Raven/Johnson Biology 8e
Chapter 02 - Answers
1. The property that distinguishes one atom (carbon for example) from another atom
(oxygen for example) is—
a. the number of electrons
b. the number of protons
c. the number of neutrons
d. the combined number of protons and neutrons
The correct answer is b—
A. Answer a is incorrect because the number of electrons orbiting a given atom can change
without changing the identity of the element.
The correct answer is b—the number of protons
B. Answer b is correct. Proton number determines the atomic number of an element. The
numbers of electrons and neutrons can vary without affecting the identity of an atom.
The correct answer is b—
C. Answer c is incorrect. Differences in the numbers of neutrons determine the isotope of an
element, but does not affect the identity of the element.
The correct answer is b —
D. Answer d is incorrect. The number of neutrons can vary between isotopes of a given
atom.
2. If an atom has one valence (outer energy level) electron, it will most likely form—
a. one polar, covalent bond
b. two nonpolar, covalent bonds
c. two covalent bonds
d. an ionic bond
The correct answer is d—
A. Answer a is incorrect. Although it is possible that this atom could form a single covalent
bond, there is no information in this question that would allow you to determine whether
that bond would be polar or nonpolar.
The correct answer is d—
B. Answer b is incorrect. There needs to be two, unpaired valence electrons in the outer
energy level of an atom in order for two covalent bonds to be formed. No information is
provided in the question to allow you to make a prediction about the polarity of the bond.
The correct answer is d—
C. Answer c is incorrect. There needs to be two, unpaired valence electrons in the outer
energy level of an atom in order for two covalent bonds to be formed.
The correct answer is d—an ionic bond
D. Answer d is correct. Ionic bonds are formed between ions of opposite charge. Ions are
generated when electrons are transferred between atoms. This transfer is more likely to
occur when there is a single, unpaired electron in the outer energy level of an atom.
Raven/Johnson Biology 8e
Chapter 02 - Answers
3. An atom with a net positive charge must have—
a. more protons than neutrons
b. more protons than electrons
c. more electrons than neutrons
d. more electrons than protons
The correct answer is b—
A. Answer a is incorrect. Although protons do carry a positive charge, they are typically
matched by an equal number of orbiting electrons, resulting in an overall neutral charge.
The correct answer is b—more protons than electrons
B. Answer b is correct. The match between the number of positive charges contributed by
the protons and the negative charges contributed by the electrons results in an atom with
an overall neutral charge. If there are fewer electrons than protons, this balance is lost and
the atom will have a net positive charge.
The correct answer is b—
C. Answer c is incorrect. Electrons are negatively charged particles, and the neutrons have
no charge. An atom with an excess of electrons over neutrons would have a net negative
charge.
The correct answer is b—
D. Answer d is incorrect. Electrons are negatively charged particles, and protons are
positively charged particles. An excess of electrons relative to protons would result in a net
negative charge for the atom.
4. The isotopes C12 and C14 differ in—
a. the number of neutrons
b. the number of protons
c. the number of electrons
d. both b and c
The correct answer is a—the number of neutrons
A. Answer a is correct. Neutron number can vary without altering the identity of an atom.
The number of neutrons is used to indicate the specific isotope number of any given atom.
The correct answer is a—
B. Answer b is incorrect. The number of protons is the property that determines the identity
of an atom. Changing proton number changes the name of the atom, not its isotope
number.
The correct answer is a—
C. Answer c is incorrect. The number of electrons depends on the proton number and can
vary, but it does not determine the isotope number of an atom.
The correct answer is a—
Raven/Johnson Biology 8e
Chapter 02 - Answers
D. Answer d is incorrect. The number of protons and electrons are important characteristics
of an atom that influence what type of atom it is, but not its isotope number.
5. An atom with more electrons than protons is called—
a. an element
b. an isotope
c. a cation
d. an anion
The correct answer is d—
A. Answer a is incorrect because the term element refers to the atoms themselves and says
nothing about the status of the electrons.
The correct answer is d—
B. Answer b is incorrect because the term isotope refers to the natural variation that exists in
the number of neutrons found within an atomic nucleus.
The correct answer is d—
C. Answer c is incorrect. The term cation refers to an atom with a net positive charge. For
this to occur there must be more protons than electrons. (Helpful Hint!—To remember the
definition of cation think about “cats” and “paws”—paws = positive.)
The correct answer is d—an anion
D. Answer d is correct. An anion is an atom with a net negative charge. These atoms get their
charge when the number of negatively charged electrons exceeds the number of protons.
6. Which of the following is NOT a property of the elements most commonly found in
living organisms?
a. The elements have a low atomic mass.
b. The elements have an atomic number less than 21.
c. The elements possess eight electrons in their outer energy level.
d. The elements are lacking one or more electrons from their outer energy level.
The correct answer is c—
A. Answer a is incorrect. All of the elements found in living organisms have a low atomic
mass so this is not a correct answer.
The correct answer is c—
B. Answer b is incorrect. The common elements found in living organisms are all found
within the first 21 elements of the periodic table.
The correct answer is c—The elements possess eight electrons in their outer energy level.
C. Answer c is correct. Elements with eight electrons in their outer energy level are said to
be inert—that is, they are unable to react chemically with other atoms. In order to form
the complex molecules found in biological organisms, the atoms must be capable of
forming chemical bonds.
The correct answer is c—
Raven/Johnson Biology 8e
Chapter 02 - Answers
D. Answer d is incorrect. Atoms that are missing one or more of the electrons from their
outer energy level are capable of forming chemical bonds. This is a vitally important
property for the formation of the complex molecules required for life.
7. Which of the following atoms would you predict could be a cation?
a. Fluorine (F)
b. Helium (He)
c. Potassium (K)
d. Boron (B)
The correct answer is c—
A. Answer a is incorrect. The atom fluorine is lacking a single electron in its outermost
energy level. This atom is more likely to gain and electron, thereby obtaining a net
negative charge.
The correct answer is c—
B. Answer b is incorrect. Helium is an inert gas. Its outer energy level is full.
The correct answer is c—Potassium (K)
C. Answer c is correct. Potassium carries one electron in its outermost energy level. This
electron is likely to be lost, leaving a net positive charge on this atom.
The correct answer is c—
D. Answer d is incorrect. Boron possesses three electrons in its outermost energy level.
These electrons are more likely to form covalent bonds instead of leaving the atom to
form an ion.
8. Refer to the element pictured. How many covalent bonds could this atom form?
a. Two
b. Three
c. Four
d. None
The answer is b—
A. Answer a is incorrect. There are more than two unpaired electrons in the outer energy
level of the atom.
The answer is b—Three
B. Answer b is correct. Covalent bonds are formed between the atoms with unpaired valence
electrons.
The answer is b—
C. Answer c is incorrect. There are fewer than four unpaired valence electrons in the outer
energy level of this atom.
The answer is b—
D. Answer d is incorrect. There are some unpaired valence electrons in the outer energy level
of this atom.
Raven/Johnson Biology 8e
9.
Chapter 02 - Answers
Refer to the element pictured. How many covalent bonds could this atom form?
a. Two
b. Three
c. Four
d. None
The correct answer is d—
A. Answer a is incorrect. There must be an unpaired valence electron available in the outer
energy level of the atom in order for a covalent bond to form.
The correct answer is d—
B. Answer b is incorrect. All the electrons are paired in the outer energy level of this atom.
The correct answer is d—
C. Answer c is incorrect. All of the electrons are paired in the outer energy level of this atom.
The correct answer is d—None
D. Answer d is correct. All of the electrons in the outer energy level of the atom are paired
up with another electron. Neon is a noble gas and is chemically inert.
10. An ionic bond is held together by—
a. shared valence electrons
b. attractions between ions of the same charge
c. charge attractions between valence electrons
d. attractions between ions of opposite charge
The correct answer is d—
A. Answer a is incorrect. Shared electrons are a property of covalent bonds, not ionic bonds.
The correct answer is d—
B. Answer b is incorrect. Like charges would repel each other. No bond could form.
The correct answer is d—
C. Answer c is incorrect. All electrons carry the same, negative charge. Since like charges
repel, there could be no attraction between electrons.
The correct answer is d—Attractions between ions of opposite charge
D. Answer d is correct. Opposite charges attract. Ions of opposite charge, like Na+ and Clwill form an ionic bond.
11. How do polar covalent bonds differ from nonpolar covalent bonds?
a. In a polar covalent bond the electrons are shared equally between the atoms.
b. In a nonpolar covalent bond there is a charge attraction between the atomic nuclei.
c. There is a large difference in electronegativity of the atoms in a nonpolar bond.
d. There is a large difference in electronegativity of the atoms in a polar bond.
The correct answer is d—
Raven/Johnson Biology 8e
Chapter 02 - Answers
A. Answer a is incorrect. Although electrons are shared in a polar covalent bond, they are not
shared equally.
The correct answer is d—
B. Answer b is incorrect. Charge attraction between atomic nuclei has nothing to do with
covalent bond formation.
The correct answer is d—
C. Answer c is incorrect. Differences in electronegativity do not exist in nonpolar bonds.
The correct answer is d—There is a large difference in electronegativity of the atoms in a polar
bond.
D. Answer d is correct. Differences in the electronegativity of atoms involved in a covalent
bond lead to the unequal sharing of electrons. Atoms with a greater affinity for electrons
will draw the electrons of a covalent bond away from the atom with the lower affinity for
electrons resulting in a charge difference across the bond.
12. A hydrogen bond can form—
a. when hydrogen is part of a polar covalent bond
b. only in water
c. between any large electronegative atoms like oxygen
d. when two atoms of hydrogen share an electron
The correct answer is a—when hydrogen is part of a polar covalent bond
A. Answer a is correct. Hydrogen has a very low electronegativity value. When hydrogen
binds to other atoms with higher electronegativity values, such as oxygen or nitrogen, it
forms a polar bond and will have a positive partial charge.
The correct answer is a—
B. Answer b is incorrect. Hydrogen bond formation can occur in many other substances, not
just water.
The correct answer is a—
C. Answer c is incorrect. Hydrogen bond formation requires both an electronegative atom
with a partial negative charge and a hydrogen atom with a partial positive charge.
The correct answer is a—
D. Answer d is incorrect. The bond formed between two hydrogen atoms is a covalent bond,
not a hydrogen bond. Hydrogen bonds require that hydrogen carry a partial positive
charge.
13. Which of the following properties of water is NOT a consequence of its ability to form
hydrogen bonds?
a. Cohesiveness
b. High specific heat
c. Ability to function as a solvent
d. Neutral pH
Raven/Johnson Biology 8e
Chapter 02 - Answers
The correct answer is d—
A. Answer a is incorrect. Hydrogen bonds are responsible for the ability of water molecules
to stick to other water molecules—a property called cohesion.
The correct answer is d—
B. Answer b is incorrect. The strength of the many hydrogen bonds found in liquid water
resists the thermal energy that would otherwise increase the temperature of the water.
The correct answer is d—
C. Answer c is incorrect. Hydrogen bond formation between water molecules and other polar
molecules is responsible for the solubility of those molecules.
The correct answer is d—Neutral pH
D. Answer d is correct. The fact that the pH of water is neutral is a consequence of the
equilibrium reaction that breaks and reforms the covalent bonds between oxygen and
hydrogen in the water molecule. Hydrogen bond formation is not involved in this
chemical reaction.
14. A substance with a high concentration of hydrogen ions is—
a. called a base
b. called an acid
c. has a high pH
d. both b and c
The answer is b—
A. Answer a is incorrect. Bases are defined as substances with a high concentration of
hydroxide ions.
The answer is b—called an acid
B. Answer b is correct. The acidity of a solution is a function of the concentration of
hydrogen ions. The greater the concentration, the lower the pH.
The answer is b—
C. Answer d is incorrect. pH values represent the negative log of the concentration of
hydrogen ions. The more hydrogen ions that are present in a solution, the lower the pH
value.
The answer is b—
D. Answer d is incorrect. Although a high concentration of hydrogen ions is associated with
an acidic solution, the pH scale describes acidic solutions with a low value, not high.
Challenge Questions
1. Elements that form ions are important for a range of biological processes. Your have
learned about the cations sodium (Na+), calcium (Ca2+), and potassium (K+) in this
chapter. Use your knowledge of the definition of a cation to identify other examples from
the periodic table.
Raven/Johnson Biology 8e
Chapter 02 - Answers
Answer—A cation is an element that tends to lose an electron from its outer energy level, leaving
behind a net positive charge due to the presence of the protons in the atomic nucleus. Electrons
are only lost from the outer energy level if that loss is energetically favorable, that is, if it makes
the atom more stable by virtue of obtaining a filled outer energy level (the octet rule). You can
predict which elements are likely to function as cations by calculating which of the elements will
possess one (or two) electrons in their outer energy level. Recall that each orbital surrounding an
atomic nucleus can only hold two electrons. Energy level K is a single s orbital and can hold two
electrons. Energy level L consists of another s orbital plus three p orbitals—holding a total of
eight electrons. Use the atomic number of each element to predict the total number of electrons
present. Examples of other cations would include: hydrogen (H), lithium (Li), magnesium (Mg),
and beryllium (Be).
2. A popular theme in science fiction literature has been the idea of silicon-based life-forms
in contrast to our carbon-based life. Evaluate the possibility of silicon-based life based on
the chemical structure and potential for chemical bonding of a silicon atom.
Answer—Silicon has an atomic number of 14. This means that there are four unpaired electrons
in its outer energy level (comparable to carbon). Based on this fact, you can conclude that silicon,
like carbon, could form four covalent bonds. Silicon also falls within the group of elements with
atomic masses less than 21, a property of the elements known to participate in the formation of
biologically important molecules. Interestingly, silicon is much more prevalent than carbon on
Earth. Although silicon dioxide is found in the cell walls of plants and single-celled organisms
called diatoms, silicon-based life has not been identified on this planet. Given the abundance of
silicon on Earth you can conclude that some other aspect of the chemistry of this atom makes it
incompatible with the formation of molecules that make up living organisms.
3. Recent efforts by NASA to search for signs of life on Mars have focused on the search for
evidence of liquid water in the planet’s history rather than looking directly for biological
organisms (living or fossilized). Use your knowledge of the influence of water on life on
Earth to construct an argument justifying this approach.
Answer—Water is considered to be a critical molecule for the evolution of life on Earth. It is
reasonable to assume that water on other planets could play a similar role. The key properties of
water that would support its role in the evolution of life are:
 The ability of water to acts as a solvent. Molecules dissolved in water could move and
interact in ways that would allow for the formation of larger, more complex molecules
such as those found in living organisms.
 The high specific heat of water. Water can modulate and maintain its temperature, thereby
protecting the molecules or organisms within it from temperature extremes—an important
feature on other planets.
 The difference in density between ice and liquid water. The fact that ice floats is a simple,
but important feature of water environments since it allows living organisms to remain in
a liquid environment protected under a surface of ice. This possibility is especially
intriguing given recent evidence of ice-covered oceans on Europa, a moon of the planet
Jupiter.