Section 5 4Polarity of Molecules Electronegativity l Definition

Section 5. 4—Polarity of Molecules

Electronegativity l Definition: The pull an atom has for the electrons it shares with another atom in a bond. l Electronegativity is a periodic trend ¡As atomic radius increases and number of electron shells increases, the nucleus of an atom has less of a pull on its outermost electrons and the electrons within a bond!

Periodic Table with Electronegativies increases decreases

Polar Bond l A polar covalent bond is when there is a partial separation of charge l One atom pulls the electrons closer to itself and has a partial negative charge. l The atom that has the electrons farther away has a partial positive charge

How to determine bond type l Find the electronegativies of the two atoms in the bond l Find the absolute value of the difference of their values ¡If the difference is 0. 4 or less, it’s a nonpolar covalent bond ¡If the difference is greater than 0. 4 but less than 1. 9, it’s a polar covalent bond ¡If the difference is greater than 1. 9, it’s an ionic bond

Two atoms sharing equally N N • Each nitrogen atom has an electronegativity of 3. 0 (from table) • They pull evenly on the shared electrons (You can tell this by calculating the difference in their electronegativities. 3 -3=0 so they share the electrons equally! • The electrons are not closer to one or the other of the atoms • This is a non-polar covalent bond

Atoms sharing almost equally H H C H Electronegativities: H = 2. 1 H C = 2. 5 │2. 5 -2. 1 │=. 4 The carbon pulls on the electrons slightly more, pulling them slightly towards the carbon Put the difference isn’t enough to create a polar bond This is a non-polar covalent bond

Sharing unevenly H C H Electronegativities: H = 2. 1 O C = 2. 5 O = 3. 5 The carbon-hydrogen difference isn’t great enough to create partial charges │2. 5 -2. 1 │=. 4 But the oxygen atoms pulls significantly harder on the electrons than the carbon does. This does create a polar covalent bond │3. 5 -2. 5 │ = 1. 0 This is a polar covalent bond

Showing Partial Charges l 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 (THE ONE WITH GREATER ELECTRONEGATIVITY) and with a “plus” tail at the partial positive atom H + C H O - H C H O

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

Let’s Practice Example: If the bond is polar, draw the polarity arrow C–H 2. 5 – 2. 1 = 0. 4 non-polar O—Cl 3. 5 – 3. 0 = 0. 5 polar F—F 4. 0 – 4. 0 = 0. 0 non-polar C—Cl 2. 5 – 3. 0 = - 0. 5 polar

Ionic Bonds l 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

Polar Bonds versus Polar Molecules l Not every molecule with a polar bond is polar itself ¡If the polar bonds cancel out then the molecule is overall non-polar. The polar bonds cancel out. No net dipole The polar bonds do not cancel out. Net dipole

The Importance of VSEPR l You must think about a molecule in 3 -D (according to VSEPR theory) to determine if it is polar or not! Water drawn this way shows all the polar bonds canceling out. H O H H But water drawn in the correct VSEPR structure, bent, shows the polar bonds don’t cancel out! Net dipole

Let’s Practice Example: Is NH 3 a polar molecule?

Let’s Practice H N H H Example: Is NH 3 a polar molecule? Electronegativities: N = 3. 0 H = 2. 1 Difference = 0. 9 Polar bonds VSEPR shape = Trigonal pyramidal Yes, NH 3 is polar Net dipole

Rules of Thumb for Molecular Polarity: l If the molecule is one of the basic VSEPR shapes (Linear, Trigonal Planar, Tetrahedron, trigonal bipyramid, and octahedral) and has all the SAME atoms connected to the central atom – the molecule is NONPOLAR l If the molecule has one of the basic VSEPR shapes and has DIFFERENT atoms then you should calculate the bond polarities to see if the molecule is polar. l The presence of a lone pair on the central atom ALMOST always make it POLAR unless the polarities cancel. (These exceptions will be addressed in AP Chem)