Atoms That Share: Covalent
Bonds
 A water molecule is an example of the single covalent
bond between oxygen and hydrogen.
 Don't look now, but you're surrounded by covalent
bonds. They're in the air you breathe and in the water
you drink. You even make them yourself when you
exhale. What are these bonds? What do they want?
 Take a look at water (H2O). H2O is a MOLECULE, a
discrete unit of atoms that are bonded together. A
molecule of water consists of an oxygen atom that is
bonded to two hydrogen atoms.
Atoms That Share: Covalent
Bonds
(Continued)
 A hydrogen atom and the oxygen atom each donate
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one valence electron to form a chemical bond.
These two valence electrons forming the bond are
shared by both atoms, resulting in a SINGLE
COVALENT BOND.
Think of this in terms of two pieces of wood that are
nailed together.
The pieces of wood are the atoms, and the
Each piece of wood shares a portion of the nails.
Atoms That Share: Covalent
Bonds
(Continued)
A water molecule is an example of the single
covalent bond between oxygen and hydrogen.
Atoms That Share: Covalent
Bonds
(Continued)
 Air, too, contains oxygen.
 Oxygen does not exist as a single oxygen atom, but as a
molecule of two oxygen atoms.
 These two oxygen atoms share two pairs of valence
electrons (four valence electrons total) between them,
forming a DOUBLE COVALENT BOND. This is true
of any double covalent bond; four valence electrons are
shared between two atoms.
Atoms That Share: Covalent
Bonds
(Continued)
 Another component of air is nitrogen. Like oxygen,
nitrogen does not exist as a single nitrogen atom, but
as a molecule made up of two nitrogen atoms. The two
nitrogen atoms in a molecule of nitrogen share three
pairs of valence electrons (six valence electrons total)
to form a TRIPLE COVALENT BOND.
Atoms That Share: Covalent
Bonds
(Continued)
 Is it possible to predict whether bonds are covalent or
not?
 A good rule of thumb is that bonds between
nonmetals (remember that hydrogen is considered a
nonmetal) are usually covalent bonds.
 For example, the carbon dioxide (CO2) molecules you
exhale are bonded together covalently.
Atoms That Share: Covalent
Bonds
(Continued)
Oxygen molecules shares two electrons to
make a double covalent bond.
Atoms That Share: Covalent
Bonds
(Continued)
In a molecule of nitrogen, two atoms share
three electrons -- a triple covalent bond
Atoms That Share: Covalent
Bonds
(Continued)
 Polar Covalent Bonds
1.
Sometimes in a covalent bond the electrons are
not shared equally between the two atoms.
2. On average, one of the atoms partially "pulls"
the bonding electrons toward itself, creating an
unequal sharing of those bonding electrons.
This is called a POLAR COVALENT BOND.
3. In order to determine whether a covalent bond
is polar or not, it's necessary to understand
electronegativity.
Atoms That Share: Covalent
Bonds
(Continued)
 Recall that electronegativity is a measure of an atom's
ability to draw its bonding electrons to itself.
 Each element has a numeric value corresponding to its
electronegativity.
 The values used here were devised by Linus Pauling,
though there are a few other scales of electronegativity
values.
Atoms That Share: Covalent
Bonds
(Continued)
Atoms That Share: Covalent Bonds
(Continued)
 Fluorine was determined to be the most
electronegative element and has an electronegativity
value of 4.0.
 Francium, the least electronegative element, has a
value of 0.7.
 It is important to remember a general trend in the
periodic table: electronegativity increases from left to
right going across a period, and it increases from the
bottom to the top of a group.
Atoms That Share: Covalent Bonds
(Continued)
 For example, in period 2, nitrogen (group 5A) has an
electronegativity value of 3.0, compared to 2.5 for
carbon (group 4A). In group 5A, phosphorous (period
3) has an electronegativity value of 2.1, compared to 3.0
for nitrogen (period 2).
Atoms That Share: Covalent Bonds
(Continued)
 In an action that resembles toddlers tugging on a toy, a
polar covalent bond occurs when one atom with a higher
electronegativity draws the bonding electrons toward itself,
pulling those electrons away from the atom with the lower
electronegativity value.
 This creates an unequal sharing of electrons known as
UNEQUAL CHARGE DISTRIBUTION, or charge
separation.
 The charge separation makes the bond polar because the
more electronegative atom becomes partially negatively
charged and the atom with the lower electronegativity
becomes partially positively charged.
Atoms That Share: Covalent Bonds
(Continued)
Consider H2 and HBr. In H2, the atoms have an equal "pull" on
the bonding electrons, making the bond NONPOLAR.
Bonded hydrogen atoms showing equal charge
distribution
Atoms That Share: Covalent Bonds
(Continued)
 In HBr, however, Br has an electronegativity of 2.8,
compared to 2.1 for hydrogen. The Br atom pulls the
bonding electrons toward itself, creating a partial
negative charge on itself and a partial positive charge
on the hydrogen atom.
Atoms That Share: Covalent Bonds
(Continued)
 The larger the absolute difference in electronegativity
between two atoms, the more polar that bond is.
 For example, the electronegativity difference of a
carbon-oxygen bond is -1.0, the result of 2.5 (the
electronegativity value for carbon) minus 3.5 (the
value for oxygen).
 The absolute value for the difference in
electronegativity is the value without the minus sign
(1.0 for a carbon-oxygen bond).
Atoms That Share: Covalent Bonds
(Continued)
 For a carbon-chlorine bond, the difference in
electronegativity is 0.5 (2.5 - 3.0 = -0.5). Thus, a
carbon-oxygen bond is more polar (1.0) than a carbonchlorine bond (0.5).
Atoms That Share: Covalent Bonds
(Continued)
Hydrogen and bromide bond with
an unequal charge distribution.