Download CHAPTER 2: ANSWERS TO SELECTED PROBLEMS Si . . . .

CHAPTER 2: ANSWERS TO SELECTED PROBLEMS
SAMPLE PROBLEMS (“Try it yourself”)
2.1
Honey is homogeneous.
2.2
The flavor of a food is an intensive property, since it does not depend on the amount of
food. Sugar has the same sweet flavor, regardless of how much sugar you taste.
2.3
Since air does not have a constant composition, it is a mixture.
2.4
Either C (carbon) or Si (silicon) is a correct answer.
2.5
This is an atom of barium (Ba).
2.6
This is an atom of chlorine (Cl). Remember that numbers of protons and electrons are
equal in a neutral atom.
2.7
The fluorine atom is an anion, because its charge is negative. The atom contains 9
protons and 10 electrons.
2.8
The mass of atom is 56 amu, and its electrical charge is +2.
2.9
This atom has 30 protons and 36 neutrons.
2.10 There are 19 carbon atoms, 19 hydrogen atoms, 7 nitrogen atoms, and 6 oxygen atoms in
one formula unit of folic acid.
2.11
Shell 1: 2 electrons
Shell 2: 8 electrons
Shell 3: 7 electrons
2.12 Since sulfur is similar to oxygen, we simply replace oxygen by sulfur in the chemical
formula. The formula of the compound is Al2S3.
2.13 Silicon and tin both have four electrons in their outer shells, so they should show similar
chemical behavior.
3.2 (this problem number is a typographical error)
2.14
.
. Si .
.
Iodine atoms have 7 valence electrons, and these electrons are in shell 5.
2.15 The Group 2A elements Be, Mg, Ca, Sr, Ba and Ra have two valence electrons. He also
has two valence electrons, but it is not placed in Group 2A.
2.16 The element has one valence electron, so it should be placed in Group 1A. This is an
atom of cesium (Cs).
END OF SECTION PROBLEMS
Section 2.1
2.1
a) This describes a compound. In general, compounds are difficult to separate into the
elements that make them up.
b) This describes a mixture. Mixtures generally look like one (or both) of their
components. In this case, the mixture looks like one of its components (gold).
c) This describes an element.
2.2
If you use a lot of salt and a little water, most of the salt will not dissolve in the water, so
you will end up with salt water (a liquid) and extra salt (a solid).
2.3
You make a heterogeneous mixture. Even though the sugar and salt look the same, they
are made from different elements, so the composition of each salt crystal is different from the
composition of each sugar crystal. A homogeneous mixture must have a constant composition.
2.4
The principal difference is that mixtures can have variable compositions, whereas
compounds have fixed compositions.
2.5
We are not told whether cotton is homogeneous or heterogeneous, so cotton could be
either a compound or a mixture.
2.6
Since muriatic acid can have different compositions, it must be a mixture.
Section 2.2
2.7
a) Possible answers are sodium, magnesium, aluminum, silicon, phosphorus, chlorine,
and argon.
b) Possible answers are oxygen, selenium, tellurium, and polonium.
c) Group 2A contains only metals, as do all of the “B” groups. (Hydrogen is usually
placed in Group 1A, so Group 1A is not a correct answer.)
d) Groups 7A and 8A contain only nonmetals.
e) Possible answers are fluorine, chlorine, bromine, iodine, and astatine. (Hydrogen is
sometimes placed in Group 7A, but it is not considered to be a halogen.)
f) Possible answers are beryllium, magnesium, calcium, strontium, barium, and radium.
2.8
There are eight elements in the third period.
2.9
a) Ga
b) Xe
c) H
Section 2.3
2.10 Statement b is true. An amu is an extremely small mass, so a penny must weigh an
enormous number of amu.
2.11 This is not a reasonable statement. An atom of fluorine weighs a tiny fraction of a gram.
The student has confused grams with amu; a fluorine atom weighs 19 amu.
2.12 Since there are five different elements in proteins, proteins contain five different types of
atoms.
Section 2.4
2.13
a) electron
b) proton
2.14
15 protons and 15 electrons
2.15
a) This is an atom of potassium.
2.16
Each fluorine atom contains 10 electrons.
2.17
a) copper (Cu)
2.18
a) 53
b) 63
c) proton and neutron
b) The electrical charge on this atom is +1.
c) +1
b) 78
Section 2.5
2.19 Choices a and c contain atoms that are isotopes. Isotopes are the same element, so they
have the same numbers of protons and the same chemical symbols. However, they have
different numbers of neutrons (and therefore different masses).
2.20 Choice c is correct. All atoms that have 34 protons will look and behave similarly,
regardless of the number of neutrons.
2.21 This is not a reasonable conclusion. The atomic weight is the average weight of all of the
selenium atoms on Earth, and it tells us nothing about the weights of individual atoms. None of
the individual atoms may actually weigh 79 amu.
Section 2.6
2.22
C12H22O11
2.23 a) One sodium atom, five carbon atoms, eight hydrogen atoms, one nitrogen atom, and
four oxygen atoms.
b) You have 15 carbon atoms.
Section 2.7
2.24
Shell 1: 2 electrons
Shell 2: 5 electrons
2.25
This is an atom of magnesium (Mg).
2.26 An orbital is a region within a shell that holds two electrons. Each shell beyond #1 is
made up of several different orbitals.
Section 2.8
2.27
a) Mg3N2
b) Be3P2
2.28
a) Selenium has six valence electrons.
b)
..
. Se
.. .
Section 2.9
2.29
A selenium atom has six valence electrons, which are located in shell 4.
2.30
a) This element has one valence electron.
b) The element is in Group 1A.
c) The element is in Period 5.
2.31
a) Possible answers are Sb and Bi (the last two elements in Group 5A).
b) Possible answers are Cs, Ba, Tl, Pb, Bi, Po, At and Rn.
2.32 Helium is placed in Group 8A even though it has only two valence electrons, because the
chemical properties of helium are similar to those of the other group 8A elements, and
completely different from those of the Group 2A elements.
CUMULATIVE PROBLEMS (Odd-numbered problems only)
2.33 Some substances that mix with water to form a homogeneous mixture are alcohol,
glycerine, and baking soda.
2.35
Air is a homogeneous mixture, because it has a uniform composition.
2.37
a) Chalk is a compound, because it has a constant composition.
b) You cannot classify quicklime without more information.
2.39
Columbium is a metal, based on its properties.
2.41
a) Fe
2.43
a) Possible answers are O, S, Se and Te.
b) Possible answers are Ge, Sn, and Pb.
c) Possible answers are As, Se, Br, and Kr.
d) Any of the metals is a correct answer: Li, Be, Na, Mg, Al, K, Ca, etc.
e) The only correct answer is Cl.
2.45
The smallest possible amount of any element (including silver) is one atom.
b) Na
c) Ag
d) Pb
2.47 Choices a and c are reasonable masses for an atom. From the periodic table, we can see
that atomic masses range from 1 amu to around 300 amu.
2.49 Choice d is correct. Atoms are extremely small, so any object that we can see with our
naked eye contains a vast number of atoms.
2.51
a) proton and neutron
b) electron
2.53
a) sulfur (S)
2.55
a) 47
2.57
a) 28 particles (9 protons, 9 electrons, and 10 neutrons)
b) There are 19 particles in the nucleus (the protons and neutrons)
b) 33
b) 60
c) proton
c) 16
c) 47
d) –2
d) 46
2.59 These two atoms are not isotopes. They have different atomic numbers, so they have
different numbers of protons, making them different elements.
2.61
Statements b and c are true.
2.63
Statement d is correct.
2.65 One formula unit contains one nitrogen atom, four hydrogen atoms, and one chlorine
atom.
2.67
C16H18N2O4S
2.69
There are 80 carbon atoms in five formula units of this compound.
2.71 Since each formula unit of penicillin G contains one sulfur atom, we must have four
formula units in our sample. Therefore, we have 16 oxygen atoms and 72 hydrogen atoms.
2.73 Formula units are the smallest possible amounts of a compound. Since one formula unit
of penicillin G contains four oxygen atoms, it is impossible to make a sample of this compound
that contains only two oxygen atoms.
2.75
Shell 1: 2 electrons
2.77
Germanium atoms have 4 valence electrons, and these electrons are in shell 4.
2.79
a) Rb
2.81
a)
K.
Shell 2: 8 electrons
Shell 3: 3 electrons
b) Rb2Se
a)
.
.Pb.
.
a)
..
:Br
.. .
2.83 Iodine is in Group 7A, so it must have 7 electrons in its outermost shell (shell 5). The
atom number of iodine is 53, so iodine must have a total of 53 electrons. There are a total of 35
electrons in shells 1, 2, 3 and 5, so iodine has 18 electrons in shell 4.
2.85 Helium has only two valence electrons. It is placed in Group 8A because it behaves like
the other Group 8A elements (and unlike the Group 2A elements).