s8pe-30802-ca
12/1/05
11:16 AM
MAZER
Page 253
Each carbon-hydrogen bond in methane is a single bond because
one pair of electrons is shared between the atoms. Sometimes atoms
may share more than one pair of electrons with another atom. For
example, the carbon atom in carbon dioxide (CO2) forms double
bonds with each of the oxygen atoms. A double bond consists of four
(two pairs of) shared electrons. Two nitrogen atoms form a triple
bond, meaning that they share six (three pairs of) electrons.
Carbon Dioxide (CO2)
O
C
O
Nitrogen (N2)
N
reading tip
Remember that each line in
the model stands for a
covalent bond—one shared
pair of electrons.
N
A group of atoms held together by covalent bonds is called a
molecule. A molecule can contain from two to many thousand
atoms. Most molecules contain the atoms of two or more elements.
For example, water (H2O), ammonia (NH3), and methane (CH4)
are all compounds made up of molecules. However, some molecules
contain atoms of only one element. The following elements exist as
two-atom molecules: H2, N2, O2, F2, Cl2, Br2, and I2.
check your reading
What is a molecule?
Polar Covalent Bonds
In an iodine molecule, both atoms are exactly the same. The shared
electrons therefore are attracted equally to both nuclei. If the two
atoms in a covalent bond are very different, however, the electrons
have a stronger attraction to one nucleus than to the other.
A covalent bond in which the electrons are shared unequally is called a
polar covalent bond. The word polar refers to anything that has two
extremes, such as a magnet with its two opposite poles.
Reading Tip
To remind yourself that
polar covalent bonds have
opposite partial charges,
remember that Earth has
both a North Pole and
a South Pole.
Water (H2O)
O
H
H
ball-and-stick model
space-filling model
In a water molecule (H2O), the oxygen atom attracts electrons
far more strongly than the hydrogen atoms do. The oxygen nucleus
has eight protons, and the hydrogen nucleus has only one proton.
The oxygen atom pulls the shared electrons more strongly toward it.
In a water molecule, therefore, the oxygen side has a slightly negative
charge, and the hydrogen side has a slightly positive charge.
VISUALIZATION
CLASSZONE.COM
Examine how electrons
move in a polar covalent
molecule.
Chapter 8: Chemical Bonds and Compounds 253
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