05.2015 1 1003 Bond Polarity
10.03 Polarity of Molecules
Dr. Fred Omega Garces Chemistry 152 Miramar College
Predicting Molecular Polarity
05.2015 2 1003 Bond 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
05.2015 3 1003 Bond Polarity
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
05.2015 4 1003 Bond 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
Symmetry and Leverage: Tug of War and Polarity
05.2015 5 1003 Bond 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.
05.2015 6 1003 Bond Polarity
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
05.2015 7 1003 Bond Polarity
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.
05.2015 8 1003 Bond Polarity
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
05.2015 9 1003 Bond Polarity
Dipoles and its mechanism Recall the criteria for Covalent - Polar Covalent - Ionic Compounds
Molecules possessing net dipole moment are polar
Δχ
05.2015 10 1003 Bond Polarity
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.
05.2015 11 1003 Bond Polarity
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.
05.2015 12 1003 Bond Polarity
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
05.2015 13 1003 Bond Polarity
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
Binary Molecules MgO
CuS FeS
HBr HF
CO NO
FF
HH
NaCl
05.2015 14 1003 Bond Polarity
How many central atoms?
Determining Molecular Polarity Flowchart (Details later)
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
05.2015 15 1003 Bond Polarity
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
No
Δχ
None
CO or O2
O3
CH2F2 CO2
H2O2
F2CCH2 NΞC-Cl
H3CCH3
H3NBH3
05.2015 16 1003 Bond Polarity
NP Determining Molecular Polarity
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
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
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
05.2015 17 1003 Bond Polarity
NP Determining Molecular Polarity
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
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
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
05.2015 18 1003 Bond Polarity
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