Essential Cell Biology 5th edition

Chemical Bonds
41
number of protons. Multiple isotopes of almost all the elements occur
naturally, including some that are unstable—and thus radioactive. For
example, while most carbon on Earth exists as carbon 12, a stable isotope
with six protons and six neutrons, also present are small amounts of
an unstable isotope, carbon 14, which has six protons and eight neutrons.
Carbon 14 undergoes radioactive decay at a slow but steady rate, a property
that allows archaeologists to estimate the age of organic material.
The atomic weight of an atom, or the molecular weight of a molecule,
is its mass relative to the mass of a hydrogen atom. This value is equal
to the number of protons plus the number of neutrons that the atom or
molecule contains; because electrons are so light, they contribute almost
nothing to the total mass. Thus the major isotope of carbon has an atomic
weight of 12 and is written as 12 C. The unstable carbon isotope just mentioned
has an atomic weight of 14 and is written as 14 C. The mass of an
atom or a molecule is generally specified in daltons, one dalton being an
atomic mass unit essentially equal to the mass of a hydrogen atom.
Atoms are so small that it is hard to imagine their size. An individual
carbon atom is roughly 0.2 nm in diameter, so it would take about 5
million of them, laid out in a straight line, to span a millimeter. One proton
or neutron weighs approximately 1/(6 × 10 23 ) gram. As hydrogen
has only one proton—thus an atomic weight of 1—1 gram of hydrogen
contains 6 × 10 23 atoms. For carbon—which has six protons and six neutrons,
and an atomic weight of 12—12 grams contain 6 × 10 23 atoms. This
huge number, called Avogadro’s number, allows us to relate everyday
quantities of chemicals to numbers of individual atoms or molecules. If
a substance has a molecular weight of X, X grams of the substance will
contain 6 × 10 23 molecules. This quantity is called one mole of the substance
(Figure 2–3). The concept of mole is used widely in chemistry as
a way to represent the number of molecules that are available to participate
in chemical reactions.
There are about 90 naturally occurring elements, each differing from the
others in the number of protons and electrons in its atoms. Living things,
however, are made of only a small selection of these elements, four of
which—carbon (C), hydrogen (H), nitrogen (N), and oxygen (O)—constitute
96% of any organism’s weight. This composition differs markedly
from that of the nonliving, inorganic environment on Earth (Figure 2–4)
and is evidence that a distinctive type of chemistry operates in biological
systems.
The Outermost Electrons Determine How Atoms Interact
To understand how atoms come together to form the molecules that
make up living organisms, we have to pay special attention to each
atom’s electrons. Protons and neutrons are welded tightly to one another
in an atom’s nucleus, and they change partners only under extreme conditions—during
radioactive decay, for example, or in the interior of the
sun or a nuclear reactor. In living tissues, only the electrons of an atom
undergo rearrangements. They form the accessible part of the atom and
specify the chemical rules by which atoms combine to form molecules.
Electrons are in continuous motion around the nucleus, but motions on
this submicroscopic scale obey different laws from those we are familiar
with in everyday life. These laws dictate that electrons in an atom can
exist only in certain discrete regions of movement—very roughly speaking,
in distinct orbits. Moreover, there is a strict limit to the number of
electrons that can be accommodated in an orbit of a given type, a socalled
electron shell. The electrons closest on average to the positively
charged nucleus are attracted most strongly to it and occupy the inner,
percent relative abundance
A mole is X grams of a substance,
where X is the molecular weight of the
substance. A mole will contain
6 × 10 23 molecules of the substance.
1 mole of carbon weighs 12 g
1 mole of glucose weighs 180 g
1 mole of sodium chloride weighs 58 g
A one molar solution has a
concentration of 1 mole of the substance
in 1 liter of solution. A 1 M solution of
glucose, for example, contains 180 g/L,
and a one millimolar (1 mM) solution
contains 180 mg/L.
The standard abbreviation for gram is g;
the abbreviation for liter is L.
Figure 2–3 What’s a mole? Some simple
examples of moles and molar solutions.
70
60
50
40
30
20
10
ECB5 e2.03/2.03
H C O N Ca
and
Mg
Na
and
K
human body
Earth's crust
P
Al
Si others
Figure 2–4 The distribution of elements
in the Earth’s crust differs radically from
that in the human body. The abundance
of each element is expressed here as a
percentage of the total number of atoms
present in a biological or geological sample
(water included). Thus, for example, more
than 60% of the atoms in the human body
are hydrogen atoms, and nearly 30% of the
atoms in the Earth’s crust are silicon atoms
(Si). The relative abundance of elements is
similar in all living things.