Covalent
bonds
and
molecules
When two or
more atoms
share
electrons, they form a
m olecule
(MOL-
e-kul). A molecule may consist of two or more atoms of
the same element (
Figu re 2.5a-c
) or two or more atoms
of different elements (
Figu re 2 .5 d ,e
). When
different
ele-
ments combine to form a molecule, we call that substance
a
compound.
Like ionic compounds, covalent compounds
have chemical properties that are different from those of
the individual elements.
A
m o lecu lar fo rm u la
indicates the number and type
of atoms that make up a molecule. In the molecular for-
mula O2, the subscript 2 indicates there are two atoms of
oxygen. In the water molecule, H2O, one atom of oxygen
shares electrons with two atoms of hydrogen. (Note that
O2
is a molecule but not a compound because it is com-
posed of identical elements.)
The shared electron pair between two atoms is called
a
covalent b o n d
. The greater the number of covalent
Covalent bond formation and molecules • Figure 2.5
When two or more atoms interact by sharing valence electrons, they form molecules. The
shared electron pairs are called covalent bonds. In some molecules—for example, oxygen b.
and nitrogen c.—two atoms may share two or three pairs of electrons and form double or
triple covalent bonds. Lines in structural formulas represent covalent bonds.
DIAGRAMS OF ATOMIC AND MOLECULAR STRUCTURE
b.
Hydrogen atoms
H
o
+
O H
Oxygen atoms
+
Nitrogen atoms
O
(
h
^
O r ©
o
+ 0
H J
o
O
o
H
o
o
,<H>.
e.
Oxygen atom
Hydrogen atoms
+
°
" ^
°
"
?
Hydrogen molecule
(
h
©
Oxygen molecule
Nitrogen molecule
-o-
O '"
" —O-
Methane molecule
h
o
o
o
©
l
h
5
o
o
o
H
Water molecule
s+
-►s f ( ®
"
s+
a.
c.
STRUCTURAL
MOLECULAR
FORMULA
FORMULA
H
H
H 2
O
=
O
O 2
N
=
N
N 2
H
I
H —
C — H
C H 4
I
H
"
/
O
" 2O
\
2
"
28 CHAPTER 2
Introductory Chemistry
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