5.04 Polarity of Molecules - San Diego Miramar College

5.04 Polarity of Molecules

Predicting Molecular Polarity

Dr. Fred Omega Garces

Chemistry 100
Miramar College

1 5.04 Bond Polarity January 10

VSEPR Table: Revisited

Valence Shell Electron-Pair Repulsion Theory (VSEPR) :

# e- pr AEn Electronic Geometry Bond Pair non-bond AEnBm Molecular Geometry
2
AE2 EA E [Coord #] BAB
2 0 AB2 Linear

3 AE3 Linear 3 0 AB3 B
2 1 AB2E
E BA..
E AE B Trigonal

Trigonal B

4 0 AB4 A

B Bent
B

B A B
B Tetrahedral
E ..
4 AE4 E AEE 3 1 AB3E
2 2 AB2E2 A
Tetrahedral B B

B Pyramidal
..

.. A Bent

B
B

2 5.04 Bond Polarity January 10

Magnetic & Molecules

Molecules and Magnets
Molecules possess magnet-like properties. Magnets have poles (+)-
end (-) end. Molecules too can behave similarity to that of magnets
with poles (dipole).

Magnets:
Magnet behavior - polar (with poles)
Non magnet behavior - nonpolar (without poles)

Molecules

3 5.04 Bond Polarity January 10

Symmetry and Leverage: Tug of War and Polarity

The Charge distribution may cancel out (nonpolar) or there may be
a net distortion (polar)

Analogy:
1. No one wins: nonpolar

2. One team wins: polar

3. a) no one wins: nonpolar b) one team wins: polar c) two team wins polar

4 5.04 Bond Polarity January 10

Not all Covalent bonds are based on Mutual Sharing

Not all covalent bonds are made by bonding electrons being
mutually shared between atoms

HH HO H O O
HH
O O HH
HH δ−
1. The oxygen δ− O
atom is proceeding 2. The oxygen O HH
to form a bond atom and the δ+ δ+
with the Hydrogen hydrogen atom HH
atom form bonds by δ+ δ+ 4. This
sharing electrons separation of
3. Because oxygen has partial charge
more protons than results in the
hydrogen, it has a polar properties
greater attraction for of water.
the bonded electron (it
has a stronger
electronegativity) this
gives oxygen a δ - charge
and hydrogen a δ + charge

5 5.04 Bond Polarity January 10

Electronegativity

Compounds: When atoms combine to form compounds, electrons can either transfer from one atom

to another, partially transfer from one atom to another or the electrons can be mutually shared
between atoms.

Transfer, partially transferred or equally shared: The behavior of the electrons is determined by
the strength by which atoms attracts the bonding electrons.

Electronegativity. The degree of which an atom attracts an electron. The greediness of an atom for
bonding electrons. Small atoms with the highest ionization energy have the highest
Electronegativity.

Most Stingy Atoms near the north
east corner of the periodic table
are the most electronegative.

Degree of polarity is based on 5.04 Bond Polarity January 10

difference of EN, or Δ χ

6

Electronegativity

When there is an unequal sharing of electrons between two atoms; the
more stingy atom draws electrons bonded to itself.

Electronegativity - Ability of an atom to draw bonded e- on to itself.

If an atom is “stingy” for bonded electrons, then that atom is more
electronegative that the atom it is bonded to.

Unequal sharing of bonding electrons lead to dipolar bonds. January 10

7 5.04 Bond Polarity

Determining Molecular Polarity Flowchart (Details later)

How many central atoms? None Δχ
2 or more
1 only

Are there lone pairs Are there lone pairs
on central atom? on any central atom?

Yes No Yes No

Polar* Are all terminal atoms Polar* Are all central atoms
the same element? the same element?

No Yes Yes No

Polar* Non-polar Are all terminal atoms Polar
the same element?

No Yes

*This is true for everything that has up to 4 regions of electron Polarity depends Non-polar
density (i.e., what we’re studying in this class). It may not be on symmetry

true for other structures, such as an octahedron.

8 5.04 Bond Polarity January 10

Dipoles and its mechanism

Recall the criteria for Covalent - Polar Covalent - Ionic Compounds

Δχ

Molecules possessing dipole moment are polar

9 5.04 Bond Polarity January 10

Polar Bonds and Polar Molecules

Polar Bonds-
Electron cloud distortion due to electronegative atom drawing
electrons on to itself.

Polar Molecules
If the electron cloud is distorted over the entire molecule (i.e.,
winner in the tug-of-war) then molecule is polar.

Nonpolar molecule
If bond is not polar or if polar bonds cancel each other (no winner in
the tug-of-war), then molecule is non-polar.

10 5.04 Bond Polarity January 10

Polar Molecules in the presence of a magnetic field

Molecules will align along a magnetic field based on their polar properties.

The H2 molecules shown on the left are not influenced by a magnetic field since January 10
H2 is a nonpolar chemical. On the other hand the HF molecule orientated
themselves such that the the partially negative fluoride are orientated towards
the positive charge in the magnetic field. This also applies to molecules with
more than two element. In H2O, the molecule is polar and it orientates itself in
the presence of a magnetic field. Note however that dichloroethyne is nonpolar
and is not influenced by a magnetic field.

11 5.04 Bond Polarity

Water, a special polar molecule: More later

Water is a special molecule that is unique compared to other As a result of this
light weight molecule. Ordinarily, light weight molecule exist as mechanism, water is a
a gas at temperatures above 0°C. Example are CH4, N2, O2, liquid at room temp, has
SO2, NH3. The special properties of water is due to its polar a lower density in its
properties and its ability to stick among itself. The molecular solid state, has a low
mechanism by which water sticks to itself is due to its polar vapor pressure among
nature and something called H-bonding. H-bonding will be others.
covered more in chapter 9.
Properties:
12 5.04 Bond Polarity
Density - low density

Surface tension - bugs
walking on water

Capillary action - method of
which trees drink

Specific Heat

Viscosity - high viscosity

Boiling point / melting point
Higher than expected:
Heat of fusion/
Vaporization

Vapor pressure - low vapor
pressure

January 10

F Determining Molecular Polarity P NP
B
FF H .. .. .. H H C N:
C O.. : B..r C C B..r : H Si H
CO O OO
H H
O
S CS

How many central atoms?

1 only 2 or more

H CO H Are there lone pairs Are there lone pairs
on any central atom?
H2N O on central atom?
C Yes No
Yes No
NH2
Polar* Are all terminal atoms Polar* Are all central atoms
the same element? the same element?

No Yes Yes No

Polar* Non-polar Are all terminal atoms Polar
the same element?

No Yes

Polarity depends Non-polar
on symmetry

13 5.04 Bond Polarity January 10

Summary

Key to determine Polarity of molecule.

• Lewis Structure

Atoms with different EN - Polar.

• VSEPR Structure

Symmetry (or distortion) of e- cloud; (tug-of-war).

Central atom (A) has lone pair of electrons.

• Polarity

Net distortion of the electron Cloud.

15 5.04 Bond Polarity January 10