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2.1.3.2PolarMolecules
H - His a pure covalent bondThe electrons are shared equally between the 2 atoms –theyhavethe same electronegativityElectronegativity - is a measure of the attractiveness of an atom for electrons in a shared pair (covalent bond)molecule is non-polarthe centre ofpositiveandnegativecharges are in the same place
H- Clelectronegativities 2.13.0Cl attracts shared electronsmore than Hthereforeit is slightly negative and H is therefore slightly positiveDifference is 0.9+H - Cl-this is apolar covalent bondand themolecule is also polar
Demonstrating Polarity
To show if a compound [usually a liquid] is polarPour it from a burette past a chargedrod[a pen rubbed vigorously on a woollen pullover is an easy way of doing this.]
If the liquid is not polar it will pass the rod un-deflectedThe stream will be deflected if it ispolarIf the charge of the rod is reversed the stream still bends – opposite end of molecule is attracted
Polar molecules
Non-Polarmolecules
Charged Rod
Are all molecules with polar bonds polar?
Let us look at a few examples and see.
H2O – What shape is it?Could be linearorangularIflinearit will be non polar because the centre of + and – would be in the O atomIf it is angularAs electron pair repulsion theory suggestsit will be polarIt is deflected by a charged rod so it is polar
O
H
H


+
+
-
Centre of positive charge
Centre ofnegative charge
O
H
H


+
+
-
Centre of +vecharge
Centre of-vecharge
Cl - Be - Clelectronegativities3.01.5 3.0Cl isδ-Be isδ+Bonds arepolar covalentCentre of+charge is in BeCentre of-vecharge is half way betweenCl’si.e.inthe Be atomCharges are not separated therefore the molecule is non-polarIt would flow un-deflected past a charged rod
+
-
-
Other molecular shapes
NH3 is pyramidal in shape and is polar and because the centres of positive and negative charges are not in the same placeWhile BF3istrigonalplanar and therefore non-polar because the centre of positive and negative charges are in the same place
N
H
H
H
B
F
F
F
Centre of positive charge
Centre of positive charge
Centre ofnegative charge
Centre ofnegative charge
Carbon dioxide is non-polar. What shape is it?O=C=OIt is linear because this is the only conformation that can put the centres of positive and negative charges in the same place.We will deal with this topic [molecular shapes] in more detail in another lesson
=O
=O
C
Centre ofnegative charge
Centre ofnegative charge
Centre of positive charge
Centre of positive charge
Ifa polar moleculeis placed in a charged field it will turn

+H - Cl-

Before switched on

+H - Cl-
After switched on
+
-


After switched on
+
-
+H - Cl-


After switched on
+
-
+H - Cl-

After switched on
+
-
+H - Cl-

After switched on
+
-
+H - Cl-

After switched on
+
-
+H - Cl-

After switched on
+
-
+H - Cl-
Type of bond
[Pure]CovalentElectronegativitiesdiffer by less than 0.4Polar CovalentElectronegativity differencegreater than0.4but less than 1.7IonicElectronegativity differencegreater than 1.7

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Polar Molecules - sciencequiz.net