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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