nonpolar. But, the oxygen atoms pulls significantly harder on the electrons than the carbon does. This is a polar covalent bond!

A polar covalent bond results in one end of the bond (and molecule) having a slight negative charge and one end having a slight positive charge. • The atom having a higher electronegativity pulls the electrons closer to itself and, so, has a partial negative charge. • The atom with the lower electronegativity has the electrons farther away and, so, has a partial positive charge.

+ - H C O H C O H H Showing Partial Charges • There are two ways to show the partial separation of charges • Use of “” for “partial” • Use of an arrow pointing towards the partial negative atom with a “plus” tail at the partial positive atom

Ionic Bonds • Ionic bonds occur when the electronegativies of two atoms are so different that they can’t even share unevenly…one atom just takes them from the other

How to determine bond type • Find the electronegativies of the two atoms in the bond • Find the absolute value of the difference of their values • If the difference is 0.4 or less, it’s a non-polar covalent bond • If the difference is greater than 0.4 but less than 1.4, it’s a polar covalent bond • If the difference is greater than 1.4, it’s an ionic bond

Let’s Practice C – H O—F F—F C—Cl Example: If the bond is polar, draw the polarity arrow

Let’s Practice 2.55 – 2.2 = 0.35 nonpolar 3.44 – 3.98 = 0.54 polar 3.98 – 3.98 = 0.0 non-polar 2.55 – 3.16 = 0.61 polar C – H O—F F—F C—Cl Example: If the bond is polar, draw the polarity arrow

Section 5.4—Polarity of Molecules