Download Structure of the Atom Reading

Key Objectives
4.2.1 IDENTIFY three types of subatomic
particles.
4.2.2 DESCRIBE the structure of atoms
according to the Rutherford atomic model.
CHEMISTRY
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Additional Resources
Q: You can X-ray a person’s hand to see inside it—but how can you see inside
an atom? You may have seen X-rays like the one of the hand shown here.
Doctors often use X-rays to see bones and other structures that cannot be
seen through the skin. Scientists tried to figure out what was inside an atom
without being able to see inside the atom. In this lesson, you will learn the
methods scientists used to “see” inside an atom.
Reading and Study Workbook, Lesson 4.2
Available Online or on Digital Media:
• Teaching Resources, Lesson 4.2 Review
• Laboratory Manual, Lab 5
• Virtual Chemistry Laboratory Manual, Labs 4-6
Subatomic Particles
What are three kinds of subatomic particles?
Key Questions
What are three kinds of
subatomic particles?
How can you describe the
structure of the nuclear atom?
Vocabulary
telectron
tcathode ray
tproton
tneutron
tnucleus
Much of Dalton’s atomic theory is accepted today. One important change,
however, is that atoms are now known to be divisible. They can be broken
down into even smaller, more fundamental particles, called subatomic
Three kinds of subatomic particles are electrons, protons,
particles.
and neutrons.
Electrons In 1897, the English physicist J. J. Thomson (1856Ľ1940) discovered the electron. Electrons are negatively charged subatomic particles.
Thomson performed experiments that involved passing electric current
through gases at low pressure. He sealed the gases in glass tubes fitted at both
ends with metal disks called electrodes. The electrodes were connected to a
source of electricity, as shown in Figure 4.4. One electrode, the anode, became
positively charged. The other electrode, the cathode, became negatively
charged. The result was a glowing beam, or cathode ray, that traveled from
the cathode to the anode.
Gas at very
low pressure
á
ź
Metal disk
(cathode)
Cathode ray
(electrons)
&
CHEMISTRY
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YO
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U Have students study the
opening photo and read the accompanying text.
Ask How could you determine what your hand looks
like under the skin without dissecting it? (X-rays, CT
scans, or MRI scans) Explain that scientists use
technology to “see” inside atoms, just as doctors
use it to “see” inside the body.
Build Background
High voltage
Figure 4.4
Cathode-Ray Tube
In a cathode-ray tube, electrons
travel as a ray from the cathode (Ź)
to the anode (à). Televisions used to
be made with a specialized type of
cathode-ray tube.
Engage
Ask Do you have an electronic item in your home
that uses a CRT? (Sample answers: a television or
a computer monitor) Explain that television and
computer CRTs have a source of rays at the small
end. The rays are projected on the large receiving
end to create the display.
Metal disk
(anode)
Vacuum pump
Atomic Structure 105
Focus on ELL
National Science Education Standards
A-1, A-2, B-1, B-4, E-2, G-2, G-3
1 CONTENT AND LANGUAGE Write the term subatomic on the board. Explain that
the prefix sub- means “below” or “a part of”. Have students brainstorm the literal
meaning of subatomic. (below atomic) Now write the words electron, proton, and
neutron on the board. Explain that in these three words, the suffix –on means
“subatomic particle”.
2 FRONTLOAD THE LESSON Have students preview Figure 4.4 and identify any
similarities and differences between the parts of a cathode ray tube and parts of
a battery. Explain that, just as a battery generates a stream of electrons that flow
through an electrical circuit, a cathode ray generates a stream of electrons that travel
as a beam.
3 COMPREHENSIBLE INPUT Draw Table 4.1 on the board, using a contrasting color
to write the mathematical signs in the Symbols column. Use the same color to write
the values of the relative charges. Point out the relative mass of an electron, and ask
students to locate the text where this figure is referenced.
Atomic Structure
105
LESSON 4.2
4.2 Structure of the Nuclear Atom
LESSON 4.2
Thomson found that a cathode ray is deflected by electrically charged
metal plates, as in Figure 4.5a. A positively charged plate attracts the cathode
ray, while a negatively charged plate repels it. Thomson knew that opposite
charges attract and like charges repel, so he hypothesized that a cathode ray is
a stream of tiny negatively charged particles moving at high speed. Thomson
called these particles corpuscles; later they were named electrons.
To test his hypothesis, Thomson set up an experiment to measure the
ratio of an electron’s charge to its mass. He found this ratio to be constant.
Also, the charge-to-mass ratio of electrons did not depend on the kind of gas
in the cathode-ray tube or the type of metal used for the electrodes. Thomson
concluded that electrons are a component of the atoms of all elements.
The U.S. physicist Robert A. Millikan (1868Ľ1953) carried out experiments to find the quantity of an electron’s charge. In his oil-drop experiment,
Millikan suspended negatively charged oil droplets between two charged
plates. He then changed the voltage on the plates to see how this affected the
droplets’ rate of fall. From his data, he found that the charge on each oil droplet was a multiple of 1.60 × 10Ź19 coulomb, meaning this must be the charge
of an electron. Using this charge value and Thomson’s charge-to-mass ratio of
an electron, Millikan calculated an electron’s mass. Millikan’s values for electron charge and mass are similar to those accepted today. An electron has one
unit of negative charge, and its mass is 1/1840 the mass of a hydrogen atom.
Foundations for Reading
BUILD VOCABULARY Have students think of words
that start with the same word parts as the three
subatomic particles described in this section:
neutron, electron, and proton. (Sample answer:
neutral, electric, protein) Words that students
already know may help them remember the
meaning of new terms.
Explore
Teacher Demo
PURPOSE To demonstrate a cathode-ray tube
and observe properties of cathode rays
MATERIALS cathode-ray tube, magnet
PROCEDURE Demonstrate a cathode-ray tube
in class. Use a magnet to deflect the beam of
particles. Review the components of a cathoderay tube, and discuss the connection to television
picture tubes and computer monitors.
EXPECTED OUTCOME Students should be able to
explain how the CRT works and see how the
cathode ray is deflected by a magnetic field.
Figure 4.5
Thomson’s Experiment
a. Thomson found that cathode
rays are attracted to metal plates
that have a positive electrical
charge. b. A cathode ray can also
be deflected by a magnet.
Infer If a cathode ray is attracted
to a positively charged plate,
what can you infer about the
charge of the particles that make
up the cathode ray?
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High voltage
a
Positive
plate
Slit
–
+
See cathode-ray
ttubes animated online.
Explain
Cathode
Vacuum pump
Negative
plate
Anode
Subatomic Particles
b
USE VISUALS Have students study Table 4.1, and
compare the masses and charges of the three
elementary particles. Students should recognize
from this comparison that the mass of an atom is
due mainly to the number of protons and neutrons
that it has. Point out that the assigned charges
for protons and electrons are relative charges.
Ask What particles make up most of the mass
of an atom? (protons and neutrons) Ask Why
are relative charges and mass useful in talking
about subatomic particles? (The actual values are
unwieldy numbers.) The absolute charge on an
electron is 1.602177 ⫻ 10⫺19 coulombs.
106 $IBQUFSt-FTTPO
Differentiated Instruction
LPR LESS PROFICIENT READERS Direct students to use Figures 4.4 and 4.5 to
describe J.J. Thomson’s experiment with cathode-ray tubes in their own words.
L1 SPECIAL STUDENTS Prior to explaining subatomic particles, divide students into
small groups. Give each group pair of inflated rubber balloons (be aware of any latex
allergies in the class) and a swatch of wool cloth. Have students rub the balloons
vigorously with the wool. Have students test the balloons against different objects
and against each other to discover how opposite charges attract and like charges
repel. Use this activity as a lead in to Thompson’s cathode ray experiment.
L3 ADVANCED STUDENTS Have students use the Internet or library to find the
original papers for the discoveries described in this chapter and write a report on
what they have learned.
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Chapter 4 • Lesson 2
Properties of Subatomic Particles
Particle
Electron
Symbol
Relative
charge
Relative mass
(mass of proton â 1)
Actual mass
(g)
eŹ
1Ź
1/1840
9.11 ñ 10Ź28
à
Proton
p
1à
1
1.67 ñ 10Ź24
Neutron
n0
0
1
1.67 ñ 10Ź24
Explore
Teacher Demo
PURPOSE To trace the history of atomic models, and
examine the role of the scientific method in the
development of such models
MATERIALS Library or Internet access
PROCEDURE Have students create a timeline that
traces the development of the atomic model. Have
them note the data that led to an existing model
being changed.
EXPECTED OUTCOME Students’ timelines should list
at least some of the atomic models shown on page
133. Students should be able to explain how certain
scientific discoveries (e.g., Rutherford’s gold foil
experiment) resulted in the revision of the prevailing
atomic model at the time.
Protons and Neutrons If cathode rays are electrons given off by atoms,
what remains of the atoms that have lost the electrons? For example, after a
hydrogen atom (the lightest kind of atom) loses an electron, what is left? You
can think through this problem using four simple ideas about matter and
electric charges. First, atoms have no net electric charge; they are electrically
neutral. (One important piece of evidence for electrical neutrality is that you
do not receive an electric shock every time you touch something!) Second,
electric charges are carried by particles of matter. Third, electric charges
always exist in whole-number multiples of a single basic unit; that is, there are
no fractions of charges. Fourth, when a given number of negatively charged
particles combines with an equal number of positively charged particles, an
electrically neutral particle is formed.
Considering all of this information, it follows that a particle with one unit
of positive charge should remain when a typical hydrogen atom loses an electron. Evidence for such a positively charged particle was found in 1886, when
Eugen Goldstein (1850Ľ1930) observed a cathode-ray tube and found rays
traveling in the direction opposite to that of the cathode rays. He called these
rays canal rays and concluded that they were composed of positive particles.
Such positively charged subatomic particles are called protons. Each proton
has a mass about 1840 times that of an electron.
In 1932, the English physicist James Chadwick (1891Ľ1974) confirmed the
existence of yet another subatomic particle: the neutron. Neutrons are
subatomic particles with no charge but with a mass nearly equal to
that of a proton. Table 4.1 summarizes the properties of these subatomic particles. Although protons and neutrons are exceedingly
small, theoretical physicists believe that they are composed of yet
smaller subnuclear particles called quarks.
Extend
Explain to students that by 1887, the British scientist
William Crookes knew that metal atoms contained
negatively charged particles. He used a cathode ray
tube containing hydrogen gas at low pressure, and
discovered that hydrogen contains positive charges.
Ask students to research the following: What
conclusion about the mass of the atom and the mass
of its protons was derived from Crookes’s findings?
(The mass of an atom is greater than its proton
content.) Which subatomic particle accounted for
this additional mass? (the neutron)
The Atomic Nucleus
How can you describe the structure
of the nuclear atom?
When subatomic particles were discovered, scientists wondered
how the particles were put together in an atom. This question was difficult to answer, given how tiny atoms are. Most
scientistsĿincluding J. J. Thomson, discoverer of the electronĿthought it likely that electrons were evenly distributed throughout an atom filled uniformly with positively charged material. In Thomson’s
atomic model, known as the “plum-pudding model,” electrons were stuck into
a lump of positive charge, similar to raisins stuck in dough. This model of the
atom turned out to be short-lived, however, due to the work of a former student of Thomson, Ernest Rutherford (1871Ľ1937), shown in Figure 4.6.
Figure 4.6 Ernest Rutherford
Born in New Zealand, Rutherford was
awarded the Nobel Prize in Chemistry
in 1908. His portrait appears on the
New Zealand $100 bill.
Atomic Structure 107
Check for Understanding
The Essential Question What structures make up an atom?
Give each student an index card. Assess students’ knowledge of subatomic particles
by reading out loud the description of each of the three kinds of particles. Students
should write the name of the particle on the card in the order in which they were
stated. Then ask students to indicate where each particle is located within the atom.
(electron, proton, and neutron; protons and neutrons are located inside the nucleus,
electrons are located outside)
ADJUST INSTRUCTION If students are having difficulty remembering which particles
are located inside the nucleus, have them review Rutherford’s atomic model by
drawing and labeling an atom.
Answers
FIGURE 4.5 The particles are negative. A negatively
charged plate repels a cathode ray.
Atomic Structure
107
LESSON 4.2
Table 4.1
Explain
&YYOU
Q: How did scientists
“see” inside an atom to
determine the structures
that are inside an atom?
USE AN ANALOGY Explain to students that the
ratio of the size of the nucleus to the size of an
atom is about 10–5. Discuss how small the nucleus
is compared to the entire atom. Explain that if
a housefly sitting on second base in a baseball
stadium represented the nucleus of an atom, the
rest of the atom would be the size of the stadium.
Evaluate
Have each student come to your desk and orally
state in one minute or less one of the discoveries by
Thomson, Millikan, or Rutherford. Then ask them
to describe how the discovery led to the current
understanding of atomic structure. Then have
students complete the 4.2 Lesson Check.
U
IRT A
Have students review Table 4.1. Ask them to create
a table or diagram to compare the characteristics of
electrons, protons, and neutrons.
CHEMISTRY
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Reteach
LAB
Figure 4.7 Rutherford’s Experiment
Rutherford’s gold-foil experiment yielded evidence of the atomic nucleus.
a. Rutherford and his co-workers aimed a beam of alpha
particles at a sheet of gold foil surrounded by a fluorescent
screen. Most of the particles passed through the foil with no
deflection at all. A few particles were greatly deflected.
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Scientists observed
the behavior of atoms in different experimental
conditions. This gave them clues as to what is inside
an atom without actual “seeing” inside an atom.
Rutherford’s Gold-Foil Experiment In 1911, Rutherford and his
co-workers at the University of Manchester, England, wanted to test the
existing plum-pudding model of atomic structure. So, they devised the
gold-foil experiment. Their test used alpha particles, which are helium
atoms that have lost their two electrons and have a double positive charge
because of the two remaining protons. In the experiment, illustrated in
Figure 4.7, a narrow beam of alpha particles was directed at a very thin
sheet of gold foil. According to the prevailing theory, the alpha particles
should have passed easily through the gold, with only a slight deflection
due to the positive charge thought to be spread out in the gold atoms.
Rutherford’s results were that most alpha particles went straight
through the gold foil, or were slightly deflected. However, what was surprising is that a small fraction of the alpha particles bounced off the
gold foil at very large angles. Some even bounced straight back toward
the source. Rutherford later recollected, “This is almost as incredible as
if you fired a 15-inch shell at a piece of tissue paper and it came back and
hit you.”
The Rutherford Atomic Model Based on his experimental results,
Rutherford suggested a new theory of the atom. He proposed that the
atom is mostly empty space, thus explaining the lack of deflection of most
of the alpha particles. He concluded that all the positive charge and almost
all the mass are concentrated in a small region that has enough positive
charge to account for the great deflection of some of the alpha particles.
He called this region the nucleus. The nucleus is the tiny central core of
an atom and is composed of protons and neutrons.
Informal Assessment
V
LESSON 4.2
CHEMISTRY
a
Fluorescent
screen
Gold foil
b. Rutherford concluded that most of the alpha
particles pass through the gold foil because
the atom is mostly empty space. The mass and
positive charge are concentrated in a small
region of the atom. Rutherford called this region
the nucleus. Particles that approach the nucleus
closely are greatly deflected.
b
Nucleus
Lead shield
Alpha particles
Source of
alpha particles
Beam of
alpha particles
Atoms of
gold foil
See Rutherford’s gold-foil
experiment animated online.
108 $IBQUFSt-FTTPO
Focus on ELL
4 LANGUAGE PRODUCTION Have students work in partners to complete the
Quick Lab. Make sure each pairing has ELLs of varied language proficiencies,
so that a more proficient student can help a less proficient one. Have students
work according to their proficiency level.
BEGINNING: LOW/HIGH Model this lab for these students. Show them your data,
sketch, and measurements. Explain your method for guessing what is in the box.
INTERMEDIATE: LOW/HIGH Ask students to brainstorm what they think they may
observe, prior to performing each step. Allow students to orally present their
results.
ADVANCED: LOW/HIGH Ask students to present to the class their sketch, data, and
interpretation of the object in one of the boxes.
108
Chapter 4 • Lesson 2
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Explore
Quick Lab
PURPOSE Th
The students
d
will determine the shape of
Quick Lab
Purpose To determine the
shape of a fixed object inside
a sealed box without opening
the box
Materials
rbox containing a regularly
shaped object fixed in place
and a loose marble
Using Inference: The Black Box
Procedure
1. Do not open the box.
2. Manipulate the box so that the marble moves around the fixed object.
3. Gather data (clues) that describe the movement of the marble.
4. Sketch a picture of the object in the box, showing its shape, size, and
location within the box.
5. Repeat this activity with a different box containing a different object.
Analyze and Conclude
1. Compare Find a classmate who had a box with
the same letter as yours, and compare your findings.
2. Apply Concepts Think about the experiments
that have contributed to a better understanding
of the atom. Which experiment does this activity
remind you of?
the hidden object by analyzing the rebound paths
of a marble rolled at the object.
SKILLS FOCUS Observing, inferring
PREP TIME 5 minutes
CLASS TIME 10 minutes
MATERIALS box containing a regularly shaped object
fixed in place and a loose marble
ADVANCE PREP Cut geometric shapes—such as a
triangle, circle, or L, from a sheet of 1-inch plastic
foam.
EXPECTED OUTCOME Students’ inferences may or
may not be different for the same object.
ANALYZE AND CONCLUDE
1.
2.
See Expected Outcome.
The activity simulates the strategy that
Rutherford used to probe the structure of metal
atoms. Like the students, Rutherford and his
coworkers were also faced with the problem of
identifying properties of an object not visible to
the eye.
NLIN
PR
S
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FOR ENRICHMENT Make a more challenging black
9.
10.
M
OBLE
box for students who have an easy time with the
simple boxes. Put two single objects in one box,
or a single object with a complex shape.
4.2 Lesso
LessonCheck
Review What are three types of subatomic
particles?
Explain How does the Rutherford model
describe the structure of atoms?
11. Review What are the charges and relative
masses of the three main subatomic particles?
12. Explain Describe Thomson’s and Millikan’s
contributions to atomic theory.
13. Compare and Contrast Compare Rutherford’s
expected outcome of the gold-foil experiment
with the actual outcome.
14. Analyze Data What experimental evidence led
Rutherford to conclude that an atom is mostly
empty space?
15. Compare and Contrast How did Rutherford’s
model of the atom differ from Thomson’s?
Atomic Structure 109
Lesson Check Answers
9. protons, neutrons, and electrons
10. A positively charged nucleus surrounded
by electrons, which occupy most of the
volume.
11. proton, positive charge, relative mass = 1;
electron, negative charge, relative mass =
1/1840; neutron, no charge, relative
mass = 1
12. Thomson passed an electric current
through sealed glass tubes filled with
gases. The resulting glowing beam
consisted of tiny negatively charged
particles moving at high speed. Thomson
concluded that electrons must be parts
of the atoms of all elements. Millikan
determined the charge and mass of the
electron.
13. Rutherford expected all the alpha particles
to pass straight through with little
deflection. He found that most alpha
particles passed straight through, but
some particles were deflected at very large
angles—and some even bounced straight
back.
14. The great majority of the alpha particles
passed straight through the gold foil.
15. Rutherford’s atomic model described the
atom as having a positively charged, dense
nucleus that is tiny compared to the atom
as a whole. In Thomson’s plum-pudding
model, electrons were stuck in a chunk of
positive charge.
Atomic Structure
109
LESSON 4.2
The Rutherford atomic model is known as the nuclear atom.
In the nuclear atom, the protons and neutrons are located
in the positively charged nucleus. The electrons are distributed
around the nucleus and occupy almost all the volume of the atom.
According to this model, the nucleus is tiny and densely packed
compared with the atom as a whole. If an atom were the size of a
football stadium, the nucleus would be about the size of a marble.
Although it was an improvement over Thomson’s model of the
atom, Rutherford’s model turned out to be incomplete. In Chapter 5,
you will learn how the Rutherford atomic model had to be revised in
order to explain the chemical properties of elements.
CHEMISTRY & YOU
CHEMISTRY
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Electron Microscopy
&
CHEMISTRY
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capable of much greater levels of magnification than
light microscopes, but their real power lies in their
resolution. Resolution refers to the degree of detail
that is revealed by a microscope. The greater a
microscope’s resolution, the more detail that can be
seen in the images it transmits. Light microscopes
rely on the natural reflective, refractive, and
absorbent behaviors of light to make objects visible
under magnification. However, these same behaviors
can inhibit the degree of resolution, regardless of
the magnitude of magnification. Thus, the resolution
of a typical compound light microscope is about
200 nm. In contrast, some SEMs can achieve a
resolution of 5 nm at high magnification.
Pose the following question to students: How does
the behavior of light inhibit the ability of a light
microscope to deliver images with high resolution?
You may need to assist students in the following
ways:
• Waves of visible light occur in wavelengths from
400 nm to 700 nm.
• The light provided to a light microscope by its
mirror or electric light source are unfocused and
spread out from the light source, while that
electron beam in an SEM is highly focused and
narrow.
• Light waves reflected by and transmitted through
the object are subject to interference from each
other and from incident light from both the
source and from the microscope’s surroundings.
• Some light waves transmitted through the
sample may be directed away from the lens of
the microscope.
Y U: TECHNOLOGY
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&YOU:
Electron Microscopy
Within 30 years of J. J. Thomson’s discovery of the
electron, scientists were studying how to produce
images of objects by using an electron beam. In 1931,
German scientists Ernst Ruska and Max Knoll built
the first electron microscope. There are two types of
electron microscopes, scanning electron microscopes
(SEM) and transmission electron microscopes (TEM).
The images shown here are from SEMs. In an SEM,
a beam of electrons is focused down to a very small
diameter and scanned across the sample. Most materials
eject electrons when the electron beam hits them. The
location of the ejected electrons is detected and used to
produce an image.
A typical light microscope is capable of magnifying
an object 1000 times. An electron microscope can
magnify an object over 100,000 times. Another
advantage of electron microscopes is their higher
resolution. Resolution is the ability to differentiate
two objects that are very close to each other. So, an
electron microscope has the ability to produce a
clearer image than a light microscope at the same
magnification. Electron microscopes do not
produce color images. The color images shown
here have false color that has been added to
the images. Electron microscopes are useful
in chemistry, but also in other fields,
such as archeolology, pharmacology
and quality assurance testing.
SEM This microscope is a
scanning electron microscope.
The image from the microscope
is seen by using a computer
screen.
BIOLOGY This diatom is a
single-celled organism that
lives in the water. The image
shown above is a dust mite
on a piece of fabric.
110
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Chapter 4 • Chemistry & You
21st Century Learning To be successful in the 21st century, students
need skills and learning experiences that extend beyond subject matter mastery.
This project helps students build the following 21st Century Learning: Creativity and
Innovation; Communication and Collaboration; Information, Media, and Technology
Literacy; Initiative and Self-Direction; and Social Skills.
Pose the following challenge to your students. Your school has invited four different
specialists in electron microscope technology to participate in a roundtable discussion
during a college fair. The specialists are there to answer questions and convey
information about the uses of electron microscopes and the types of careers that
utilize this type of technology.
Ask for four student volunteers to be the team of four “experts” in electron microscopy
in the following fields: biology, forensics, biochemistry, and food science. The remaining
students will form four interview panels. Each panel will select one of the four
specialists to interview. The interview panels should prepare a set of questions and their
expected answers on the specific uses of electron microscopy in the expert’s field, as
well as pros and cons of its use. The team of “experts” will prepare a set of expected
questions and their answers on which to base their preparation for their roles.
USE VISUALS Have students examine the image of
the scientists working with the electron microscope.
Point out how a person sees an image made by the
electron microscope; the scientists in the photo are
not looking into eyepieces, but see a projection on
a computer screen. Explain that the type of electron
microscope a person uses is typically dependent on
the type of information he or she needs to know.
A microscope is typically selected based on its
resolution, magnification, depth of field, field of
view, illumination method, degree of automation,
and type of image produced.
BIOCHEMISTRY A chromosome
from the nucleus of a eukaryotic cell
FORENSICS The image on the left is from a light
microscope, the image on the right from an SEM. The SEM
image shows a clearer image of the fingerprint left on the
page because oil from our fingers produces a different
intensity of ejected electrons than paper or the ink.
Extend
Connect to
TECHNOLOGY
Emphasize the time period that the electron
microscope was invented and the technology
associated with the apparatus. Tell students that
cathode ray tubes, magnets, electron beams, and
vacuum chambers are all integral parts of the
electron microscope. In pairs, have students research
other technological advances that came about using
the same or similar components. (Televisions, hand
held radios, X-ray machines and lasers all were
invented around the same time as the electron
microscope and as a result of the same technology.)
MATERIALS SCIENCE A tip of a pencil is pointing to a tiny
pressure sensor (mauve square) used in a car’s air bag.
When the sensors detect rapid deceleration, they trigger
the inflation of the air bag.
Take It Further
1. Infer Why would a forensic investigator want to
analyze gunshot residue using an electron microscope?
2. Compare Research the differences between an
SEM and a TEM.
Chemistry & You 111
Differentiated Instruction
Answers
L1 STRUGGLING STUDENTS Have students draw pictures of an ordinary object
as they imagine it would appear under an ordinary microscope and then as they
imagine it would appear under an electron microscope.
TAKE IT FURTHER
ELL ENGLISH LANGUAGE LEARNERS Have magnifying glasses available in class,
as well as a compound microscope. Allow students time to examine and use both
tools.
L3 ADVANCED STUDENTS Assign students to write a summary explaining how an
electron microscope provides a far greater degree of magnification and a better
resolution than an ordinary compound microscope. Summary should include how
Thomson’s research on cathode rays propelled electron microscope technology.
1.
An electron microscope would allow scientists
to see details in the gunshot residue that might
not be visible by other microscopes.
2.
An SEM is a scanning electron microscope.
In an SEM, an electron beam is passed over
the surface of an object. An SEM produces an
image of the surface of on an object. A TEM is
a transmission electron microscope. In a TEM,
an electron beam is directed through an object,
producing an image of what is inside an object.
Chemistry & You
111
CHEMISTRY & YOU
Explain