They can share their electrons figure 211 each

they can share their electrons
(Figure 2.11)
. Each hydrogen
atom is now associated with 2 electrons in what amounts
to a completed valence shell. Two or more atoms held to-
gether by covalent bonds constitute a molecule , in this
case a hydrogen molecule.
Figure 2.12a shows several ways of representing a hydrogen
molecule. Its molecular formula , H
2
, simply indicates that the
molecule consists of two atoms of hydrogen. Electron sharing
can be depicted by an electron distribution diagram or by a
Lewis dot structure , in which element symbols are surrounded
by dots that represent the valence electrons (H:H). We can also
use a structural formula , H ¬ H, where the line represents a
single bond , a pair of shared electrons. A space-filling model
comes closest to representing the actual shape of the molecule.
Oxygen has 6 electrons in its second electron shell and
therefore needs 2 more electrons to complete its valence
shell. Two oxygen atoms form a molecule by sharing two
pairs of valence electrons
(Figure 2.12b)
. The atoms are thus
joined by a double bond (O “ O).
Figure 2.12 Covalent bonding in four molecules. The
number of electrons required to complete an atom’s valence shell
generally determines how many covalent bonds that atom will form.
This figure shows several ways of indicating covalent bonds.
O O
H
H
H H
C
H
H
O
H H
H
Electron
Distribution
Diagram
Name and
Molecular
Formula
Lewis Dot
Structure and
Structural
Formula
Space-
Filling
Model
H
O H
H
C H
H
H
H
O O
(a) Hydrogen (H
2
).
Two hydrogen atoms
share one pair of
electrons, forming
a single bond.
(b) Oxygen (O
2
).
Two oxygen atoms
share two pairs of
electrons, forming
a double bond.
(c) Water (H
2
O).
Two hydrogen
atoms and one
oxygen atom are
joined by single
bonds, forming a
molecule of water.
(d) Methane (CH
4
).
Four hydrogen
atoms can satisfy
the valence of
one carbon
atom, forming
methane.
H H
•
•
O
H
H
•
•
•
•
• •
• •
C
H
H
H H
•
•
•
•
• •
• •
O
•
•
•
•
• •
• •
O
• •
• •

CHAPTER 2
The Chemical Context of Life 39
Each atom that can share valence electrons has a bonding
capacity corresponding to the number of covalent bonds the
atom can form. When the bonds form, they give the atom a
full complement of electrons in the valence shell. The bond-
ing capacity of oxygen, for example, is 2. This bonding capac-
ity is called the atom’s valence and usually equals the number
of unpaired electrons required to complete the atom’s outer-
most (valence) shell. See if you can determine the valences of
hydrogen, oxygen, nitrogen, and carbon by studying the elec-
tron distribution diagrams in Figure 2.9. You can see that the
valence of hydrogen is 1; oxygen, 2; nitrogen, 3; and carbon, 4.
However, the situation is more complicated for elements in
the third row of the periodic table. Phosphorus, for example,
can have a valence of 3, as we would predict from the pres-
ence of 3 unpaired electrons in its valence shell. In some mol-
ecules that are biologically important, however, phosphorus
can form three single bonds and one double bond. Therefore,
it can also have a valence of 5.