Essential Cell Biology 5th edition

42 CHAPTER 2 Chemical Components of Cells
Figure 2–5 An element’s chemical
reactivity depends on the degree to
which its outermost electron shell is filled.
All of the elements commonly found in
living organisms have outermost shells that
are not completely filled. The electrons in
these incomplete shells (here shown in red )
can participate in chemical reactions with
other atoms. Inert gases (yellow), in contrast,
have completely filled outermost shells
(gray) and are thus chemically unreactive.
atomic number
electron shell
element I II III IV
1 Hydrogen (H)
2 Helium (He)
6 Carbon (C)
7 Nitrogen (N)
8 Oxygen (O)
10 Neon (Ne)
11 Sodium (Na)
12 Magnesium (Mg)
15 Phosphorus (P)
16 Sulfur (S)
17 Chlorine (Cl)
18 Argon (Ar)
19 Potassium (K)
20 Calcium (Ca)
QUESTION 2–1
A cup containing exactly 18 g, or
1 mole, of water was emptied into
the Aegean Sea 3000 years ago.
What are the chances that the same
quantity of water, scooped today
from the Pacific Ocean, would
include at least one of these ancient
water molecules? Assume perfect
mixing and an approximate volume
for the world’s oceans of 1.5 billion
cubic kilometers (1.5 × 10 9 km 3 ).
most tightly bound shell. This innermost shell can hold a maximum of
two electrons. The second shell is farther away from the nucleus, and
can hold up to eight electrons. ECB5 e2.05/2.05 The third shell can also hold up to eight
electrons, which are even less tightly bound. The fourth and fifth shells
can hold 18 electrons each. Atoms with more than four shells are very
rare in biological molecules.
The arrangement of electrons in an atom is most stable when all the
electrons are in the most tightly bound states that are possible for them—
that is, when they occupy the innermost shells, closest to the nucleus.
Therefore, with certain exceptions in the larger atoms, the electrons of an
atom fill the shells in order—the first before the second, the second before
the third, and so on. An atom whose outermost shell is entirely filled
with electrons is especially stable and therefore chemically unreactive.
Examples are helium with 2 electrons (atomic number 2), neon with 2 + 8
electrons (atomic number 10), and argon with 2 + 8 + 8 electrons (atomic
number 18); these are all inert gases. Hydrogen, by contrast, has only
one electron, which leaves its outermost shell half-filled, so it is highly
reactive. The atoms found in living organisms all have outermost shells
that are incompletely filled, and they are therefore able to react with one
another to form molecules (Figure 2–5).
Because an incompletely filled electron shell is less stable than one that
is completely filled, atoms with incomplete outer shells have a strong
tendency to interact with other atoms so as to either gain or lose enough
electrons to fill the outermost shell. This electron exchange can be
achieved either by transferring electrons from one atom to another or
by sharing electrons between two atoms. These two strategies generate
the two types of chemical bonds that can bind atoms strongly to
one another: an ionic bond is formed when electrons are donated by one
atom to another, whereas a covalent bond is formed when two atoms
share a pair of electrons (Figure 2–6).
An H atom, which needs only one more electron to fill its only shell, generally
acquires this electron by sharing—forming one covalent bond with
another atom. The other most common elements in living cells—C, N,
and O, which have an incomplete second shell, and P and S, which have
an incomplete third shell (see Figure 2–5)—also tend to share electrons;
these elements thus fill their outer shells by forming several covalent
bonds. The number of electrons an atom must acquire or lose (either by
sharing or by transfer) to attain a filled outer shell determines the number
of bonds that the atom can make.