Prentice Hall Chemistry
(c) 2005
Section Assessment Answers
Chapter
4
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4.1 Section Assessment
1. [See textbook for question.]
Democritus believed that atoms were
indivisible (you couldn’t split them into
pieces, hence the name “atomos”),
and they were indestructible.
Do we still believe these things to
be true?
No. Atoms can be split.
J. J. Thomson discovered the
electron in c. 1897.
The “atom bomb” gets its explosive
power from splitting uranium or
plutonium atoms in half.
Democritus
460 B.C. – 370 B.C.
4.1 Section Assessment
1. [See textbook for question.]
Democritus believed that atoms were
indivisible (you couldn’t split them into
pieces, hence the name “atomos”),
and they were indestructible.
Do we still believe these things to
be true?
No. Atoms can be destroyed.
Antimatter, discovered in 1932 by
Carl D. Anderson, destroys matter
when it touches it.
Matter and antimatter explode with a
hundred times more force than
nuclear bombs as they annihilate
each other.
Democritus
460 B.C. – 370 B.C.
4.1 Section Assessment
2. [See textbook for question.]
Dalton used experimental methods, whereas Democritus had
used only imagination and reasoning.
Dalton also had much better knowledge of the elements, and he
studied the mass ratios in which elements combined to make
compounds.
Some of
Ancient Greek
Democritus
“elements”
460 B.C. – 370 B.C.
fire
Modern
“States of
Matter”
hot gas or
plasma
air
gas
water
liquid
earth
solid
Dalton’s
elements
hydrogen
oxygen
nitrogen
carbon
sulfur
phosphorus
John Dalton
1766-1844
4.1 Section Assessment
3. [See textbook for question.]
The scanning tunneling microscope can generate pictures where
individual atoms can be distinguished.
For instance, on page . . . 103, each iron atom in the
picture appears as a single . . . cone.
Scanning Tunneling Microscope
1981
4.1 Section Assessment
4. [See textbook for question.]
i) All matter is composed of tiny, indivisible particles called
“atoms”. Is everything he said here still believed to be true?
ii) Atoms of the same element are identical. The atoms of any
one element are different from those of another element.
Is everything he said here still believed to be true?
iii) Atoms of different elements can physically mix together or can
chemically combine in simple whole-number ratios to form
compounds. Is everything he said here still believed to be true?
iv) Chemical reactions occur when atoms are separated, joined,
or rearranged. Atoms of one element, however, are never
changed into atoms of another element as a result of chemical
reaction. Is everything he said here still believed to be true?
4.1 Section Assessment
5. [See textbook for question.]
According to Dalton, atoms can not be changed from one element
to another. For example, a carbon atom will always be a carbon
atom, forever.
Of course, we know better than that now. We now know that in
addition to chemical reactions, which merely rearrange atoms,
there are also nuclear reactions, which DO change atoms from
one element to another.
For instance, carbon-14 atoms will spontaneously turn into
nitrogen-14 atoms by the process of radioactive decay.
Does that mean that Dalton was an idiot?
NO WAY! Dalton was brilliant.
Even Einstein was wrong about certain things.
4.1 Section Assessment
6. [See textbook for question.]
5 x 10-2 nm to 2 x 10-1 nm
That’s the short version of the answer.
If you know how to get that answer, then click the red button
below to skip to #7.
You can also click the red button if you just don’t care about #6.
If you don’t understand how to get the answer to #6 and you do
want to learn how, then click the green button instead.
Skip to #7
I want to learn about #6!
Okay. Buckle your seatbelt.
4.1 Section Assessment
6. [See textbook for question.]
The book says, on page 103, at the beginning of the 3rd
paragraph, that “The radii of most atoms fall within the range of 5
x 10-11 m to 2 x 10-10 m.”
First of all, we’d better make sure we know what “radii” means.
The width of a circle is called its “diameter”.
Half the width of a circle is called its
“radius”.
The radius is the distance from the
center of the circle to the edge of
the circle.
“Radii” is just the plural of “radius”.
And now, for some
stuff that you would
already know if I had
made you learn chapter
three . . .
A yardstick is three feet long.
An American football field is 100 yards
long.
These days, America is
the only country that
measures distance in
yards.
The rest of the world
uses the metric system.
In the metric system, the
meter is used instead of
the yard.
A meter is a little bit
longer than a yard.
An ordinary ruler is twelve inches long.
0
1
INCHES
2
5
4
3
7
6
8
9
10
11
12
cm
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
. . . You will notice that it is about 30 centimeters long.
A paperclip is
about one
centimeter
wide.
0
If you flip it around . . .
If you look closely, you will notice that there are ten tiny marks
between each centimeter.
These marks are one millimeter apart.
If you look closely, you will notice that there are ten tiny marks
between each centimeter.
These marks are one millimeter apart.
The wire that a paperclip is made out of is about one millimeter thick.
A millimeter is pretty small, isn’t it?
A one-millimeter metal wire is still
much thicker than a human hair,
though.
According to the tables on page 74, how big is a nanometer?
It says that 1 m = 109 nm
109 is 1,000,000,000, which is one billion.
Therefore, a nanometer is one one-billionth of a meter.
The tables also say that 1 m = 103 mm
103 is 1,000, which is one thousand.
Therefore, a millimeter is one one-thousandth of a meter.
If 1m = 109 nm and 1 m = 103 mm, then . . .
109 nm = 103 mm
10(9-3) nm = 1 mm
106 nm = 1 mm
103
103
1 mm = 1,000,000 nm
A nanometer is one one-millionth of a millimeter.
That’s small.
If you got confused during
the math on that last slide,
don’t worry.
I was just tyring to show
you how ridiculously small a
nanometer (nm) is.
What metric unit of length is just
a bit longer than three feet?
1 meter = 1 m
How wide is a paper clip?
1 centimeter = 1 cm
How thick is the wire that a paper
clip is made of?
1 millimeter = 1 mm
How big is a nanometer?
1 nm = one one-billionth
of a meter
1 nm = one one-millionth
of a millimeter
GREAT!
Now that you know your
units, let’s do the math . . .
4.1 Section Assessment
6. [See textbook for question.]
The book says, on page 103, at the beginning of the 3rd
paragraph, that “The radii of most atoms fall within the range of 5
x 10-11 m to 2 x 10-10 m.”
Unfortunately, we need to give our answer in nm, not m.
Page 103 gives us good information, but it’s in the wrong UNITS.
We need to CONVERT meters (m) into nanometers (nm).
We need to do a UNIT CONVERSION.
4.1 Section Assessment
6. [See textbook for question.]
5 x 10-11 m
x
109 nm
= 5 x 10(-11 + 9) nm
1m
1
= 5 x 10(-2) nm
2 x 10-10 m
1
x
109 nm
= 2 x 10(-10 + 9) nm
1m
= 2 x 10(-1) nm
5 x 10-2 nm to 2 x 10-1 nm
4.1 Section Assessment
7. [See textbook for question.]
So what the heck do you do with these numbers?
63.5 g
= 10.5 x 10-23 g/atom
6.02 x 1023 atoms
10.54817276
6.02 ) 63.5
= 1.05 x 10-22 g/atom
4.2 Section Assessment
8. [See textbook for question.]
The three kinds of subatomic particles are
the electron, the proton, and the neutron.
All atoms are made merely of different combinations of these
three fundamental building blocks.
Since you are made of atoms, that means that you are made of
protons, neutrons, and electrons.
4.2 Section Assessment
9. [See textbook for question.]
From his experiments, Rutherford concluded that an atom is made
of a positively-charged nucleus surrounded by a region of empty
space in which electrons orbit that nucleus.
Rutherford believed that an atom’s nucleus was very tiny
compared to the atom as a whole, and that, in spite of this, the
nucleus is where most of the atom’s mass is.
Thus, according to Rutherford’s interpretation of his experimental
results, atoms are made mostly of empty space.
This implies that all things made of matter, anything that is solid,
liquid, or gas, is actually made mostly of empty space.
4.2 Section Assessment
10. [See textbook for question.]
(Table 4.1 at the top of page 106 summarizes this nicely for you.)
Particle
Symbol
Charge
electron
e-
1-
proton
neutron
p+ (or H+!)
n0
Relative mass
1/1840
1+
1
0
1 (actually a little more than that)
4.2 Section Assessment
11. [See textbook for question.]
Thomson, through his cathode ray tube experiments, discovered
the electron.
Millikan, through his oil drop experiments, determined the charge
and mass of the electron.
4.2 Section Assessment
12. [See textbook for question.]
Rutherford expected that the alpha particles he shot at the gold
foil would pass through it with little deflection.
Instead, he found that, although most did shoot straight through
with little or no deflection, some alpha particles were deflected at
very large angles, and some alpha particles even bounced back
toward the alpha particle source.
He likened this to shooting a cannonball at a piece of facial tissue
and having it bounce back.
Rutherford had no idea that there was anything dense and heavy
enough in an atom to bounce an alpha particle back to where it
came from.
4.2 Section Assessment
13. [See textbook for question.]
The great majority of the alpha particles went straight through the
gold foil, as though nothing were there.
4.2 Section Assessment
14. [See textbook for question.]
Thomson thought the atom was a mass of positive charge with
negative electrons embedded in its outer surface.
Rutherford’s model didn’t envision the atom being a big ball of
positive charge, but, rather, a tiny speck of positive charge in the
middle of an almost perfectly empty region of space.
Rutherford did not envision electrons as stuck in anything, but,
rather, as whizzing through space, in “orbit” around the positive
nucleus. He envisioned the atom as a tiny solar system, in which
the nucleus was like the sun and the electrons were like planets.
#’s 15 – 24 are “Practice
Problems”, so the answers are
in the back of the book.
Look on page R84.
If you still don’t understand
how you’re supposed to
figure out the answers, ask
Mr. Barnes to explain.
4.3 Section Assessment
25. [See textbook for question.]
The number of protons in an atom determines what element it
belongs to.
Number of protons is called “atomic number”.
For instance, all gold atoms have exactly 79 protons in them.
Any atom that has 26 protons in it is, by definition, an iron atom.
4.3 Section Assessment
26. [See textbook for question.]
number of neutrons = mass number – atomic number
This is true because
mass number = number of protons + number of neutrons . . .
and atomic number = number of protons
4.3 Section Assessment
27. [See textbook for question.]
Isotopes of a given element vary only in the number of neutrons in
each atom.
For instance, most carbon atoms are from the isotope carbon-12,
but a few are from the isotope carbon-14.
Carbon-12 atoms and carbon-14 atoms all have 6 protons, but
carbon-12 atoms have only 6 neutrons each, whereas
carbon-14 atoms have 8 neutrons each.
Incidentally, the two extra neutrons in carbon-14 make that
isotope radioactive . . . But we’ll discuss radioactivity later . . .
4.3 Section Assessment
28. [See textbook for question.]
Atomic mass is calculated by what is called a “weighted average”
method (no pun intended).
The mass of each known isotope of the element is multiplied by
the % abundance of that element.
Because of this, rare isotopes tend to have a very small effect
upon the average atomic mass of an element.
Also, the average atomic mass of the element tends to be very
close to the atomic mass of the most common isotope of that
element.
4.3 Section Assessment
29. [See textbook for question.]
By looking at the periodic table, one can predict the chemical and
physical properties of an element by its location on the table.
Here are a few examples of this:
*Elements in the same vertical column tend to have the same
number of “valence electrons”, and, therefore, similar bonding
properties.
*Elements to the left of the “staircase” are almost all metals,
elements to the right almost all nonmetals, and elements touching
the staircase mostly metalloids.
*Elements with high electronegativies and ionization energies tend
to be clustered in the upper right hand corner of the periodic table.
[Don’t burden your brain too much with these examples just yet.
We’ll be looking at them more closely in chapter 6.]
4.3 Section Assessment
30. [See textbook for question.]
A platinum-194 atom has a total of 194 protons and neutrons,
combined, in its nucleus.
(194 is the “mass number” of platinum-194.)
Since all platinum atoms have 78 protons, then the symbol for a
platinum-194 atoms would be . . .
194
78
Pt
4.3 Section Assessment
31. [See textbook for question.]
The average atomic mass of an element is typically not a whole
number precisely because it is an average atomic mass.
Averages tend to have decimal fractions hanging of the ends of
them.
An average atomic mass is the weighted average of the atomic
masses of all the naturally-occurring isotopes of the element.
Even if the atomic masses of the isotopes were whole numbers
(which they aren’t), the weighted average of those atomic masses
would almost certainly never be a whole number.
[Don’t worry about this question too much. If you get it, great. If
you don’t, no biggie. There are more important concepts for you
to master in this class than this one.]
4.3 Section Assessment
32. [See textbook for question.]
a.
Protons:
Neutrons:
Electrons:
6
3
Li
3
3
3
7
3
Li
If an atom does have an electric
charge, then it will be written
where the flashing blue squares
are.
3
(Lithium’s atomic number is 3.)
4
(# of neutrons =
mass number – atomic number)
3
(no electrical charge listed, so
# of electrons = # of protons =
atomic number)
4.3 Section Assessment
32. [See textbook for question.]
b.
Protons:
42
20
Ca
44
20
Ca
20
20
Neutrons: 22
24
Electrons:
20
20
4.3 Section Assessment
32. [See textbook for question.]
c.
78
34
Se
80
34
Se
Protons:
34
34
Neutrons:
44
46
Electrons:
34
34
4.3 Section Assessment
33. [See textbook for question.]
Be (beryllium)
Mg (magnesium)
Ba (barium)
Sr (strontium)
Ra (radium)
If two elements are in the same vertical column of the
periodic table, they probably have similar properties.
Because of this, columns are also known as “families”.
The above-named elements comprise the family known
as the . . . “alkaline earth metals”.
The End
Yay for atoms!
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