Electronegativity and Bond Polarity

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Electronegativity and Bond
Polarity
Bond Polarity
• So far we have assumed that when atoms share a
pair of electrons they share the electrons equally.
• However, different atoms have different
attraction for the electrons to be shared, and so
do not share equally.
Electronegativity
• Electronegativity is a measure of the ability of an
atom to attract the electrons in a bond.
• What are the trends that we see for
electronegativity in the periodic table?
• Comparisons between electronegativity values
can be used to make generalizations about the
type of bonding and types of atoms forming a
bond.
Electronegativity
• Elements such as fluorine, oxygen and nitrogen
have high electronegativities, whereas metals
have low electronegativities.
• For atoms in a molecule to share the bonding
electrons equally, the electronegativities must be
identical.
▫ This description is true for the diatomic molecules
of an element such as N2, O2, Cl2, H2 and so on.
Electronegativity
• When the electronegativities are similar, the
sharing of bonding electrons is approximately
equal.
▫ Such as carbon and hydrogen
• The greater the difference in the
electronegativities of the atoms in a compound,
the more uneven will be the sharing of electrons
between them.
Electronegativity
• The extreme of unequal sharing is the formation
of ions.
• When ions are formed, one ion loses its valence
electrons completely and the other gains valence
electrons.
• When the difference in electronegativities is
great (approximately 1.8 and above), the
compound is most likely to be ionic.
Electronegativity
• Similarly, the closer the electronegativity values
of the two atoms the more likely they are to form
a covalent compound by sharing electrons.
• The bonds formed between electronegativity
differences of between 0.5 and 1.8 are more
likely to be polar covalent, while those with an
electronegativity difference of zero will form
pure covalent bonds.
Electronegativity
• Most bonds are somewhere along a bonding type
continuum.
• For this reason electronegativity values are only
used as a general guide for identifying bonding
type.
Electronegativities of Selected
Elements
Sample Problem
• Use electronegativity values and the location of
the element in the periodic table to identify the
compound made by each pair of elements as
either covalent or ionic.
▫ Sodium and sulfur
▫ Sulfur and oxygen
▫ Lead and oxygen
Sample Problem
Element
Pair
EN
Difference
in EN
Classification
Bonding
Sodium
and sulfur
0.9
1.6
Metal
Ionic
Sulfur and
oxygen
2.5
Lead and
oxygen
1.8
2.5
Non-metal
1.0
3.5
3.5
Non-metal
Covalent
Non-metal
1.7
Metal
Non-metal
Ionic
Bond Polarity
• If the electrons are shared unevenly in a
covalent bond, the bond is said to be a
polar covalent bond, or a permanent
dipole.
• Such a bond can be identified using the symbol δ
(delta).
• In particular δ- and δ+ are used to indicate a
slight negative and a slight positive charge
respectively.
Bond Polarity
• If a polar covalent bond occurs in a diatomic
molecule, one part of the molecule will be
more negative than the other, due to having
a larger share of the bonding electrons.
▫ This is the case with molecules such as HCl and
HBr.
• The molecule is then described as a polar
molecule.
Bond Polarity
• When there is more than one polar covalent
bond in a molecule, the shape of the molecule
must be considered.
• It is possible to have molecules that
contain polar bonds but overall are nonpolar—the permanent dipoles cancel each
other out.
Bond Polarity
• An example of such a molecule is methane.
• This molecule contains four polar C-H bonds,
with carbon being slightly more electronegative than
hydrogen.
• In three dimensions, the tetrahedral
arrangement of the bonds in this molecule
means that the slight positive charges of the
hydrogen atoms are perfectly balanced out
by each other and are cancelled out by the
partial negative charge on the carbon atom.
Bond Polarity
• In fact, any molecule that is perfectly
symmetrical will be non-polar overall.
Methane
Bond Polarity
• Chloromethane (CH3Cl) is a similar molecule
to methane, but it is polar.
• Note that this molecule is not symmetrical in
three dimensions and so the dipoles cannot
cancel each other out.
• Chlorine (3.0) is more electronegative than
either carbon (2.5) or hydrogen (2.1) and so
draws the bonding electrons towards it.
Chloromethane
Carbon dioxide
• Another important example of a non-polar
molecule that nevertheless contains polar bonds
is carbon dioxide.
Practice Problems
• Identify which atom in each of the following
bond pairs will carry a slight negative charge and
which a slight positive charge.
a)
b)
c)
d)
e)
C-H
B-O
P-Cl
S-H
Si-O
Answers
a)
b)
c)
d)
e)
C slight negative charge, H slight positive charge
O slight negative charge, B slight positive charge
Cl slight negative charge, P slight positive charge
S slight negative charge, H slight positive charge
O slight negative charge, Si slight positive charge
Practice Problems
Molecule
Name
Oxygen (O2)
Dibromomethane
(CH2Br2)
Carbon
disulfide
(CS2)
Ammonia
(NH3)
Structural
Formula
Are bonds
polar?
Is the
Is the
molecule
molecular
symmetrical? polar
overall?
Answer

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