Essential Cell Biology 5th edition

Chemical Bonds
43
+
atoms
+
SHARING OF
ELECTRONS
+
atoms
+
TRANSFER OF
ELECTRON
Figure 2–6 Atoms can attain a more
stable arrangement of electrons in their
outermost shell by interacting with one
another. A covalent bond is formed when
electrons are shared between atoms. An
ionic bond is formed when electrons are
transferred from one atom to the other. The
two cases shown represent extremes; often,
covalent bonds form with a partial transfer
(unequal sharing of electrons), resulting in a
polar covalent bond, as we discuss shortly.
+ +
+ +
molecule
positive
ion
negative
ion
QUESTION 2–2
covalent bond
ionic bond
Because the state of the outer electron shell determines the chemical
properties of an element, when the elements are listed in order of their
atomic number we see a periodic recurrence of elements that have similar
properties. For example, an element with an incomplete second shell
containing one electron will behave ECB5 e2.06/2.06 in a similar way as an element that
has filled its second shell and has an incomplete third shell containing
one electron. The metals, for example, have incomplete outer shells with
just one or a few electrons, whereas, as we have just seen, the inert gases
have full outer shells. This arrangement gives rise to the periodic table of
the elements, outlined in Figure 2–7, in which the elements found in living
organisms are highlighted in color.
Covalent Bonds Form by the Sharing of Electrons
All of the characteristics of a cell depend on the molecules it contains.
A molecule is a cluster of atoms held together by covalent bonds, in
which electrons are shared rather than transferred between atoms. The
shared electrons complete the outer shells of the interacting atoms. In the
simplest possible molecule—a molecule of hydrogen (H 2 )—two H atoms,
each with a single electron, share their electrons, thus filling their outermost
shells. The shared electrons form a cloud of negative charge that
is densest between the two positively charged nuclei. This electron density
helps to hold the nuclei together by opposing the mutual repulsion
between the positive charges of the nuclei, which would otherwise force
them apart. The attractive and repulsive forces are precisely in balance
atomic number
1
H
1
Li
11
Na
23
19
K
39
Rb
Cs
Fr
atomic weight
Be
12
Mg
24
20
Ca
40
Sr
Ba
Ra
Sc
Y
La
Ac
Ti
Zr
Hf
Rf
23
V
51
Nb
Ta
Db
24
Cr
52
42
Mo
96
25
Mn
55
26
Fe
56
27
Co
59
28
Ni
59
29
Cu
64
30
Zn
65
5
B
11
Ga
Tc Ru Rh Pd Ag Cd In Sn Sb
W Re Os Ir Pt Au Hg Tl Pb Bi Po
Al
6
C
12
14
Si
28
7
N
14
15
P
31
8
O
16
16
S
32
34
He
Ne
Ar
Ge As Se Br Kr
79
Te
9
F
19
17
Cl
35
53
I
127
At
Xe
Rn
A carbon atom contains six protons
and six neutrons.
A. What are its atomic number and
atomic weight?
B. How many electrons does it
have?
C. How many additional electrons
must it add to fill its outermost
shell? How does this affect carbon’s
chemical behavior?
D. Carbon with an atomic weight of
14 is radioactive. How does it differ
in structure from nonradioactive
carbon? How does this difference
affect its chemical behavior?
Figure 2–7 When ordered by their atomic
number into the periodic table, the
elements fall into vertical columns in which
the atoms have similar properties. This
is because the atoms in the same vertical
column must gain or lose the same number
of electrons to attain a filled outer shell, and
they therefore behave similarly when forming
bonds with other atoms. Thus, for example,
both magnesium (Mg) and calcium (Ca) tend
to give away the two electrons in their outer
shells to form ionic bonds with atoms such as
chlorine (Cl), which need extra electrons to
complete their outer shells.
The chemistry of life is dominated by
lighter elements. The four elements
highlighted in red constitute 99% of the
total number of atoms present in the human
body and about 96% of our total weight.
An additional seven elements, highlighted
in blue, together represent about 0.9% of
our total number of atoms. Other elements,
shown in green, are required in trace
amounts by humans. It remains unclear
whether those elements shown in yellow are
essential in humans or not.
The atomic weights shown here are
those of the most common isotope of each
element. The vertical red line represents a
break in the periodic table where a group of
large atoms with similar chemical properties
has been removed.