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2
Chapter 2
Atoms, Elements, and Compounds
2.1
A
LE
Solutions to Section 2-1 Core Problems
Which of the following will make a homogeneous mixture, and which will make a
heterogeneous mixture? Assume that the two components are stirred well.
a) A teaspoon of salt and a cup of water
Homogeneous mixture: the salt will dissolve completely in the water to form a
solution, and the composition of the solution will be the same throughout.
b) A teaspoon of pepper and a cup of water
S
Heterogeneous mixture: the pepper will not dissolve in the water, but will instead
remain as flakes of pepper. The composition of the mixture will be different in
different places.
2.3
Which of the following statements describe an intensive property, and which describe an
extensive property?
R
a) The glass in my front window is transparent.
FO
Intensive: the glass is transparent regardless of how big the window is. This is a
property of the substance (the glass), and it so does not depend on the size of the
glass.
b) The glass in my front window weighs 1.2 kg.
Extensive: smaller pieces of glass would weigh less, larger pieces would weigh more.
This property is specific to this object and depends on the size of the sample
c) The glass in my front window is 2 mm thick.
Extensive: windows made of the same glass could be thicker or thinner. This
property is specific to this object and is size-dependent.
T
d) The glass in my front window does not dissolve in water.
O
Intensive: the glass does not dissolve in water. This is a property of the substance,
not just of the size of the object.
N
2.5
Chalk is a naturally occurring mineral. In ancient times, people discovered that if chalk
is heated, it breaks down into a fluffy white powder (called quicklime) and a colorless,
odorless gas (called fixed air). A sample of chalk from anywhere on Earth will break
down to produce 56% quicklime and 44% fixed air.
a) Based only on this information, is chalk an element, a compound, or a mixture? Or
can you tell? Explain your reasoning.
Chalk can break down into other substances, so it can’t be an element. But it also has
a fixed composition (always gives the same proportions of those substances), so it
24
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Atoms, Elements, and Compounds
must be a compound. If it were a mixture, then the proportions of the component
substances would be variable.
b) Based on this information, is quicklime an element, a compound, or a mixture? Or
can you tell? Explain your reasoning.
2.7
A
LE
Quicklime is produced from the breakdown of a pure substance (chalk), so it’s likely
to be a pure substance and not a mixture (though in some cases a compound can break
down into a mixture of compounds or elements). However, we don’t have any
information on whether quicklime itself can be broken down into yet other substances
or not, so we can’t decide whether it is an element or a compound.
Sulfur and oxygen are both nontoxic. When these substances are mixed and heated, they
can combine to make a poisonous gas. Based on this information, is this combination
likely to be a compound, or is it probably a mixture? Explain your reasoning.
S
Mixtures of substances have physiological effects similar to those of the component
substances. Since the individual substances are nontoxic, their mixtures are also expected
to be nontoxic. Compounds, on the other hand, usually have very different
properties, sometimes including toxicity, than their component elements. Therefore,
the combination described must produce a compound, not a mixture.
Write the name of each of the following elements.
R
2.9
No way around this but to memorize; these elements are listed in Table 2.2.
e) Co cobalt
2.11
b) N nitrogen
FO
a) C carbon
c) Cl chlorine
d) Mg magnesium
f) Se selenium
Each of the following elements has a symbol that does not match its English name. What
is the symbol for each of these? Again, these must be memorized from Table 2.2.
a) iron Fe
b) sodium Na
c) silver Ag
d) lead Pb
Solutions to Section 2-2 Core Problems
Fill in the blanks in the following statements with the names of the appropriate subatomic
particles.
T
2.13
O
Our options for each blank are proton, neutron, or electron, and we know the properties
of each (Table 2.3).
N
a) Electrons have a negative charge.
Protons are positive and neutrons are neutral.
b) Protons and neutrons have roughly the same mass.
Each of these particles has a mass of about 1 amu, while the mass of an electron is
much smaller.
25
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Chapter 2
c) The number of protons determines which element an atom is.
The element identity of an atom is determined by the number of protons in its
nucleus.
A
LE
d) Electrically neutral atoms have the same number of protons and electrons.
“Electrically neutral” means that the atom has zero net charge. Since protons and
electrons have equal but opposite charges, the number of protons must be equal to the
number of electrons in an electrically neutral atom.
2.15
You have an atom that contains 16 protons, 17 neutrons, and 16 electrons.
a) What is the mass number of this atom?
The mass number is the sum of the neutrons and protons: 16 + 17 = 33.
b) What is the atomic number of this atom?
S
The atomic number is the number of protons (16 in this atom.)
c) What is the approximate mass of this atom in amu?
d) What element is this?
R
Since protons and neutrons each have a mass of approximately 1 amu, the mass
number gives the approximate mass (to the nearest 1 amu) of an atom. The
approximate mass of this atom is 33 amu.
FO
The element identity of an atom is determined by the number of protons (16 in this
atom). The element with atomic number 16 is sulfur.
2.17 The atomic number of silver is 47. How many protons, neutrons and electrons are there
in an electrically neutral atom of silver-107?
The atomic number of an element is (always) the number of protons in its atoms, so this
atom has 47 protons.
T
To get the number of neutrons, subtract the number of protons from the mass number:
107 – 47 protons = 60 neutrons.
In any electrically neutral atom, the number of electrons and the number of protons must
be equal: 47 electrons for a neutral atom of silver.
What is the significance of the number 16 in the symbol 16O?
O
2.19
The upper-left position is reserved for the mass number (number of neutrons +
number of protons).
N
Solutions to Section 2-3 Core Problems
2.21
Which of the following is a true statement? Refer to Figure 2.6 as you answer this
question.
Figure 2.6 illustrates the electron shells for an atom of argon (Ar). Argon has 18
electrons, arranged in three shells. The shells represent areas where the electrons are most
26
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Atoms, Elements, and Compounds
likely to be found, but in reality an electron does not stay on the “surface” of an electron
shell—in fact, there is no real surface; the illustration only represents maximum
probability areas for the electrons.
a) Electrons in the first shell must remain inside the region labeled shell 1.
A
LE
Not true; the shell isn’t a cage, it’s an area of probability of finding an electron.
b) Electrons in the first shell must remain on the surface of the shell.
Definitely not true. Electrons can go anywhere inside or outside the shell.
c) Electrons in the first shell can sometimes be outside the region labeled shell 1.
This is the only true statement.
2.23
Write the electron arrangement for each of the following elements.
S
In an electrically neutral atom, the number of electrons = the number of protons = the
atomic number, so we can easily find out how many electrons each element has in its
atoms. These elements are both among the first 18 elements, so their electrons occupy the
lowest possible shells.
a) nitrogen
R
The shells are keyed to the periodic table: we can remember that shell 1 holds 2 electrons
because He, the first noble gas, has 2 electrons. We can remember that shell 2 holds 8
electrons because Ne, the next noble gas after He, has 8 more electrons than He (10 – 2 =
8.)
FO
Each N atom has a total of 7 electrons, filling the shells systematically.
shell 1: 2 electrons
shell 2: 5 electrons
This uses up all the electrons, so no further shells are needed.
b) magnesium
T
Each Mg atom has a total of 12 electrons, filling the shells systematically.
shell 1: 2 electrons
shell 2: 8 electrons
shell 3: 2 electrons
This uses up all the electrons, so no further shells are needed.
2.25
The electron arrangement of selenium (Se) is
shell 1: 2 electrons
shell 2: 8 electrons
shell 3: 18 electrons shell 4: 6 electrons
N
O
a) How many valence electrons does an atom of selenium contain?
By definition, valence electrons are the electrons in the outermost shell (in this case,
shell 4). Selenium has 6 valence electrons. (As explained in Section 2-9, we could
also have determined this by seeing that Se is in Group 6A of the periodic table.)
27
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Chapter 2
b) Draw the Lewis structure of a selenium atom.
2.27
A
LE
The Lewis structure includes the element symbol and the valence electrons,
represented as dots placed around the chemical symbol. (The paired and single
electrons can be shown in any of the four positions around the atom.)
a) How many valence electrons are there in an atom of carbon (C)?
By definition, valence electrons are the electrons in the outermost shell (in this case,
shell 2). Carbon has four valence electrons. (As we will see in Section 2-9, for maingroup elements, the number of valence electrons is the group number. Carbon is in
Group 4A, so it has four valence electrons.)
b) Draw the Lewis structure of a carbon atom.
C
Lead (Pb) has four valence electrons. Which elements in Table 2.6 should show similar
behavior to lead, based on their numbers of valence electrons?
R
2.29
S
The Lewis structure includes the element symbol and the valence electrons,
represented as dots.
FO
Elements with the same number of electrons in their outermost occupied shells often have
similar chemical properties. The other elements in Table 2.6 with four valence electrons
are Carbon (C) and Silicon (Si).
Solutions to Section 2-4 Core Problems
2.31
Using the periodic table, find elements that match each of the following descriptions:
a) a nonmetal in Group 6A
T
Nonmetals are on the right of the heavy “stair step” line, not adjacent to the line, (the
elements shaded in blue in Figure 2.8), and Group 6A is the Oxygen family. O, S,
and Se are all nonmetals in Group 6A. (Te and Po are also in Group 6A, but are
metalloids.)
b) a metal in the same group as carbon
N
O
Carbon is C, the element with atomic number 6. Metals (the elements shaded in tan in
Figure 2.8) are to the left of the heavy “stair-step” line, not adjacent to the line, and
groups are vertical columns. Si and Ge are metalloids in the same group as carbon.
The metals in the group are Sn and Pb.
c) a metalloid in the same period as calcium
Periods are horizontal rows; calcium is in Period 4. The metalloids (elements adjacent
to the stair-step line, in green-shaded blocks in Figure 2.8) in this period are Ge and
As.
28
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Atoms, Elements, and Compounds
d) an element that is shiny and conducts electricity
A
LE
These are characteristics of metals, so we can choose any metal in the entire periodic
table (any element shaded in tan and to the left of the stair-step line, not adjacent to
the line, in Figure 2.8): Li, Be, Al, Sn, Bi and any elements under them, as well as
any element from a B group.
e) a halogen in Period 3
Periods are horizontal rows. Halogens are elements in Group 7A (except H). There is
only one possible answer for this, where Period 3 and Group 7A intersect: Cl.
f) an alkaline earth metal
The alkaline earth metals are Group 2A: Be, Mg, Ca, Sr, Ba, Ra.
Columbium is a name that was once used for one of the chemical elements. Columbium is
a shiny, silvery solid that conducts electricity well and can bend without shattering. Is
columbium a metal, a metalloid, or a nonmetal?
S
2.33
The properties listed are characteristics of a metal (see Table 2.8). (Columbium is now
called niobium; the name columbium was derived from the mineral source, columbite.)
Based on its position on the periodic table, how many valence electrons does an atom of
selenium (Se) contain? Which shell do they occupy?
R
2.35
2.37
FO
For representative elements, the group number is the number of valence electrons (for Se,
Group 6A). Also for representative elements, the period number is the valence shell (for
Se, Period 4). So selenium has 6 valence electrons in shell 4.
A representative element has the following electron arrangement:
shell 1: 2 electrons
shell 4: 8 electrons
shell 2: 8 electrons
shell 5: 1 electron
shell 3: 18 electrons
Without looking at the periodic table, answer the following questions:
T
a) How many valence electrons does this element have?
By definition, valence electrons are the electrons in the outermost occupied shell in an
atom. In this case, the outermost shell is shell 5, with 1 valence electron.
N
O
b) What is the group number for this element?
By definition, representative elements are those in the A groups. For representative
elements, the group number = the number of valence electrons. This element is in
Group 1A (the alkali metals).
c) Which period is this element in?
For representative elements, the period number is the valence shell. This element is
in Period 5.
29
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Chapter 2
2.39
Draw Lewis structures for each of the following atoms. You may use the periodic table.
Lewis structures represent valence electrons as dots around the chemical symbol. For
each element, we’ll use the group number to determine the number of valence electrons.
A
LE
a) K Group 1A, one valence electron:
b) Pb Group 4A, four valence electrons:
c) Br Group 7A, seven valence electrons:
2.41
Using the periodic table, give an example of each of the following:
a) a metal that has five valence electrons
S
We’re looking for a metal (the elements to the left of the stair-step line,, not adjacent
to the line, shaded in tan in Figure 2.8) in Group 5A: Bi.
b) a representative element that has its valence electrons in the sixth shell
For representative elements, the period number and the valence shell number are the
same, so we can use any representative element in Period 6: Cs, Ba, Tl, Pb, Bi, Po,
At, Rn.
R
c) a transition element that is in the fourth period
FO
The fourth period is the fourth row of the periodic table, and the transition elements
(also called transition metals) are the B groups: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
Zn.
d) an element that should show similar chemical behavior to oxygen
T
Elements in the same group show similar chemical behavior (and the closer two
elements are within a group, the more similar their chemical behavior will be). The
element that should have the most similar chemical behavior to oxygen is sulfur, S,
but other Group 6A elements should also show similar chemical behavior: Se, Te, Po.
(The last two, being metalloids, are likely to be less similar to O than S or Se would
be, since O, S and Se are nonmetals.)
Solutions to Section 2-5 Core Problems
Which of the following pairs of atoms are isotopes?
O
2.43
Isotopes have the same atomic number (# protons), and are therefore the same element,
but have different mass numbers.
N
a)
39
K and 40K
Isotopes: both atoms are the same element but they have different mass numbers.
30
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Atoms, Elements, and Compounds
b) An atom with seven protons and eight neutrons, and an atom with eight protons and
eight neutrons.
2.45
A
LE
Not isotopes: The atoms do have different mass numbers (15 and 16), but they are
also different elements because they have different numbers of protons (the atom with
seven protons is nitrogen, and the atom with eight protons is oxygen).
A beaker is filled with a large number of atoms, all of which have 34 protons and 44
neutrons. Which of the following would look and behave similarly to this collection of
atoms?
a) a group of atoms that have 33 protons and 45 neutrons
b) a group of atoms that have 35 protons and 44 neutrons
c) a group of atoms that have 34 protons and 45 neutrons
S
Chemical behavior is determined by the number of protons (atomic number), regardless
of the number of neutrons and the mass number. The only group of atoms that would
behave similarly is (c), the group that also has 34 protons in its nuclei. (Another way
of looking at this is that the original sample is a sample of Se, and the other two samples
are a) As and b) Br.)
The atomic weight of gold (from the periodic table) is 197.0 amu. Based on this fact,
which of the following conclusions can be drawn?
R
2.47
FO
The atomic weight is an average of all naturally-occurring atoms of an element. It doesn’t
tell us anything specific about which isotopes exist or in what proportions, but it does
give us an idea of what mass numbers are reasonable or likely (we would expect to see
mass numbers that are close to the average atomic weight).
a) All gold atoms weigh 197 amu.
We cannot conclude this; for example, a mixture of 50% 196Au and 50% 198 Au
would give the same average.
b) Most gold atoms weigh 197 amu.
T
We cannot conclude this; there don’t have to be any atoms at all with a mass of 197
amu.
O
c) More gold atoms weigh 197 amu than any other mass.
We cannot conclude this; there don’t have to be any atoms at all with a mass of 197
amu.
N
d) We cannot conclude anything about the masses of individual gold atoms.
This is the only safe answer (though we could predict that gold atoms with masses
very different from 197 are unlikely to exist—we would not expect to see atoms of
gold-160 or gold-220, for example.)
31
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Chapter 2
Solutions to Section 2-6 Core Problems
2.49
How much does each of the following weigh? Be sure to use the appropriate unit for each
answer.
A
LE
The average atomic weight of an element, in amu or atomic mass units, is given in the
periodic table. What’s new in this section is the definition of a mole of an element: a
mole is the atomic weight of an element expressed in grams, and contains 6.022 ×1023
atoms of that element. In every case, the average mass of an atom of an element, given
in amu, is the same value as the mass of a mole of atoms of that element, given in
grams. The number is the same; we just use amu for individual atoms, molecules or
formula units and g for moles of a substance.
Remember: amu for atoms, g for moles!
a) One atom of chlorine
S
The atomic mass of chlorine, as given in the periodic table, is 35.45 amu. Remember
that this is the average mass of an atom of chlorine–there are no individual atoms of
chlorine that actually have a mass of 35.45 amu. We would expect an individual atom
of chlorine to weigh 35 or 37 amu (as listed in Table 2.9), not 35.45.
b) One mole of chlorine
2.51
R
If the average mass of an atom of chlorine is 35.45 amu, then the mass of a mole of
chlorine is 35.45 g.
Convert the following masses into moles.
FO
a) 82.77 g of Na
We look in the periodic table to find the atomic mass of Na (sodium), 22.99 amu.
From the definition of a mole, if one atom of Na has a mass of 22.99 amu, then one
mole of Na has a mass of 22.99 grams. We can use this relationship to write two
conversion factors:
1 mol Na
22.99 grams Na
or
22.99 grams Na
1 mol Na
O
T
Then we choose the form of the conversion factor that makes the units cancel
properly, and set up the calculation (rounding our answer to 3 significant figures):
1 mol Na
82.77 g Na 
 3.60 mol Na
22.99 g Na
N
b) 2.31 g of Cu
From the periodic table, the mass of one mole of copper is 63.55 g, which gives us
two conversion factors:
1 mol Cu
63.55 g Cu
or
63.55 g Cu
1 mol Cu
Then we choose the form of the conversion factor that makes the units cancel
properly, and set up the calculation (rounding our answer to 3 significant figures):
32
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Atoms, Elements, and Compounds
2.31 g Cu 
1 mol Cu
63.55 g Cu
 0.0363 mol Cu
c) 2.5 × 108 g of Pb
A
LE
From the periodic table, the mass of one mole of Pb is 207.2 g. Our two conversion
factors are:
1 mol Pb
207.2 g Pb
or
207.2 g Pb
1 mol Pb
We choose the form of the conversion factor that makes the units cancel properly, and
set up the calculation (rounding our answer to 2 significant figures):
2.5 108 g Pb 
Convert the following numbers of moles into masses.
S
2.53
1 mol Pb
 1.2  106 mol Pb
207.2 g Pb
a) 2.33 mol of Zn
From the periodic table, the mass of one mole of Zn is 65.38 g. Our two conversion
factors are:
R
65.38 g Zn
1 mole Zn
or
65.38 g Zn
1 mole Zn
FO
We choose the form of the conversion factor that makes the units cancel properly, and
set up the calculation (rounding our answer to 3 significant figures):
2.33 mol Zn 
65.38 g Zn
1 mol Zn
 152 g Zn
b) 0.02155 mol of Br
From the periodic table, the mass of one mole of Br is 79.90 g. Our two conversion
factors are:
We choose the form of the conversion factor that makes the units cancel properly, and
set up the calculation (rounding our answer to 4 significant figures):
0.02155 mol Br 
79.90 g Br
1 mol Br
 1.722 g Br
N
O
T
1 mol Br
79.90 g Br
or
79.90 g Br
1 mol Br
33
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Chapter 2
Solutions to Section 2-7 Core Problems
2.55
A formula unit of table sugar (sucrose) contains 12 carbon atoms, 22 hydrogen atoms,
and 11 oxygen atoms. Write the chemical formula of table sugar.
2.57
A
LE
We write the element symbols for the elements in the order given, and put a subscript
after each symbol to show how many atoms of that element are present: C12H22O11.
Monosodium glutamate (MSG) is a commonly used food additive that has the chemical
formula NaC5H8NO4.
a) How many atoms of each element are in one formula unit of MSG?
Remember that any element listed without a number is assumed to have the subscript
“1.” There are one atom of sodium (Na), five atoms of carbon (C), eight atoms of
hydrogen (H), one atom of nitrogen (N), and four atoms of oxygen (O).
b) If you have three formula units of MSG, how many carbon atoms do you have?
2.59
S
Each formula unit contains five atoms of carbon, so three formula units contain a
total of 5 × 3 = 15 atoms of carbon.
How much does each of the following weigh? Be sure to use the appropriate units.
R
a) one formula unit of Na2CO3
To calculate the mass of a formula unit of a substance, we add up the masses of the
atoms that make up the formula unit. For Na2CO3:
=
=
=
2 × 22.99 amu = 45.98 amu
= 12.01 amu
3 × 16.00 amu = 48.00 amu
= 105.99 amu
FO
2 Na atom
1 C atoms
+ 3 O atoms
b) one mole of Na2CO3
If one formula unit of Na2CO3 has a mass of 105.99 amu, then one mole of Na2CO3
has a mass of 105.99 g.
Convert each of the following masses into moles.
T
2.61
N
O
General strategy: in any problem asking us to relate the mass of a sample to the moles of
substance it contains, we will need to (1) add up the formula mass, using the atomic
weight values from the periodic table; (2) recognize that the formula mass in amu and
the molar mass in grams have the same numerical value, and give that value the units of
grams/mole; and then (3) use the molar mass to convert appropriately.
a) 13.47 g of KMnO4
First, we add up the formula weight of KMnO4:
1K
1 Mn
+4O =
4 × 16.00 amu
= 39.10 amu
= 54.94 amu
= 64.00 amu
= 158.04 amu
34
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Atoms, Elements, and Compounds
If one formula unit of KMnO4 has a mass of 158.04 amu, then one mole of KMnO4
has a mass of 158.04 grams. We can use this relationship to write two conversion
factors:
A
LE
1 mol KMnO 4
158.04 g KMnO 4
or
158.04 g KMnO 4
1 mol KMnO 4
Then we set up the calculation, choosing the correct form of the concentration
conversion factor to make the units cancel properly, and rounding our answer to 4
significant figures:
13.47 g KMnO 4 
1 mol KMnO 4
 0.08523 mol KMnO 4
158.04 g KMnO 4
b) 12.99 g of phenylalanine (C9H11NO2)
First, we calculate the formula mass of phenylalanine:
108.09 amu
11.09 amu
14.01 amu
32.00 amu
165.19 amu
S
= 9 × 12.01 amu =
= 11 × 1.008 amu =
=
= 2 × 16.00 amu =
=
R
9 C atoms
11 H atoms
1 N atom
+ 2 O atoms
If one molecule of phenylalanine has a mass of 165.19 amu, then one mole of
phenylalanine has a mass of 165.19 grams. This provides us with two conversion
factors:
FO
1 mol phenylalanine
165.19 g phenylalanine
or
165.19 g phenylalanine
1 mol phenylalanine
Then we choose the form of the conversion factor that makes the units cancel
properly, and set up the calculation (rounding our answer to 4 significant figures):
12.99 g phenylalanine 
1 mol phenylalanine
 0.09903 mol phenylalanine
165.19 g phenylalanine
T
c) 6.47 kg of aspirin (C9H8O4)
N
O
First we calculate the formula mass of aspirin:
9 C atoms
8 H atoms
+ 4 O atoms
=
=
=
9 × 12.01 amu
8 × 1.008 amu
4 × 16.00 amu
=
=
=
=
108.09 amu
8.06 amu
64.00 amu
180.15 amu
If one molecule of aspirin has a mass of 180.15 amu, then one mole of aspirin has a
mass of 180.15 grams. This provides us with two conversion factors:
1 mol aspirin
180.15 g aspirin
or
180.15 g aspirin
1 mol aspirin
35
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Chapter 2
Then we choose the form of the conversion factor that makes the units cancel
properly, and set up the calculation, making sure to convert the given kg to g and
rounding our answer to 3 sf:
2.63
1000 g
1 mol

 35.9 mol aspirin
1 kg
180.15 g
A
LE
6.47 kg aspirin 
Convert each of the following numbers of moles into a mass.
General strategy: in any problem asking us to relate the mass of a sample to the moles of
substance it contains, we will need to (1) add up the formula mass, using the atomic
weight values from the periodic table; (2) recognize that the formula mass in amu and the
molar mass in grams have the same numerical value, and give that value the units of
grams/mole; and then (3) use the molar mass to convert appropriately.
We add up the formula mass of KClO4:
=
= 39.10 amu
= 35.45 amu
4 × 16.00 amu = 64.00 amu
= 138.55 amu
R
1 K atom
1 Cl atom
+ 4 O atoms
S
a) 0.318 mol of KClO4
If one formula unit of KClO4 has a mass of 138.55 amu, then one mole of KClO4 has
a mass of 138.55 grams. We can use this relationship to write two conversion factors:
FO
1 mol KClO 4
138.55 g KClO 4
or
138.55 g KClO 4
1 mol KClO 4
Then we choose the correct form of the conversion factor to make the units cancel
properly, and set up the calculation (rounding our answer to 3 significant figures):
0.318 mol KClO 4 
138.55 g
1 mol
 44.1 g KClO 4
b) 13.77 mol of leucine (C6H13NO2)
T
First we need to add up the molecular mass of leucine:
N
O
6 C atoms
13 H atoms
1 N atom
+ 2 O atoms
= 6 × 12.01 amu = 72.06 amu
= 13 × 1.008 amu = 13.10 amu
= 14.01 amu
= 2 × 16.00 amu = 32.00 amu
= 131.17 amu
If one molecule of leucine has a mass of 131.17 amu, then one mole of leucine has a
mass of 131.17 g. We can use this relationship to write two conversion factors:
1 moe leucine
131.17 g leucine
or
131.17 g leucine
1 mol leucine
36
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Atoms, Elements, and Compounds
Then we choose the form of the conversion factor that makes the units cancel
properly, and set up the calculation (rounding our answer to 4 significant figures):
2.65
1 mol leucine
 0.09903 mol leucine
131.17 g leucine
A
LE
12.99 g leucine 
Complete the following statements by inserting a number into each space.
a) To make PbCl2, you must use 1 mol of Pb for every 2 mol of Cl.
The mole ratio of elements in a compound is the same as the atom ratio given in the
formula. The formula shows that there are 2 atoms of Cl for every 1 atom of Pb, so
there must also be 2 moles of Cl for every 1 mole of Pb.
b) To make PbCl2, you must use 207.2 g of Pb for every 70.90 g of Cl.
Solutions to Concept Questions
S
For the mass relationship, we take the mole relationship we determined in part a) and
use it to calculate the mass of each element. One mole of Pb has a mass of 207.2 g,
and 2 moles of Cl has a mass of 2 × 35.45 = 70.90 g.
* indicates more challenging problems.
a) What is the fundamental difference between a mixture and a compound?
R
2.67
FO
Both mixtures and compounds are made up of two or more substances, but mixtures
have variable composition (any proportions of the substances can be mixed, within
certain limits) while compounds have constant composition (only certain very
specific amounts of two or more elements will combine to form a compound).
b) What is the fundamental difference between an element and a compound?
Both elements and compounds are pure substances (cannot be separated into two or
more other substances by physical processes). However, compounds can be broken
down into two or more other substances by chemical processes, while elements
cannot.
O
T
Another way to express the fundamental difference between compounds and elements
is that in an element, every atom has the same number of protons, while a
compound must by definition by composed of at least two different elements
(atoms with different numbers of protons).
N
2.69
Physiological saline is a mixture of salt and water that can be used in intravenous
injections. It always contains 0.9% salt and 99.1% water. Is physiological saline a
compound? Explain your answer.
Start by looking at the way “compound” and “mixture” are defined, and the
characteristics of each. Physiological saline is a mixture, not a compound. Here are three
possible ways to explain or rationalize this:
(1) While the term “physiological saline” is only applied to a mixture that has the
composition above, those percentages are agreed upon by humans, they aren’t a
37
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Chapter 2
law of nature. (It’s essentially a labeling law—similar to the rule for the maximum
amount of fat a product could contain and still be labeled “low-fat.”) Salt and water
can be mixed in any proportions to make saltwater.
A
LE
(2) The salt and water are not chemically bonded to each other; they can still be
easily separated by physical methods (for example, letting the water evaporate
away from the salt). This is a characteristic of mixtures.
(3) The mixture of salt and water looks like one of its components (water) and has
properties that are a combination of those of the components, a common
characteristic of a mixture. Compounds, in contrast, typically have properties that
are very different from those of the component elements.
2.71
A student says, “A carbon atom is not the smallest possible amount of carbon, because
atoms are built from smaller particles.” How would you respond to this student?
S
While it’s true that atoms are built from smaller particles, these smaller particles are not
“carbon,” just as a brick is not a house, a piece of fingernail is not a hand, and a grain of
sand is not a beach. If you take apart an atom of carbon into smaller particles, it’s
not carbon anymore.
Why do chemists express the masses of atoms using the atomic mass units, rather than a
normal metric unit such as grams or milligrams?
R
2.73
2.75
FO
We often choose units based on their convenience for a given object. We don’t express
the mass of a grain of rice in tons, the length of a sofa in miles, or the volume of the sun
in milliliters. An atom is so small that its mass in grams or milligrams or even
nanograms is an extremely small, inconvenient number to work with. The whole
purpose of the atomic mass unit (amu) is to easily work with the masses of
individual atoms and subatomic particles.
a) What is an electron shell?
T
In any atom, the electrons are arranged in definite layers, referred to as “shells,”
starting from the nucleus and proceeding outward. These shells are regions
within which the electrons are most probably found.
b) What is an orbital, and what is the relationship between an orbital and a shell?
N
O
An orbital is a region of an electron shell that can hold two electrons. Orbitals
are subdivisions of shells (every shell except shell 1 contains multiple orbitals.)
38
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Atoms, Elements, and Compounds
The drawing below represents the arrangement of the atoms in niacin. Write the chemical
formula for niacin.
A
LE
2.77
2.79
S
The structure contains six carbon atoms, five hydrogen atoms, two oxygen atoms and one
nitrogen atom. The chemical formula is C6H5NO2. (By convention, the formulas of most
compounds of carbon and hydrogen with other elements are written with carbon first,
hydrogen second, and other elements in alphabetical order after that; this will be
discussed in Chapters 3 and 9.)
How is one mole of carbon similar to one mole of lead? How do they differ?
R
A mole is a number of items, just as a dozen is a number of items. A mole of one element
will contain the same number of atoms as a mole of any other element. One mole of
carbon and one mole of lead each contain the same number of atoms, just as one
dozen carbon atoms is the same number of atoms as a dozen lead atoms.
FO
However, that’s about the only thing they have in common. The two samples will have
different masses (because the atomic weights of the two elements are different); they
will contain different numbers of electrons, protons and neutrons (because the
number of each particle in an individual atom of carbon or lead is different); they
will have different chemical and physical properties (because they are different
chemical elements); etc.
T
(There is one other thing carbon and lead have in common—since they are both in Group
4A, they have the same number of valence electrons. So both samples will contain the
same number of valence electrons.)
2.81
You have samples of two different elements. Sample #1 contains more moles than sample
#2 does. Which of the following statements are true?
N
O
a) Sample #1 must contain more atoms than sample #2 does. Since the problem
specifies that these are samples of elements, this must be true; “moles” in reference
to samples of elements must mean atoms, so if sample #1 contains more moles, it
must contain more atoms.
One way to think of this is to substitute the word “dozen” for the word “mole.” If
sample #1 contains more dozens (of atoms) than sample #2, it must also contain more
atoms.
39
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Chapter 2
NOTE:
If these were samples of compounds, this statement would not necessarily be true. For
example, a sample of 2 moles of CO2 contains fewer atoms than a sample of 1 mole
of C6H5O2N.)
A
LE
b) Sample #1 must weigh more than sample #2 does. False. It’s easy to find a
counterexample of this: let’s say sample #1 contains 2 moles of Li (or 2 × 6.94 =
13.88 g of Li), and sample #2 contains 1 mole of Na (or 22.99 g of Na). The masses
of the sample could go either way—it depends on the particular elements present in
the samples.
c) Sample #1 must take up more space than sample #2 does. False. For example, if
sample #1 is a solid but sample #2 is a gas, then sample #2 may well take up more
space, even if it has fewer moles.
You want to make one mole of glycerin (C3H6O3). How many moles of each element must
you use? How can you tell?
S
2.83
R
The chemical formula of a compound tells us the number of atoms of each element
present in one formula unit, and also how many moles of each element are present
in one mole of the compound. The formula C3H6O3 indicates that one mole of
glycerin contains three moles of carbon, six moles of hydrogen, and three moles of
oxygen.
Solutions to Summary and Challenge Problems
2.85
FO
* indicates more challenging problems.
Which of the following are true statements?
a) Compounds generally look different from any of the elements they are made from.
True. The properties of compounds, including their appearances, are generally
(though not necessarily always) very different from the appearances of the component
elements.
b) Elements are made from atoms, but compounds are not.
T
False. Elements and compounds are both made from atoms. The difference is that in a
sample of an element, all the atoms have the same number of protons, while a
compound by definition contains atoms of at least two different elements.
N
O
c) You can always tell whether a substance is a compound or a mixture by looking at it.
False. Mixtures often look very much like one or more of their component pure
substances (for example, salt water looks like water). You can’t necessarily tell
whether a sample is a mixture or a pure substance by looking at it.
d) Wood is a heterogeneous mixture.
True. Even to the naked eye, and much more so under a microscope, wood has
several different-looking components and is a heterogeneous mixture.
40
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Atoms, Elements, and Compounds
e) You cannot make an element by combining two other substances.
True. Elements cannot be made from, or broken down into, anything else by physical
or chemical processes.
An atom contains 29 protons, 36 neutrons, and 29 electrons.
A
LE
2.87
a) What is the atomic number of this atom? The atomic number of any atom is the
number of protons it contains, so this atom’s atomic number is 29.
b) What is the mass number of this atom? The mass number of any atom is the total
number of protons and neutrons it contains, so this atom’s mass number is 29 + 36 =
65.
S
c) What is the approximate mass of this atom? The mass number is the approximate
mass number in amu (atomic mass units), so the approximate mass of this atom is 65
amu. (This is because the mass of an electron is very small, so most of the mass of an
atom is due to the protons and neutrons, each of which has an approximate mass of 1
amu).
d) What element is this? Looking at the periodic table, we see that the element with
atomic number 29 is Cu, copper.
R
e) What group is this element in? “Groups” are vertical columns in the periodic table.
The element with atomic number 29 is in Group 1B (under the numbering system
used in this book).
2.89
FO
f) What period is this element in? “Periods” are horizontal rows in the periodic table.
The element with atomic number 29 is in Period 4.
Iodine-131 is used to diagnose and treat thyroid conditions. How many protons,
neutrons, and electrons are in an electrically neutral atom of iodine-131?
The number of protons in each atom any element is equal to the atomic number. We use
the periodic table to find that the atomic number of iodine (I) is 53, so an atom of iodine
contains 53 protons. (This would be true for any atom of any isotope of iodine.)
O
T
The number 131 in this context is the mass number of a specific isotope of iodine. The
mass number of an isotope is the sum of the protons and neutrons in the nucleus of each
atom of that isotope. We can rearrange the relationship
protons + neutrons = mass number
N
to get
mass number – protons = neutrons
131 – 53 = 78 neutrons
Protons and electrons have equal and opposite charges. Therefore, in any electrically
neutral atom, the number of electrons must be equal to the number of protons—in this
case, 53 electrons.
41
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Chapter 2
*2.91 Lipitor (atorvastatin calcium) is a chemical compound that decreases the concentration
of low-density lipoprotein (so-called “bad cholesterol”) in the blood. It has the chemical
formula CaC66H68F2N4O10.
a) How many hydrogen atoms are in one formula unit of Lipitor?
A
LE
The chemical formula of a compound tells us how many atoms of each element are
present in one formula unit. One formula unit of Lipitor contains 68 hydrogen
atoms.
b) If you have 20 formula units of Lipitor, how many fluorine atoms do you have?
From the chemical formula, we can see that one formula unit of Lipitor contains 2
fluorine atoms. Therefore, 20 formula units of Lipitor would contain 40 F atoms:
20 formula units of Lipitor 
2 F atoms
 40 F atoms
1 formula unit of Lipitor
1 formula unit Lipitor
 50 formula units of Lipitor
4 N atoms
R
200 N atoms 
S
c) A sample of Lipitor contains a total of 200 nitrogen atoms. How many formula units
of Lipitor are in this sample? One formula unit of Lipitor contains 4 nitrogen atoms.
To get to 200 nitrogen atoms would require 50 formula units of Lipitor:
d) A sample of Lipitor contains 300 oxygen atoms. How many carbon atoms does it
contain?
FO
From the formula, we find that there are 10 O atoms for every 66 C atoms. We can
use this relationship as a conversion factor:
300 O atoms 
2.93
66 C atoms
 1980 C atoms
10 O atoms
Give one example of each of the following:
a) an element that has five valence electrons
T
This would apply to any Group 5A element, so take your pick: N, P, As, Sb, Bi.
b) an element that has two electrons in shell 4 (Hint: It can have electrons in other
shells.)
N
O
The easy one here is Ca, with its two valence electrons. But generally elements 21
(Sc) through 30 (Zn) also have two electrons in shell 4, because the electrons that
are being added to those elements are going into shell 3. (There are a couple of
exceptions for reasons that are outside the scope of this text.)
c) an element that has eight electrons in shell 2 (See the hint to part b.)
Ne is the first element that has eight electrons in shell 2. But every element after Ne
also has eight electrons in shell 2, since that’s the limit—every later element in the
periodic table is adding electrons into higher shells.
42
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Atoms, Elements, and Compounds
d) an element that has a total of eight electrons
*2.95 Lead has the following electron arrangement:
shell 1: 2 electrons
shell 4: 32 electrons
A
LE
There’s only one answer for this one: only oxygen, element 8, has exactly 8 electrons
in its neutral atoms.
shell 2: 8 electrons
shell 5: ?? electrons
shell 3: 18 electrons
shell 6: ?? electrons
Use the atomic number of lead and its position in the periodic table to figure out the
number of electrons in shells 5 and 6.
S
See Sections 2-3 and 2-4. For the representative (A Group) elements, the group number
(Group 4A for lead, Pb) equals the number of valence electrons (4 valence electrons in a
lead atom), and for all elements, the period number (Period 6 for lead) equals the number
of the outermost shell (shell 6 for lead). Therefore lead must have four electrons in shell
6. From there we can work backward to figure out how many electrons are in shell 5. In
shells 1-4, there are a total of 60 electrons (2 + 8 + 18 + 32 = 60), and Shell 6 accounts
for 4 more, a total of 64 electrons in shells 1-4 and 6. Lead is element number 82, and
therefore each atom has 82 protons and 82 electrons; 82 – 64 = 18 electrons in shell 5.
a) What is the formula weight of PbSO4?
R
2.97
Look in the periodic table to find the atomic weights of Pb, S and O:
= 207.2 amu
= 32.06 amu
4 × 16.00 amu = 64.00 amu
= 303.3 amu (rounded to 1 decimal place)
FO
1 Pb atom
1 S atom
+ 4 O atoms =
This value (303.3 amu/formula unit) is also the molar mass (303.3 g/mol).
b) If you have 31.3 g of PbSO4, how many moles of this compound do you have?
We can use the molar mass from part a to write two conversion factors:
T
1 mol PbSO 4
303.3 g PbSO 4
or
303.3 g PbSO 4
1 mol PbSO 4
O
Then we can use the appropriate conversion factor to calculate the moles of PbSO4:
31.3 g PbSO 4 
1 mol PbSO 4
303.3 g PbSO 4
 0.103 mol PbSO 4 (rounded to 3 sf)
N
c) If you have 0.2275 mol of PbSO4, how many grams of this compound do you have?
We use the molar mass we determined in part a:
0.2275 mol PbSO 4 
303.3 g PbSO 4
1 mol PbSO 4
 69.00 g PbSO 4 (rounded to 4 sf)
43
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Chapter 2
*2.99 You have 5 mol of Al. If you want to make AlCl3, how many moles of Cl must you use?
The formula AlCl3 tells us that there are three moles of Cl for every one mole of Al:
3 mol Cl
15 mol Cl
1 mol Al
A
LE
5 mol Al 
*2.101 You have 1.00 mol of nitrogen. How many moles of oxygen will you need in order to
make each of the following compounds (with nothing left over)?
In each case, we use the atom ratio from the chemical formula as a mole ratio.
NOTE:
The atom ratios or mole ratios derived from a chemical formula are assumed to be exact
numbers, and do not affect significant figure decisions.
In this formula, there is one O atom for every one N atom, so we will need
1.00 mol O to combine with 1.00 mol N:
1.00 mol N 
1 mol O
 1.00 mol O
1 mol N
1.00 mol N 
R
In this formula, there are two O atoms for every one N atom, so we will need
2.00 mol O to combine with 1.00 mol N:
b) NO2
2 mol O
 2.00 mol O
1 mol N
In this formula, there is one O atom for every two N atoms, so we will need
0.500 mol O to combine with 1.00 mol N:
FO
c) N2O
S
a) NO
1 mol O
 0.500 mol O
2 mol N
N
O
T
1.00 mol N 
44
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