Polarity

Y12 Chemistry

Having poles

Opposite ends In molecules its respect to charge

Covalent bonds can be either polar or non-polar

Polarity

Electronegativity

The ability of an atom to attract

bonding electrons to itself

Every atom is neutral….

But the positive nucleus attracts bonding electrons depending on; The amount of positive charge on the nucleus of

the atom The radius of the atom

Electronegativity

Electronegativities

2.1H

1.0Li

1.5Be

2.0B

2.5C

3.0N

3.5O

4.0F

0.9Na

1.2Mg

1.5Al

1.8Si

2.1P

2.5S

3.0Cl

0.8K

1.0Ca

Increase across a period

Incr

ease

up a

gro

up

Flourine has the highest

electronegativity

Occur because the more electronegative atom

has a greater “pull” on the bonded electrons This leads to a slightly negative charge The other atom is slightly positive +

Polar Bonds

H Clδ+¿ δ −

Calculating the electronegativity difference

between two atoms will tell us if a bond is ionic, polar covalent or non polar covalent.

Ionic Covalent Continuum

<0.5 0.5 – 1.6 >1.6

IonicPolar covalent

Non polar covalent

A bond which has a positive end and a negative end is

said to have an electrical dipole The direction of the dipole is from positive to negative

Dipoles

H Clδ+¿ δ −

Many molecules contain more than one bond

Each bond may or may not be polar

Sometimes the dipoles will be balanced and sometimes they will form a net dipole in one direction

A molecule with a net dipole is a polar molecule

Polar molecules

Molecules with one bond are easy If the bond is polar the molecule is polar

Predicting polarity

H Clδ+¿ δ −

In molecules with more than two atoms and

polar bonds, the molecule will be non-polar only if the polarities of the bonds cancel each other out

Predicting Polarity

C OOCO2 is non polar

because the shape is symmetrical so the polar bonds cancel

each other out

Predicting Polarity

Identify all bonds which are polar

Establish the shape /

symmetry of the molecule

Decide whether the polarities will

cancel

Symmetrical

shapes

Linear

Trigonal planar

Tetrahedral

Unsymmetrical shapes

Bent

Trigonal pyramid

Symmetry of shapes

4 C – Cl bonds All C Cl

CCl4

Shape is symmetric

al

Polar bonds cancel

Non - polar

3 C – Cl bonds and 1 C - H 3 C Cl and 1 C H

CHCl3

Shape is symmetrica

l

Polar bonds do not

cancel outpolar

3 N – H bonds All N H

NH3

Shape is unsymmetric

al

Dipoles do not cancel

outpolar

 

Polar BondsNon-polar bondsBonded non-metal atoms identicalBonded non-metal atoms differentElectronegativity differenceNo electronegativity differencePolar moleculeNon-polar moleculeNet dipolePolar bonds cancelPolar bonds don’t cancelPolar bonds asymmetrically arrangedPolar bonds symmetrically arranged

Construct a flow-chart to identify polar molecules using the following statements:

Bonded non-metal atoms identical

Bonded non-metal atoms different

No electronegativity difference

Non-polar bonds

Electronegativity difference

Polar Bonds

Polar bonds symmetrically arranged

Polar bonds cancel

Non-polar molecule

Polar bonds asymmetrically arranged

Polar bonds don’t cancel

Net dipole

Polar molecule