P A R T I T H E C E L L

The Chemical Basis
of Life

C H A P T E R

2

O U T L I N E

2.1 The Nature of Matter

• Matter is composed of 92 naturally occurring elements, each composed of atoms.•16

• Atoms have subatomic particles: neutrons, protons, and electrons.•17

• Atoms of the same type that differ by the number of neutrons are called isotopes.•18

• Atoms react with one another by giving up, gaining, or sharing electrons.•19

• Bonding between atoms results in molecules with distinctive chemical properties and shapes.•21

2.2

Water’s Importance to Life

• The existence of organisms is dependent on the chemical and physical properties of water.•22

• Organisms are sensitive to the hydrogen ion concentration [H

•

] of solutions, which can be indicated using the pH scale.•25

You are probably aware that the effects of exposure to extreme levels of radiation in humans can range from sunburns, to hair loss, to cancer,

and maybe even to death. What is it about exposure to radioactivity that causes such damage to living cells? The atoms that make up all of the
molecules in living cells are composed of subatomic particles. When we refer to a “radioactive atom,” what we really mean is that the atom is
not stable and has a tendency to release subatomic particles. If subatomic particles are released from an atom, they can cause major damage.

When organisms are exposed to radioactivity, loose subatomic particles penetrate their cells and cause cellular molecules to form free

radicals. These free radicals damage other molecules such as DNA within the cell. When DNA is damaged, a mutation results. Cancer can be

the result of mutations; hence, exposure to enough radioactivity often causes cancer in humans. A high enough level of radiation causes so

much damage to cells that they (and the individual) die. The cells most susceptible to free radical damage are those that naturally divide the

fastest, such as skin, hair, and digestive cells.

Chemistry is the very essence of all organisms. You may have never thought of your body as simply a pile of chemicals, or considered how the

chemistry of your body changes when you eat or drink certain foods, or realized the changes that occur in cells when they are exposed to

radioactivity. Understanding these basic principles of chemistry will greatly enhance your ability to understand biology and provide a

foundation on which to build further knowledge.

2.1 The Nature of Matter

Kiss your sweetheart, pat your dog, catch a bus, mow your lawn—everything you touch is matter. Matter refers to anything that takes up space and has
mass. It is helpful to know that matter can exist as a solid, a liquid, or a gas. Then we can realize that not only are we matter, but so are the water we drink
and the air we breathe.

All matter, both nonliving and living, is composed of elements. Formally speaking, an element is a substance that cannot be broken down into

another substance by ordinary chemical means. There are only 92 naturally occurring elements, and each of these differs from the others in its
properties. (A property is a physical or chemical characteristic, such as density, solubility, melting point, and reactivity.)

Both the Earth’s crust and all organisms are made up of elements, but these elements occur in different proportions (Fig. 2.1). Six

elements—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—are of special significance to us. They make up about 98% of the body weight
of most organisms. The acronym CHNOPS helps us remember these six elements, whose properties are essential to the uniqueness of living things, from
cells to organisms.

Atomic Structure

The atomic theory states that elements consist of tiny particles called atoms. Because each element consists of only one kind of atom, the same name is
given to an element and its atoms. This name is represented by one or two letters, called the atomic symbol. For example, the symbol H stands for a
hydrogen atom, and the symbol Na (for natrium in Latin) stands for a sodium atom.

From our discussion of elements, you might expect each atom to have a certain mass. The mass number of an atom is dependent upon the presence

of three types of -subatomic particles: neutrons, which have no electrical charge; protons, which have a positive charge; and electrons, which have a
negative charge. Protons and neutrons are located within the center of an atom, which is called its nucleus, while electrons move about the nucleus.

Figure 2.2 shows the arrangement of the subatomic particles in a helium atom, which has only two electrons. In -Figure 2.2a, the stippling shows the

probable location of electrons, and in Figure 2.2b, the circle represents the approximate -location of electrons. If we could draw an atom the size of a
baseball stadium, the nucleus would be like a gumball in the center of the stadium, and the electrons would be tiny specks whirling about in the upper
stands. Most of an atom is empty space. Usually, we can only indicate where the electrons are expected to be. In our analogy, the electrons might very well
stray outside the stadium at times.

In effect, the mass number of an atom is just about equal to the sum of its protons and neutrons. Protons and neutrons are assigned one atomic

mass unit each. Electrons are so small that their mass is assumed to be zero in most calculations. The term mass number is used, rather than atomic
weight because mass is constant while weight changes according to the gravitational force of a planet. The gravitational force of the Earth is greater than
that of the moon; therefore, substances weigh less on the moon even though their mass has not changed.

All atoms of an element have the same number of protons. This is called the atom’s atomic number. The number of protons makes an atom

unique. In Figure 2.3, the atomic number is written above the atomic symbol. The mass number is written below the atomic symbol. For example, the
carbon atom is shown in this way:

The Periodic Table

Once chemists discovered a number of the elements, they began to realize that the elements’ chemical and physical characteristics -recur in a predictable
manner. The periodic table (Fig. 2.3) was developed as a way to display the elements, and therefore the atoms, according to these characteristics. Every
atom is in a particular period (the horizontal rows) and in a particular group (the vertical columns). The atoms in group 8 are called the noble gases
because they are gases that rarely react with another atom. Notice that helium is a noble gas.

The atomic number written above the atomic symbol tells you the number of positively charged protons, and also the number of negatively charged

electrons if the atom is electrically neutral. To determine the usual number of neutrons, subtract the number of protons from the mass number, written
below the atomic symbol, and take the closest whole number.

Isotopes

Isotopes are atoms of the same element that differ in the number of neutrons. In other words, isotopes have the same number of protons, but they have
different mass numbers. A nucleus with excess neutrons is unstable and may decay and emit radiation. The radiation given off by radioactive isotopes
can be detected in various ways. Most people are familiar with the use of a Geiger counter to detect radiation.

Uses of Radioactive Isotopes•The importance of chemistry to biology and medicine is nowhere more evident than in the many uses of radioactive
isotopes. The chemical behavior of a radioactive isotope is essentially the same as that of the stable isotopes of an element. This means that you can put
a small amount of radioactive isotope in a sample and it becomes a tracer by which to detect molecular changes.

Specific tracers are used in imaging the body’s organs and tissues. For example, positron-emission tomography (PET) is a way to determine the

comparative activity of tissues. Radioactively labeled glucose that emits a subatomic particle known as a positron is injected into the body. The
radiation given off is detected by sensors and analyzed by a computer. The result is a color image that shows which tissues took up glucose and are
metabolically active (Fig. 2.4a,b). Although not shown in Figure 2.4, a PET scan can help diagnose a malfunctioning thyroid, a brain t umor,
Alzheimer disease, epilepsy, or whether a stroke has occurred.

Radioactive substances in the environment can harm cells, damage DNA, and cause cancer. The release of radioactive particles following a

nuclear power plant accident can have far-reaching and long-lasting effects on human health. However, the effects of radiation can also be put to good
use (Fig. 2.5). The ability of radiation to kill cells is often used to increase shelf life of fruits (Fig. 2.5a) and to destroy cancer cells (Fig. 2.5b). Packets of
radioactive isotopes can be placed in the body so that the subatomic particles emitted destroy only cancer cells, with little risk to the rest of the body.

Radiation from radioactive isotopes has been used for many years to sterilize medical and dental equipment. Now the possibility exists that it

can also be used to sterilize the U.S. mail to free it of possible pathogens, such as anthrax spores introduced by terrorists.

Arrangement of Electrons in an Atom

Although it is not possible to determine the precise location of an individual electron at any given moment, it is useful to construct models of atoms that
show electrons at discrete energy levels about the nucleus (Fig. 2.6). It seems reasonable to suggest that negatively charged electrons are attracted to the
positively charged nucleus, and therefore it takes an increasing amount of energy to push them farther away from the nucleus. Electrons in outer shells,
therefore, contain more energy than those in inner shells.

Each energy level contains a certain number of electrons. In the models shown in Figure 2.6, the energy levels (electron shells) are drawn as

concentric rings about the nucleus. For atoms up through number 20 (i.e., calcium), the first shell closest to the nucleus can contain two electrons;
thereafter, each additional shell can contain eight electrons. For these atoms, each lower level is filled with electrons before the next higher level
contains any electrons.

The sulfur atom, with an atomic number of 16, has two electrons in the first shell, eight electrons in the second shell, and six electrons in the third,

or outer, shell. Revisit the periodic table (see Fig. 2.3), and note that sulfur is in the third period. In other words, the period tells you how many shells an
atom has. Also note that sulfur is in group 6. The group tells you how many electrons an atom has in its outer shell.

If an atom has only one shell, the outer shell is complete when it has two electrons. If an atom has two or more shells, the outer shell is most stable

when it has eight electrons; this is called the octet rule. As mentioned previously, atoms in group 8 of the periodic table are called the noble gases
because they do not ordinarily undergo reactions. Atoms with fewer than eight electrons in the outer shell react with other atoms in such a way that each
has a completed outer shell after the reaction. Atoms can give up, accept, or share electrons in order to have eight electrons in the outer shell. In other
words, the number of electrons in an atom’s outer shell, called the valence shell, determines its chemical reactivity. The size of an atom is also
important. Both carbon (C) and silicon (Si) atoms are in group 4, and therefore can bond with four other atoms in order to achieve eight electrons in their
outer shells. But carbon is in period 2 and silicon is in period 3. The smaller atom, carbon, can bond to other carbon atoms and form long-chained
molecules, while the larger silicon atom is unable to do so.

Types of Chemical Bonds

A group of atoms bonded together is called a molecule. When a molecule contains atoms of more than one element, it can be called a compound. For
example, sodium (Na) can combine with chlorine (Cl) to form sodium chloride (NaCl), the compound we know as table salt.

Ionic Bonding

The reaction between sodium and chlorine atoms is an example of how charged atoms called ions form. Consider that sodium (Na), with only one
electron in its third shell, usually gives up an electron (Fig. 2.7a). Once it does so, the second shell, with eight electrons, becomes its outer shell.
Chlorine (Cl), on the other hand, tends to take on an electron, because its outer shell only has seven electrons. If chlorine gets one more electron it has
a completed outer shell. So, when a sodium atom and a chlorine atom react, an electron is transferred from sodium to chlorine. Now both atoms have
eight electrons in their outer shells.

This electron transfer causes these atoms to become ions. The sodium ion has one more proton than it has electrons; therefore, it has a net

charge of •1 (symbolized by Na

•

). The chloride ion has one more electron than it has protons; therefore, it has a net charge of •1 (symbolized by

Cl

•

). Negatively charged ions often have names that end in ―ide,‖ and thus Cl

•

is called a chloride ion. In the periodic table, atoms in groups 1 and

2 and groups 6 and 7 become ions when they react with other atoms. Atoms in groups 2 and 6 always transfer two electrons. For example, calcium
becomes Ca

2•

, while oxygen becomes O

2•

.

Ionic compounds are held together by an attraction between negatively and positively charged ions, called an ionic bond. A sodium chloride

crystal illustrates the solid form of a salt (Fig. 2.7b). Salts can exist as a dry solid, but when placed in water, they release ions as they dissolve. NaCl
separates into Na

•

and Cl

•

. Ionic compounds are most commonly found in this dissociated (ionized) form in biological systems because these systems

are 70–90% water.

Covalent Bonding

A covalent bond results when two atoms share electrons in order to have a completed outer shell. In a hydrogen atom, the outer shell is complete when
it contains two electrons. If hydrogen is in the presence of a strong electron acceptor, it gives up its electron to become a hydrogen ion (H

•

). But if this

is not possible, hydrogen can share with another atom, and thereby have a completed outer shell. For example, one hydrogen atom can share with
another hydrogen atom. In this case, the two orbitals overlap and the electrons are shared between them—that is, you count the electrons as belonging to
both atoms:
Rather than drawing an orbital model like the above, scientists often use simpler ways to indicate molecules. A structural formula uses straight lines, as
in H±H. A molecular formula omits the lines that indicate bonds and simply shows the number of atoms involved, as in H

2

.

Sometimes, atoms share more than two electrons to complete their octets. A double covalent bond occurs when two atoms share two pairs of

electrons, as in this molecule of oxygen gas:
In order to show that oxygen gas (O

2

) contains a double bond, the structural formula would be written as ONO.

It is also possible for atoms to form triple covalent bonds, as in nitrogen gas (N

2

), which can be written as NPN. Single covalent bonds between

atoms are quite strong, but double and triple bonds are even stronger.

The molecule methane results when carbon binds to four other atoms (Fig. 2.8). In methane, each bond actually points to one corner of a

tetrahedron. The best model to show this arrangement is a ball-and-stick model (Fig. 2.8c). Space-filling models (Fig. 2.8d) come closest to showing the
actual shape of a molecule. The shapes of molecules help dictate the functional roles they play in organisms.

Chemical Reactions

Chemical reactions are very important to organisms. For example, we have already noted that the process of photosynthesis enables plants to make
molecular energy available to themselves and other organisms. An overall equation for the photosynthetic reaction indicates that some bonds are broken
and others are formed:
This equation says that six molecules of carbon dioxide react with six molecules of water to form one glucose molecule and six molecules of oxygen.
The reactants (molecules that participate in the reaction) are shown on the left of the arrow, and the products (molecules formed by the reaction) are
shown on the right. Notice that the equation is ―balanced‖—that is, the same number of each type of atom occurs on both sides of the arrow.

Note the glucose molecule in the equation above. Glucose is more complex than any molecule discussed so far. The arrangement of its atoms is

shown below:

2.2

Water’s Importance to Life

Life began in water, and it is the single most important molecule on Earth. All organisms are 70–90% water; their cells consist of membranous
compartments enclosing aqueous solutions. Water has unique properties that make it a life-supporting substance. These properties of water stem
from the structure of the molecule.

The Structure of Water

Atoms differ in their electronegativity—that is, their affinity for electrons in a covalent bond. For example, in water, oxygen shares electrons with
two hydrogen atoms. The covalent bonds are angled, and the molecule is bent roughly into a shape because the oxygen is more electronegative than
the hydrogens. The shared electrons spend more time orbiting the oxygen nucleus than the hydrogen nuclei, and this unequal sh aring of electrons
makes water a polar molecule. The point of the (oxygen) is the negative (•) end, and the two hydrogens are the positive (•) end (Fig. 2.9a).

The polarity of water molecules causes them to be attracted to one another. The positive hydrogen atoms in one molecule are attracted to the

negative oxygen atoms in other water molecules. This attraction is called a hydrogen (H) bond, and each water molecule can engage in as many as four
H bonds (Fig. 2.9b). The covalent bond is much stronger than an H bond, but the sheer number of H bonds in water makes the H bond strong overall.
The properties of water are due to its polarity and its ability to form hydrogen bonds.

Properties of Water

Water is so familiar that we take it for granted, but without water, life as we know it would not exist. The properties of water that support life are
solvency, cohesion and adhesion, high surface tension, high heat capacity, high heat of vaporization, and varying density.

Water is a solvent.•Because of its polarity and H-bonding ability, water dissolves a great number of substances. Molecules that are attracted to
water are said to be hydrophilic (hydro, water; phil, love). Nonionized and nonpolar molecules that are not attracted to water are said to be
hydrophobic (hydro, water; phob, fear).

When a salt such as sodium chloride (NaCl) is put into water, the negative ends of the water molecules are attracted to the s odium ions, and

the positive ends of the water molecules are attracted to the chloride ions. This attraction causes the sodium ions and the chloride ions to break up,
or dissociate, in water:

Water dissolves many polar nonionic substances, such as glucose, by forming H bonds with them. When ions and molecules disperse in water,

they move about and collide, allowing reactions to occur. As mentioned, cellular fluids are aqueous solutions of various substances, and so are the
oceans. Without the dissolving power of water, aquatic organisms could not take up the substances they need from the water.

Water is cohesive and adhesive.•Because of hydrogen bonding, water molecules cling together, and water exists as a liquid under ordinary conditions
of temperature and pressure. The strong cohesion of water molecules is apparent because water flows freely, and yet the molecules do not separate from
each other. The positive and negative poles of water molecules cause water to adhere to polar surfaces; therefore, water exhibits adhesion.

Liquid water is an excellent transport system, both outside and within living organisms. Unicellular organisms rely on external water to transport

nutrient and waste molecules, but multicellular organisms often contain internal vessels in which water transports nutrients and wastes. For example, the
liquid portion of our blood, which transports dissolved and suspended substances about the body, is 90% water. Cohesion and adhesion allow water to
fill a tubular vessel. In plants, because of cohesion and adhesion, water is able to rise to the top of even very tall trees (Fig. 2.10). Water evaporating

from the leaves creates a tension that pulls a continuous water column up from the roots to the leaves of the plant.

Water has a high surface tension.•The stronger the force between molecules in a liquid, the greater the surface tension. As with cohesion,
hydrogen bonding causes water to have a high surface tension. This property makes it possible for humans to skip rocks on wat er. An insect called
a water strider can even walk on the surface of a pond without breaking through (Fig. 2.11). The high surface tension of water is important to
other forms of surface aquatic life as well. For example, it keeps plant debris resting on the surface, providing shelter and food for those
organisms that live near the surface.

Water has a high heat capacity.•The many hydrogen bonds that link water molecules allow water to absorb heat without greatly changing in
temperature. Water’s high heat capacity is important not only for aquatic organisms but for all organisms. Because t he temperature of water rises
and falls slowly, terrestrial organisms are better able to maintain their normal internal temperatures and are also protected from rapid temperature
changes.

Water also has a high heat of vaporization: It takes a great deal of heat to break the hydrogen bonds in water so that it becomes gaseous and

evaporates into the environment. If the heat given off by our metabolic activities were to go directly into raising our body temperature, death would
follow. Instead, the heat is dispelled as sweat evaporates (Fig. 2.12).

Because of water’s high heat capacity and high heat of vaporization, temperatures along the Earth’s coasts are moderate. During the summer, the

ocean absorbs and stores solar heat, and during the winter, the ocean releases it slowly. In contrast, the interior regions of the continents experience
abrupt changes in temperature.

Water is less dense as ice.•Unlike other substances, water expands as it freezes, which explains why cans of soda burst when placed in a freezer and
how roads in northern climates become bumpy because of ―frost heaves‖ in the winter. It also means that ice is less dense than liquid water, and
therefore ice floats on liquid water (Fig. 2.13).

If ice did not float on water, it would sink, and ponds, lakes, and perhaps even the ocean would freeze solid, making life impossible in the water

and also on land. Instead, bodies of water always freeze from the top down. When a body of water freezes on the surface, the ice acts as an insulator to
prevent the water below it from freezing. This protects aquatic organisms so that they can survive the winter. As ice melts in the spring, it draws heat
from the environment, helping to prevent a sudden change in temperature that might be harmful to life.

Acids and Bases

Figure 2.14 shows that when water dissociates, it releases an equal number of hydrogen ions (H

•

) and hydroxide ions (OH

•

) as in the following

reaction:

Acidic Solutions (High H

•

Concentration)

Lemon juice, vinegar, tomatoes, and coffee are all acidic solutions. What do they have in common? Acidic solutions have a sharp or sour taste, and
therefore we sometimes associate them with indigestion. To a chemist, acids are substances that dissociate in water, releasing hydrogen ions (H

•

).

For example, an important acid is hydrochloric acid (HCl), which dissociates in this manner:

HCl

H

•

• Cl

•

Dissociation is almost complete; therefore, HCl is called a strong acid. If hydrochloric acid is added to a beaker of water, the number of hydrogen ions
(H

•

) increases greatly (Fig. 2.15).

Basic Solutions (Low H

•

Concentration)

Milk of magnesia and ammonia are common basic solutions that most people have heard of. Basic solutions have a bitter taste and feel slippery when in
water. To a chemist, bases are substances that either take up hydrogen ions (H

•

) or release hydroxide ions (OH

•

). For example, an important base is

sodium hydroxide (NaOH), which dissociates in this manner:

NaOH

Na

•

• OH

•

Dissociation is almost complete; therefore, sodium hydroxide is called a strong base. If sodium hydroxide is added to a beaker of water, the number of
hydroxide ions increases (Fig. 2.16).

It is not recommended that you taste any strong acid or base, because they are quite destructive to cells. Any container of household cleanser, such

as ammonia, has a poison symbol and carries a strong warning not to ingest the product.

pH and the pH Scale

pH

1

is a mathematical way of indicating the number of hydrogen ions in a solution. The pH scale is used to indicate the acidity or basicity

(alkalinity) of a solution. The pH scale ranges from 0 to 14 (Fig. 2.17). A pH of 7 represents a neutral state in which the h ydrogen ion and
hydroxide ion concentrations are equal, as in pure water. A pH below 7 is acidic because the hydrogen ion concentration, commonly expressed in
brackets as [H

•

], is greater than the hydroxide concentration, [OH

•

]. A pH above 7 is basic because [OH

•

] is greater than [H

•

]. Further, as we

move down the pH scale from pH 7 to pH 0, each unit has 10 times the acidity [H

•

] of the previous unit. As we move up the scale from 7 to 14,

each unit has 10 times the basicity [OH

•

] of the previous unit.

The pH scale was devised to eliminate the use of cumbersome numbers. For example, the hydrogen ion concentrations of these solutions are on

the left, and the pH is on the right:

The effect of pH on organisms is dramatically illustrated by the phenomenon known as acid deposition. When fossil fuels are burned, sulfur

dioxide and nitrogen oxides are produced, and they combine with water in the atmosphere to form acids. These acids then come in contact with
organisms and objects, leading to damage or even death (Fig. 2.18).

Buffers and pH

In the human body, pH needs to be kept within a narrow range in order to maintain health. A buffer is a chemical or a combination of chemicals that
keeps pH within normal limits. Buffers resist pH changes because they can take up excess hydrogen ions (H

•

) or hydroxide ions (OH

•

). Many

commercial products, such as Bufferin, shampoos, and deodorants, are buffered as an added incentive for us to buy them.

The pH of our blood is usually about 7.4, and this normal level is maintained in part by a buffer consisting of a combination of carbonic acid and

bicarbonate ions. Carbonic acid (H

2

CO

3

) is a weak acid that minimally dissociates. The following reaction shows how carbonic acid dissociates and can

re-form:
When hydrogen ions (H

•

) are added to blood, this reaction occurs:

H

•

• HCO

3

•

•• ••H

2

CO

3

When hydroxide ions (OH

•

) are added to blood, this reaction occurs:

OH

•

• H

2

CO

3

•• ••HCO

3

•

• H

2

O

These reactions prevent any significant change in blood pH.

T H E C H A P T E R I N R E V I E W

Summary

2.1

The Nature of Matter

Definitions to remember:

• Matter: Takes up space, has mass, and is composed of elements

• Element: A fundamental constituent of matter; contains atoms

• Nucleus: The center of an atom, contains protons (+) and neutrons

• Mass number: Protons plus neutrons

• Isotopes: Atoms of same type that differ in the number of neutrons

• Atomic number: Number of protons equals number of electrons when atom is neutral

• Electrons (•)

First shell: Complete with two electrons

Other shells: Complete with eight electrons assuming an atomic number of 20 or below.

Valence shell: Number of electrons in outer shell determines reactivity of atom.

• Octet rule: Atoms react with one another in order to have a completed outer shell with eight electrons

• Ionic bond: Attraction between oppositely charged ions. Ions form when atoms lose or gain one or more electrons to achieve a completed outer

shell.

• Covalent bond: Sharing of electrons between two atoms. There are single covalent bonds (sharing one pair of electrons), double (sharing two

pairs of electrons), and triple (sharing three pairs of electrons).

• Polar covalent bond: Sharing of electrons is not even and as a result, a molecule has a negative pole and a positive pole.

• Hydrogen bond: Weak attraction between slightly positive hydrogen atoms of one molecule and slightly negative oxygen or nitrogen atoms within

same or different molecules.

2.2

Water’s Importance to Life

Water is a polar molecule. Its polarity and hydrogen bonding account for its unique properties, which can be summarized as follows:

Acids and Bases

Water dissociates to produce an equal number of hydrogen ions and hydroxide ions. This is neutral pH. This illustration shows the range of acidic and
basic solutions:
Cells are sensitive to pH changes. Biological systems contain buffers that help keep the pH within a normal range.

Thinking Scientifically

If you watch a bird dive into the water, you will see that the surface of the water is smooth and continuous as the bird enters. However, when the bird flies
back into the air, many drops of water fly into the air as well (see Fig. 2.9). Explain this observation based on properties of water.

Testing Yourself

Choose the best answer for each question.

1. Which of the following is not a component of an atom?

a. proton

c. neutron

b. positron

d. electron

2. The most abundant element by weight in the human body is

a. carbon.

c. oxygen.

b. hydrogen.

d. nitrogen.

3. This rule states that the outer electron shell is most stable when it contains eight electrons.

a. stability rule

c. octet rule

b. atomic rule

d. shell rule

4. How many electrons does nitrogen require to fill its outer shell?

a. 0

c. 2

b. 1

d. 3

5. Molecules held together by _________ bonds tend to dissociate in biological systems due to the water content in those systems.

a. covalent

c. hydrogen

b. ionic

d. nitrogen

6. Water flows freely, but does not separate into individual molecules because water is

a. cohesive.

c. hydrophobic.

b. hydrophilic.

d. adhesive.

7. Water can absorb a large amount of heat without much change in temperature because it has

a. a high surface tension.

b. a high heat capacity.

c. ten times as many hydrogen ions.

d. ten times as many hydroxide ions.

Go to www.mhhe.com/maderessentials for more quiz questions.

Bioethical Issue

Acid precipitation is produced when atmospheric water is polluted by sulfur dioxide and nitrous oxide emissions. These emissions are mostly produced by
the burning of fossil fuels. Because the emissions are airborne, the acid precipitation may occur far from where the fossil fuels are being burned. For
example, most of the acid precipitation in Scandinavian countries comes from emissions produced in Great Britain, central Europe, and Russia. Similarly,
much of the acid precipitation in southeastern Canada results from industrial activity in the midwestern and eastern United States.

Do you think the countries producing pollution should compensate their neighbors for the damage being done? Are countries at least obligated to

minimize the effects of their air pollutants on other countries?

Under the Kyoto Protocol of 1997, some industrialized nations made legally binding commitments to reduce fossil fuel emissions. The countries that

have signed realize that their industrial activities affect other countries. While they are not compensating other countries for damage done by their
pollutants, they have agreed to take steps to limit the emission of pollutants. Although the United States produces more fossil fuel emissions per capita
than any other country in the world, it opted out of the Kyoto Protocol in 2001.

Understanding the Terms

acid•25
atom•17

atomic number•17
atomic symbol•17
base•25
buffer•26
compound•19
covalent bond•20
electron•17
electronegativity•22
electron shell•18

element•16
hydrogen (H) bond•22
hydrogen ion (H

•

)•25

hydrophilic•23
hydrophobic•23
hydroxide ion (OH

•

)•25

ion•20
ionic bond•20
isotope•18
mass number•17
matter•16
molecule•19
neutron•17
nucleus•17
octet rule•19
pH•26
pH scale•26
polar•22
product•21
proton•17
reactant•21
salt•20
tracer•18
valence shell•19

Match the terms to these definitions:

a. _______________

Subatomic particle that moves in shells around the nucleus of an atom.

b. _______________

Anything that takes up space and has mass.

c. _______________

A molecule that has positive and negative ends.

d. _______________

Having an affinity for water.

e. _______________

Atoms of the same element that differ in the number of neutrons.

f. _______________

Substance having an attached radioactive isotope that allows a researcher to track its whereabouts in a biological system.

g. _______________

An atom that has become charged due to electron transfer.

Figure 2.1Elements in the Earth’s crust and living organisms.
If analyzed at the level of atoms, the Earth’s crust has a very different composition from human beings. a. By weight, human beings are mostly composed of oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. b. The Earth’s
crust, containing rock, soil, sand, and other materials, is mostly oxygen, silicon, and aluminum atoms.

The pH of your stomach is acidic enough to dissolve steel.
Radiation causes cancer, but it can also be used to cure cancer.

The chemical composition of human blood closely resembles that of seawater.

Figure 2.2•Two models of helium (He).
Atoms contain subatomic particles, which are located as shown in these two simplified models of helium. Protons are positively charged, neutrons have no charge, and electrons are negatively charged. Protons and neutrons are found
within the nucleus, and electrons are outside the nucleus. a. This model shows electrons as 
 a negatively charged cloud around the nucleus. b. In this model, 
 the average location of electrons is sometimes represented by a circle.

Figure 2.3•A portion of the periodic table.
In the periodic table, the elements, and therefore the atoms that compose them, are in the order of their atomic numbers but arranged in periods (horizontal rows) and groups (vertical columns). All the atoms in a particular group have
certain chemical characteristics in common. The six elements previously mentioned (CHNOPS) are highlighted in red.

Figure 2.4•PET scans.
In a PET scan, red indicates areas of greatest metabolic activity, and blue means areas of least activity. Computers analyze the data from different sections of an organ. a. A longitudinal scan of the thyroid resembles its overall shape. 
 b. A cross
section of the brain is a powerful diagnostic tool for the physician.

Figure 2.5•High levels of radiation.
a. Radiation kills microbes. After irradiation, peaches (bottom) no longer spoil and can be kept for a longer length of time. 
 b. Physicians treat patients with high levels of radioactive isotopes to kill cancer cells.
Figure 2.6•Atoms of the six elements, CHNOPS.
Electrons orbit the nucleus at particular energy levels (electron shells): The first shell contains up to two electrons, and each shell thereafter can contain up to eight electrons (until we consider atoms with an atomic number above 20).
Each shell is to be filled before electrons are placed in the next shell. Why does carbon have only two shells while phosphorus and sulfur have three shells?
Figure 2.7•Formation of sodium chloride.
a. During the formation of sodium chloride, an electron is transferred from the sodium atom to the chlorine atom. At the completion of the reaction, each atom has eight electrons in the outer shell, but each also carries a charge as shown.
b. In a sodium chloride crystal, ionic bonding between Na

•

and Cl

•

causes the atoms to assume a three-dimensional lattice shape.

Figure 2.8•Shapes of covalently bonded molecules.
An electron model (a) and a structural model (b) show that methane (CH

4

) contains one carbon atom bonded to four hydrogen atoms. c. The ball-and-stick model shows that when carbon bonds to four other atoms, as in methane, each

bond actually points to one corner of a tetrahedron. d. The space-filling model is a three-dimensional representation of the molecule.

Figure 2.9•The structure of water.
a. The space-filling model shows the shape of a water molecule. Oxygen attracts the shared electrons more than hydrogen atoms do, and this causes the molecule to be polar: The oxygen carries a slight negative charge and the hydrogens
carry a slight positive charge. b. The positive hydrogens form hydrogen bonds with the negative oxygen in nearby molecules. Each water molecule can be joined to other water molecules by as many as four hydrogen bonds.
Figure 2.10•Cohesion and adhesion of water 
 molecules.
How does water rise to the top of tall trees? Water-filled vessels extend from the roots to the leaves. When water evaporates from the leaves, this water column is pulled upward due to the cohesion of water molecules with one another
and the adhesion of water molecules to the sides of the vessel.
Figure 2.11•Surface tension of water.
A water strider can flit across a pond on widespread legs because the surface tension of water is strong enough to support it.
Figure 2.12•Heat of 
 vaporization.
At room temperature, water is a liquid. a. Water boils at 100°C, vaporizes, and takes up a large amount of heat. b. It takes much body heat to vaporize sweat, which is mostly liquid water, and this helps keep our bodies cool when the
temperature rises.
Figure 2.13•Properties of ice.
a. The geometric requirements for hydrogen bonding of water molecules cause ice to be less dense than liquid water. b. Therefore, bodies of water freeze from the top down, and organisms in ponds and lakes are protected during the
winter.
Figure 2.16•Addition of sodium hydroxide (NaOH), a base.
NaOH releases OH

•

as it dissociates. The addition of NaOH to water results in a solution with more OH

•

than H

•

.

Figure 2.14•Dissociation of water molecules.
Dissociation produces an equal number of hydrogen ions (H

•

) and hydroxide ions (OH

•

). (These illustrations are not meant to be mathematically accurate.)

Figure 2.15•Addition of hydrochloric acid (HCl).
HCl releases hydrogen ions (H

•

) as it dissociates. The addition of HCl to water results in a solution with more H

•

than OH

•

.

Figure 2.17•The pH scale.
The proportionate amount of hydrogen ions to hydroxide ions is indicated by the diagonal line. Any solution with a pH above 7 is basic, while any solution with a pH below 7 is acidic.
Figure 2.18•Effects of acid deposition.
The burning of gasoline derived from oil, a fossil fuel, leads to acid deposition, which causes (a) trees to die and (b) statues to deteriorate.

1

pH is defined as the negative log of the hydrogen ion concentration [H

•

]. A log is the power to which 10 must be raised to produce a given number.

Properties

Chemical Reasons

EffectWater is a solvent.

Polarity Water facilitates chemical reactions.

Water is cohesive and adhesive.

Hydrogen bonding; polarity Water serves as a transport medium.

Water has a high surface tension. Hydrogen bonding

The surface tension of water is hard to break.

Water has a high heat capacity.

Hydrogen bonding

Water protects organisms from rapid changes in temperature.

Water has a high heat of vaporization.

Hydrogen bonding

Water helps organisms resist overheating.

Water is less dense as ice.

Hydrogen bonding changes Ice floats on liquid water.

Check Your Progress

1. Contrast mass number with atomic number.
2. 
 List some uses of radioactive isotopes in biology and medicine.
3. How do group 3 elements differ in the periodic table? How do period 3 elements differ?

Answers:•1. Mass number is approximately the sum of the protons and neutrons in an atom. Atomic number is the number of protons in an atom.•
 2. Uses of radioactive isotopes include imaging of body parts (e.g., PET scans, thyroid
imaging), sterilization of medical equipment, cancer therapy, and increased storage life of produce.•3. The third horizontal row in the periodic table contains atoms that sequentially increase by one the number of electrons in the outer shell.
Group 3 elements (the third vertical column in the table) contain atoms that all have three electrons in the outer shell.

Check Your Progress

1. Contrast an ionic bond with a covalent bond.
2. 
 Methane is formed by covalent bonding between carbon and hydrogen. How many hydrogen atoms would be bonded to the carbon atom?

Answers:•1. An ionic bond is created when one atom gives up electron(s) and another gains electron(s) so that both atoms have their outer shells filled. A covalent bond is formed when two atoms share electrons to fill their outer shell.•2.
Carbon requires four electrons to fill its outer shell, so it bonds with four hydrogen atoms.

Check Your Progress

Contrast an acid with a base.

Answer:•An acid dissociates in water to release hydrogen ions. A base either takes up hydrogen ions or releases hydroxide ions.

UV radiation can cause skin cancer.