Molecules and solutions
ANSWER KEY
Checkup
1 WHAT IS A MOLECULE? (pp. 40–50)
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solely for classroom use with Observatory.
1. Are the following substances molecules? Explain your answer.
a) NaCl
Yes, it is a molecule because it contains two different atoms: sodium and chlorine.
b) O2
Yes, it is a molecule because it contains two oxygen atoms.
c) Au
No, it is not a molecule; it is only one atom: gold.
d) Co
No, it is not a molecule; it is only one atom: cobalt.
e) CO
Yes, it is a molecule because it contains two different atoms: carbon and oxygen.
f) H2SO4
Yes, it is a molecule because it contains several different atoms: two hydrogen atoms,
one sulphur atom and four oxygen atoms.
2. What is the natural tendency of each of the following elements with regard to gaining or losing
electrons?
a) potassium
Potassium tends to lose an electron.
b) oxygen
Oxygen tends to gain two electrons.
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Molecules and solutions
c) aluminum
Aluminum tends to lose three electrons.
d) krypton
Krypton is a noble gas, so it does not tend to gain or lose electrons.
3. Metals give up electrons. Consequently, do they tend to form positive or negative ions?
They tend to form positive ions.
4. Calculate the sum of the charges for each of the following electron transfers, and indicate the
net charge of the resulting ion.
a) A calcium atom loses two
electrons.
c) An iodine atom gains one electron.
(+20) protons
(+53) protons
+ (–18) electrons
+ (–54) electrons
–1
An I – ion is obtained.
+2
A Ca ion is obtained.
2+
b) A nitrogen atom gains three electrons.
d) An aluminum atom loses three electrons.
(+7) protons
(+13) protons
+ (–10) electrons
+ (–10) electrons
–3
An N
3−
+3
3+
ion is obtained.
An Al ion is obtained.
5. Do the models below represent atoms or ions? Explain your answers.
a)
b)
It is an atom because it contains an equal
It is an ion because it contains more protons
number of protons and electrons.
than electrons.
6. Iron has the ability to form one of two ions: Fe2+ or Fe3+. How many protons and electrons does
each of these ions contain?
Fe2+: 26 protons and 24 electrons
Fe3+: 26 protons and 23 electrons
9. Which of the following substances contain at last one ionic bond?
HCl, NaOH, CO2, KBr, CaO, AlCl3, NH3
(NaOH- OH is a polyatomic ion), KBr, CaO, AlCl3
10. Draw a Lewis structure for each of the following molecules to show how their ionic bonds are
formed.
You do not need to worry about drawing the product, just show me how the bond is
formed.
a) LiF
b) CaBr2
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c) Li3N
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d) MgO
11. What is the charge of each of the ions in the following molecules?
a) NaBr Na+ and Br−
b) CaO
Ca2+ and O2−
c) Li2S
Li+ and S2−
d) AlF3
Al3+ and F−
12. Which of the following substances contain at least one covalent bond?
CH3COOH, CS2, CoCl2, Fe2O3, H2O, CH4, AgCl
CH3COOH, CS2, H2O, CH4
13. Draw a Lewis structure for each of the following molecules to show how their covalent bonds
are formed.
a) Cl2
c) CO2
b) CBr4
d) HF
14. What does the subscript 2 mean in the molecule MgCl2?
It means that this molecule contains two chlorine atoms.
15. Write the chemical formula of the molecule resulting from the union of each of the following
pairs of substances.
a) potassium and sulphur
K2S
b) chlorine and copper
CuCl2
c) chromium and fluorine
CrF3
d) the ions Mg
2+
and
SO42−
MgSO4 (SO4 is a polyatomic ion)
16. Apply the rules of nomenclature to name the following substances.
a) NaBr Sodium bromide
b) PCl5
Phosphorus pentachloride
c) SiCl4
Silicon tetrachloride
d) K3N
Potassium nitride
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e) Al2O3 Aluminum oxide
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ANSWER KEY
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CHAPTER 2
Molecules and solutions