10.03 polarity of moleculesfaculty.sdmiramar.edu/faculty/sdccd/fgarces/z... · vsepr table:...

05.2015 1 1003 Bond Polarity

10.03 Polarity of Molecules

Dr. Fred Omega Garces Chemistry 152 Miramar College

Predicting Molecular Polarity


VSEPR Table: Revisited

Valence Shell Electron-Pair Repulsion Theory (VSEPR) : # e- pr

AEn Electronic Geometry Bond Pair

[Coord #] non-bond AEnBm Molecular Geometry

2 AE2 EE A Linear

2 0 AB2 B A B Linear

3 0 AB3 B AB

B Trigonal

3

AE3

EE

EA

Trigonal

2 1 AB2E AB

B

..

Bent 4 0 AB4

BA

BB

B

Tetrahedral 3 1 AB3E

BA

BB

..

Pyramidal

4

AE4

A EEE

E

Tetrahedral 2 2 AB2E2 A

BB

..

..

Bent


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


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

Symmetry and Leverage: Tug of War and Polarity


Not all Covalent bonds are based on Mutual Sharing

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

O

H HOH H

O

H H

O

H H

δ+ δ+

δ−

O

H H

O

H H

OH H

δ+ δ+

δ−

1. The oxygen atom is proceeding to form a bond with the Hydrogen atom

2. The oxygen atom and the hydrogen atom form bonds by sharing electrons

3. Because oxygen has more protons than hydrogen, it has a greater attraction for the bonded electron (it has a stronger electronegativity) this gives oxygen a δ - charge and hydrogen a δ + charge

4. This separation of partial charge results in the polar properties of water.


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

difference of EN, or Δ χ


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.


How many central atoms?

Are there lone pairs on central atom?

Are there lone pairs on any central atom?

1 only 2 or more

Polar* Are all terminal atoms the same element?

Yes No

Polar*

No

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

true for other structures, such as an octahedron.

Non-polar

Yes

Polar*

Yes

Are all central atoms the same element?

Polar

No

Are all terminal atoms the same element?

Yes

Non-polar Polarity depends on symmetry

Yes No

Determining Molecular Polarity Flowchart (Details later)

No

Δχ

None


Dipoles and its mechanism Recall the criteria for Covalent - Polar Covalent - Ionic Compounds

Molecules possessing net dipole moment are polar

Δχ


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


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 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.


Water, a special polar molecule: More later

Water is a special molecule that is unique compared to other light weight molecule. Ordinarily, light weight molecule exist as a gas at temperatures above 0°C. Example are CH4, N2, O2, SO2, NH3. The special properties of water is due to its polar properties and its ability to stick among itself. The molecular mechanism by which water sticks to themselves is due to its polar nature and something called H-bonding. H-bonding will be covered more later in this chapter.

As a result of this mechanism, water is a liquid at room temp, has a lower density in its solid state, has a low vapor pressure among others.

Properties:

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





1 only 2 or more


Yes No

Polar*

No



Non-polar

Yes

Polar*

Yes


Polar

No


Yes


Yes No


No

Δχ

None

Binary Molecules MgO

CuS FeS

HBr HF

CO NO

FF

HH

NaCl




Δχ

None

Binary Molecules

MgO CuS

FeS

HBr

HF

CO

NO

FF HH

NaCl

0.0 0.5 1.0 1.9 0.6 2.1 0.7 2.3





1 only 2 or more


Yes No

Polar*

No



Non-polar

Yes

Polar*

Yes


Polar

No


Yes


Yes No

Determining Molecular Polarity Flowchart

No

Δχ

None

CO or O2

O3

CH2F2 CO2

H2O2

F2CCH2 NΞC-Cl

H3CCH3

H3NBH3


NP Determining Molecular Polarity




1 only 2 or more


Yes No

Polar*

No

Non-polar

Yes

Polar*

Yes


Polar

No


Yes


Yes No

No

FB

F F ....

HC

HO ::

........ C CBr Br

C SS

H C N :

OOO

H

SiH H

H

H C O

O

H

C

H2N NH2

O

P

C O


Molecular Polarity: Examples

Linear BeH2 (electron deficient molecule) Trigonal BH3 (electron deficient molecule)

Tetrahedral CH4

Pyramidal NH3

Bent H2O Tetra SO2 Trig

O3 NO3- CS2 H2O2

C3H4 CH3Cl SO2 COCl2 SO3 OF2 C2H2 H2CO PF3 CI4 HCN NF3 Assignment on VSEPR and Polarity Determination


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.

10.03 polarity of moleculesfaculty.sdmiramar.edu/faculty/sdccd/fgarces/z... · vsepr table:...

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