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Concept evaluations by chapter
CHAPTER 1
EST
ANSWER KEY
Atoms and elements
1. A teacher asks her students to represent the following chemical reaction, using Dalton’s atomic
model:
2 H2 + O2  2 H2O
Here are four of the representations the teacher received:
a)
c)
b)
d)
Which representation follows Dalton’s model?
Representation b)
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2. Thomson made many observations using cathode ray tubes.
a) What are cathode rays composed of?
Cathode rays are composed of electrons.
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b) Based on his discoveries, Thomson modified Dalton’s atomic model. What was wrong with
Dalton’s model?
Thomson discovered that the atom is not indivisible, contrary to Dalton’s belief, because it is
possible, and relatively easy, to detach negative particles (electrons) from atoms.
c) Complete the diagram opposite by adding
the signs of the charges needed to deflect the
cathode rays downward.
+
–
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3. The diagram opposite represents the results
of one of Rutherford’s experiments. The
arrows show the possible trajectories of
alpha particles when a sheet of gold foil is
bombarded with them.
a) What are alpha particles?
They are positive particles emitted by certain
radioactive substances.
b) Which arrow represents the trajectory most often observed during the experiment? Explain
your answer.
The trajectory observed most often is that of arrow B. Because atoms are mostly empty space, the
majority of the alpha particles pass through the gold foil without being deflected.
c) Why was Rutherford so surprised by the results of his experiment?
Because he did not expect any alpha particles to bounce back off the gold foil.
d) Based on his discoveries, Rutherford modified Thomson’s atomic model. What was wrong
with Thomson’s model?
The deflection of some alpha particles indicated that they had struck an object that was both dense
and positively charged—an idea that did not fit with the image of a “positive dough” in Thomson’s
atomic model. Moreover, the fact that most of the rays were not deflected implied that atomic nuclei
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4. Bohr studied the light emitted by various elements by decomposing it to observe their visible
emission spectra.
a) What do the bands of colour in the emission spectra of the elements represent?
Each band represents the energy emitted by an electron when it jumps from an orbit that is farther
from the nucleus back to its original orbit.
b) Why do different elements emit bands of different colours?
Because the distances between the orbits and the atomic nucleus are different from one element to
another. Movement between orbits therefore represents different amounts of energy, resulting in
bands of different colours.
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and electrons are relatively far from one another.
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c) Based on his discoveries, Bohr modified Rutherford’s atomic model. What was wrong with
Rutherford’s model?
The emission of specific colour spectra implied the emission of energy in specific quantities. The
electrons must therefore occupy specific locations around the nucleus. Rutherford had previously
assumed that electrons circled the nucleus in random trajectories.
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EST
5. Neon is an element with the atomic number 10 and a relative atomic mass of 20.28 u.
a) Since the atomic number of neon is 10, the atoms of this element contain 10 protons and
10 electrons. Given that the mass of a proton is approximately 1 u and the mass of an
electron is negligible compared to that of a proton, why is the relative atomic mass of neon
not approximately 10 u?
Because the nucleus of a neon atom also contains neutrons and the mass of each neutron is
approximately 1 u.
b) Neon has three natural isotopes:

neon-20, with a relative atomic mass of approximately 20 u

neon-21, with a relative atomic mass of approximately 21 u

neon-22, with a relative atomic mass of approximately 22 u
Which of these three isotopes is most commonly found in nature? Explain your answer.
The most common isotope is neon-20 because the relative atomic mass is an average that takes into
account the proportion of the various isotopes in nature and the relative atomic mass of neon is
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closer to 20 than to the other two values.
c) Draw a simplified atomic model of each of the natural isotopes of neon.
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Atoms and elements
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d) The simplified atomic model improved on the Rutherford-Bohr model. What was wrong with
the Rutherford-Bohr model?
The Rutherford-Bohr atomic model did not take into account the presence of neutrons in the nucleus,
unlike the simplified atomic model.
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6. Where in the periodic table can you find the following:
a) an alkaline earth metal?
In the second column
b) a noble gas?
In the last column
c) a halogen?
In the second-last column
d) a metal?
To the left of the staircase
e) an atom containing only one
electron shell?
In the first row
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7. Six different elements are represented below according to the Rutherford-Bohr atomic model.
C.
Sodium
E.
Nitrogen
B.
Fluorine
D.
Boron
F.
Magnesium
Neon
a) Write the name of each of the six elements on the line under the model.
b) Write the atomic numbers and the chemical symbols for the six elements in the appropriate
boxes in the table below.
11
12
Na
Mg
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Atoms and elements
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7
9
10
B
N
F
Ne
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A.
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c) Do all these elements belong to the same period? Explain your answer.
No, they do not all belong to the same period because they do not all have the same number of
electron shells. Some of them (boron, nitrogen, fluorine and neon) have two electron shells; others
(sodium and magnesium) have three.
d) Which of these elements are good conductors of electricity? Explain your answer.
Sodium and magnesium are good conductors because they are both metals.
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EST
8. Mendeleev developed his periodic classification of the elements at a time when some of the
elements had not yet been discovered. Since he believed in the periodicity of properties, he
formulated certain predictions about the properties of unknown elements. For example, he
predicted that the atomic mass of an element with the atomic number 32 would be 72 u and
that its density would be 5.5 g/mL. Refer to the periodic table and to Appendix 1 of the student
book to see whether Mendeleev’s predictions were accurate, and write your findings below.
The element with the atomic number 32 is germanium. Its atomic mass is 72.61 u, and its density is
5.35 g/mL. Mendeleev’s predictions were therefore very close to the experimental values.
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9. Which characteristic of the atom explains its chemical properties?
The number of valence electrons
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EST
10. What are the atomic numbers of the following elements?
a) Its nucleus contains six protons and eight neutrons.
Its atomic number is 6.
b) An atom contains 10 electrons and 11 neutrons.
Its atomic number is 10.
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CHAPTER 1
Atoms and elements
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11. If you know the AZ E notation of an atom of an element, how can you determine:
a) the number of electrons it contains?
The number of electrons is equal to the atomic number of the element, represented by the symbol Z.
b) the number of neutrons it contains?
The number of neutrons is equal to the difference between the mass number (A) and the atomic
number (Z).
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EST 12. Which of the following elements are isotopes of the same element? Explain your answer.
22
10E
a) 2012 E
b) 2211 E
c)
d) 1912 E
Elements a) and d) are isotopes of the same element because they have the same atomic number (Z).
They are therefore two forms of the same element but with different mass numbers.
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13. What properties do all the isotopes of a single element share?
The isotopes of a single element all have the same chemical properties.
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14. Natural silver is made up of two isotopes in almost equal proportions. Their atomic masses are
107 u and 109 u, respectively. What is the relative atomic mass of silver?
It is approximately 108 u.
EST
15. Boron, which has an atomic mass of 10.81 u, is a mixture of two isotopes, which occur
in a ratio of 20 to 80 percent. The mass number of one of these isotopes is 11.
a) What is the mass number of the other isotope?
The mass number of the other isotope is 10.
b) Which of the two isotopes is more common?
The more common isotope is boron-11.
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A
EST 16. Identify the following elements and write their names using Z E notation.
a) an element with 12 protons and 12 neutrons
b) an element with 16 neutrons and a mass number of 31
c) an element that is a liquid at 25°C and that has a
relative atomic mass of 80 u
24
12
31
15
Mg
P
80
35
Br
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17. Draw a Rutherford-Bohr atomic model of the
element in the second row and the sixth
column of the periodic table. Remember to
write the name of the element.
Oxygen
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18. Draw a Lewis structure for each of the following elements. Remember to write the names of
the elements.
a) I am a noble gas belonging b) I am the lightest halogen.
to the third period.
Argon
Fluorine
c) I am the smallest of the
atoms with four valence
electrons.
Carbon
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19. Draw a Rutherford-Bohr atomic model for each of the following elements. Remember to write
the names of the elements.
b) I am an alkali metal with
three electron shells.
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a) I am the first element in
the group of alkali metals.
Lithium
Sodium
c) I am a halogen with one
electron shell more than
fluorine.
Chlorine
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CHAPTER 1
Atoms and elements
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20. Draw a simplified atomic model for each of the following elements. Remember to write the
names of the elements.
a) I am the smallest metal
b) I am an atom with six
c) I am the smallest of the
with three valence
valence electrons and
alkaline earth metals.
electrons.
three electron shells.
27
Aluminum (13
Al)
32
Sulphur (16
S)
Beryllium (49Be)
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21. Are the following statements true or false? If a statement is false, correct it.
a) A mole corresponds to a certain quantity of matter.
True
b) The molar mass of molecular nitrogen (N2) is 14.01 g.
False. The molar mass of N2 is 28.02 g/mol.
c) The mass of 6.02  1023 molecules of water is 18.02 g.
True
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22. One of the lakes of Tibet contains the second largest lithium deposit in the world. This element
is used extensively today in cellphone batteries and will also be used, in the near future, in
batteries for hybrid or electric cars. In 2008, one battery out of every four sold in the world was
manufactured in China. China therefore has a certain economic interest in Tibet. Mining the
resource is relatively simple: a salt (lithium carbonate) dissolved in the lake water is recovered,
and the lithium is extracted.
a) Given that 0.2 mol of lithium carbonate weighs 14.8 g, which of the following chemical
formulas correctly represents a molecule of this salt? Show your calculations.
A. LiCO3
B. Li2CO3
C. LiCO2
D. Li3CO
If 0.2 mol weighs 14.8 g, then 1 mol weighs 74 g.
The molar mass of LiCO3 is: 6.94 g + 12.01 g + (3  16.00 g) = 66.95 g
The molar mass of Li2CO3 is: (2  6.94 g) + 12.01 g + (3  16.00 g) = 73.89 g
The molar mass of LiCO2 is: 6.94 g + 12.01 g + (2  16.00 g) = 50.95 g
The molar mass of Li3CO is: (3  6.94 g) + 12.01 g + 16.00 g = 48.83 g
The chemical formula with the molar mass closest to 74 g is Li2CO3.
b) How many molecules does a 370-g sample of this salt contain? Show your calculations.
If 1 mol of this salt weighs 74 g, then 370 g equal 5 mol.
If 1 mol contains 6.02  1023 molecules, then 5 mol contain 5  6.02  1023,
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or 3.01  1024 molecules.
A 370-g sample of lithium carbonate contains 3.01  1024 molecules.
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Atoms and elements