Molecules can be classified as being polar or non-polar.
a) Polar molecules must have at least one polar bond (covalent bond between two atoms of different electronegativity) and these polar bonds cannot be arranged symmetrically. This means the dipole moments of the polar bonds do not cancel out eg. NH3 is a polar molecule because it contains three polar N-H bonds and a non-bonding pair of electrons on the central atom and because of this asymmetric arrangement of polar bonds the molecule is polar. Another example is HCN which is a linear molecule H-CºN and the two bonds are of different polarity so the dipole moments do not cancel out (the arrangement is not symmetric). Similarly CHCl3 is polar because although the tetrahedral shape can be called symmetrical, the molecule does not have a symmetric arrangement of polar bonds (so the dipole moments do not cancel out).

b) Non-polar molecules are those that either
(i) have no polar bonds eg. O2
or (ii) have polar bonds which are arranged symmetrically so that the dipole moments of the polar bonds cancel out. eg. CF4 has four polar C-F bonds but these are all equivalent and in a symmetric (tetrahedral) shape.
Some common examples of polar and non-polar molecules are
|| polar substances
non-polar substances
water, ethanol, methanol, ethanoic acid, ammonia, hydrogen chloride
cyclohexane, hydrocarbons (eg. petrol, wax, turps), tetrachloromethane

State whether each of the following molecules (whose shapes you gave on p 40) are polar or non-polar .
CH4, NH3, H2S, H2CO,
PCl3, CS2, CH3OH, PCl5,
SF6, H2SO4 XeF2 XeF4

Physical properties of polar substances

  • If an electrically charged object is placed near a stream of a polar liquid, the polar molecules will align themselves so that the oppositely charged end of the molecule is attracted to the rod. Non-polar molecules are not charged and there is no deflection of the liquid stream. This deflection of the liquid can be used to test whether the liquid is polar or not.
  • Polar molecular substances dissolve in polar solvents (eg. ethanol dissolves in water) whereas non-polar substances dissolve in non-polar solvents (eg. wax, a hydrocarbon, and solid iodine both dissolve readily in cyclohexane but not in water). This solubility behaviour is referred to as “like dissolves like”. Note that water is sometimes referred to as the universal solvent because as a polar solvent it not only dissolves many polar (and ionic) substances but is also able to dissolve some non-polar substances (eg I2) to a limited extent.
  • Molecular substances do not conduct electricity as they do not have electrically charged particles (electrons or ions) that are free to move under the influence of an electric field. However, many solutions of molecular substances conduct electricity because the molecule undergoes a hydrolysis reaction with the water to produce ions. Examples include weak acids such as CH3COOH partially dissociating in water and the hydrolysis of non metal chlorides such as PCl3.
PCl3 + 3H2O ® H3PO4 + 3H+ + 3Cl-
Romanu2�sap � mso-tab-count:10'> The bond angles are 120o.

4. Bent (or V-shaped)- occurs when the central atom is linked to only two other atoms but also contains at least one other non-bonding pair of electrons. It is commonly found in molecules of Group 16 atoms such as O and S eg. H2S and SO2
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Bond angles 109.5o.
  1. 5. Linear - occurs when the central atom is linked to two other atoms but there are no additional non-bonding electrons. Commonly found with molecules having two double bonds (eg. CO2) and is the only option for molecules containing only two atoms eg. HCl, O2.
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In a linear molecule the bond angle is 180o
6. Other shapes are less common, but occur when the central atom has more than 8 electrons.
These shapes are summarised in the table on the next page.
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Note that where there are six regions of charge the angles are all 90o. The shape of the molecule
with six bonded pairs is described as octahedral because the resulting 3D shape has 8 faces.


Give the shape of all the Lewis structures drawn in the exercise on page 32.
CH4, NH3, H2S,
H2CO, PCl3, CS2,
CH3OH, PCl5, SF6,
H2SO4, XeF2 XeF4
NO2, SF3+, SF5,
SO32, CO32, NH4+,