Electronegativity
• In a covalent bond between identical atoms
eg. H2 and Cl2 the electrons are shared equally
between the two atoms
• In covalent bonds between different atoms eg.
HCl and CO2 one of the atoms attracts the
electrons more than the other
• In HCl it is found the electrons are more
attracted to the chlorine atom
• As the electrons spend more time nearer the
Chlorine atom the chlorine atom gets a slight
negative charge and the hydrogen atom gets a
slight positive charge
• The small amount of charge is indicated by the
Greek symbol “delta” δ
• The ability of an atom to attract electrons to
itself in a covalent bond is called its
Electronegativity
• Electronegativity is the relative attraction
that an atom in a molecule has for the shared
pair of electrons in a covalent bond
• Each atom has a certain “pulling power” on
electrons
• Linus Pauling an American chemist found that
Fluorine has four times the “pulling power” of
calcium
• Flourine has an electronegativity value of 4.0
Calcium has an electronegativity of 1.0 all
other elements have electronegativity values
relative to these
A copy of this table is in your
mathematical tables
• Elements with low electronegativity values are
said to be Electropositive
Polar covalent bonds
• Because molecules such as HCl contain atoms
with different electronegativity values and
each atom has a slight charge the molecule is
said to be a polar molecule
Remember
• There are some molecules that have polar
bonds but are not polar molecules
• This happens when the centre of gravity of the
partial negative charges coincides with the
partial positive charges
Uses of electronegativity Values
1. We can use our knowledge of
electronegativity values and the shapes of
molecules to predict whether covalent bonds
are polar or non polar
Significance of polarity in molecules
• If water were not polar it would be a gas at
room temperature and life could not exist!
• Water is known to be an excellent solvent this
is because most ionic and polar covalent
substances will dissolve in water as there is an
attraction between the charges in the water
molecules and the charges in the ionic and
covalent molecules (detailed explanation p62)
Like dissolves like
• Polar and ionic substances will dissolve in
polar liquids such as water (HCL, NH3)
• Non polar substances such as methane, oil
and most plastics will not dissolve in water but
will dissolve in non polar liquids such as
tetrachloroethene or hexane
• Dry cleaners use tetrachloroethene to dissolve
oily stains from clothes
The purple iodine crystals below will not
dissolve in water but will dissolve in carbon
tetrachloride this is because iodine is non
polar
2. We can use electronegativity values to predict
whether bonds are ionic or covalent
An electronegativity difference greater than 1.7
indicates ionic bonding
An electronegativity difference of less than 1.7
indicates polar covalent bonding
An electronegativity difference of 0 indicates a
pure covalent bond
NB !!! A few exceptions
•
•
•
•
Lithium hydride LiH
Sodium Hydride NaH
Potassium Hydride KH
Calcium Hydride CaH2
• Are all ionic and disobey the “rule of thumb”
Try the following Questions
• P68 s,t,u 5.2 5.3