Bonding, Molecular Shape & Structure

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E.g. F-F (4.0 – 4.0 = 0) is non-polar covalent

H-F (4.0 – 2.1 = 1.9) is polar covalent

LiF (4.0 – 1.0 = 3.0) is ionic

+

-

Slide 23

Slide 24

Dipole Moment occurs in any polar covalent bond, because of an unequal sharing of the electron pair between two atoms

E.g. Which of the following bonds is most polar: S-Cl,

S-Br, Se-Cl or Se-Br?

S-Cl (3.0 – 2.5) = 0.5

S-Br (2.8-2.5) = 0.3

Se-Cl (3.0-2.4) = 0.6

Se-Br (2.8-2.4) = 0.4

Therefore, Se-Cl is the most polar!

We should be able to reach the same conclusion using the Periodic Table,

Cl is furthest to the right and to the top of the Periodic Table, so is the most electronegative. Se is furthest to the left (‘metallic like’) and towards the bottom. Therefore, difference in electronegativity should be the greatest!

Slide 25

Electronegativity difference decreases as bond length increases

Dipole Moment: µ = Qr

Dipole moment is defined as the magnitude of charge (Q)

multiplied by the distance between the charges;

units are D (Debye) = 3.36 x 1030 C.m

Prof. Peter Debye

Noble Prize 1936

Slide 26

When proton & electron 100 pm apart, the dipole moment is 4.80 D

4.8 D is a key reference value! It represents a pure charge of +1 and -1, which are 100 pm (100pm = 1Å) apart. The bond is said to be 100% ionic!

H-F; µ = 1.82 D (measured) bond length = 0.92 Å

If 100% ionic,

µ = 92/100 (4.8 D) = 4.42 D

% ionic = 1.82/4.42 x 100 = 41 % ionic

H-Cl; µ = 1.08 D (measured) bond length = 1.27 Å

If 100% ionic,

µ = 127/100 (4.8 D) = 6.10 D

% ionic = 1.08/6.10 x 100 = 18 % ionic

H-Br; µ = 0.82 D (measured) bond length = 1.41 Å

If 100% ionic,

µ = 141/100 (4.8 D) = 6.77 D

% ionic = 0.82/6.77 x 100 = 12 % ionic

Slide 27

Polar Molecules = Molecules with permanent dipole moments

HCl has only one covalent bond (which is polar). Therefore, its dipole moment = H-Cl bond dipole

In a molecule with two or more polar bonds, each bond has a dipole moment contribution = bond dipole

Net dipole moment = vector sum of its bond dipoles

Linear Molecules: CO2 is Non-polar

Because CO2 dipoles are orientated in opposite directions.

The dipoles have equal magnitudes; they cancel

Net dipole = 0

Slide 28

Symmetrical molecules (e.g. CCl4, CH4) are non-polar. The four dipoles are of equal magnitude and neutralize one another at the center of a tetrahedron