Chemistry I
Study Guideline – Unit Three
Atomic Structure
______________________________________________________________________________________________________________________________
By the end of this chapter, you should be able to:
1. Describe the nature and significance of scientific modeling in general and
several of the atoms
2. Compare and Contrast the models of the atom including Dalton’s, Thomson’s
and Rutherford’s.
3. Explain how atoms are related to electricity.
4. Describe how J. J. Thomson investigated the charge-to-mass ratio of an
electron and a proton using a CRT.
5. Relate the nature of radioactivity to Ernst Rutherford’s alpha scattering
experiment and how it showed the existence of the nucleus.
6. Identify the characteristics of Alpha, Beta and Gamma radiation.
7. Name and describe three subatomic particles.
8. Define atomic number, mass number, isotopes and ions.
9. Define and Explain (average) atomic mass.
10. Determine the number of protons, neutrons and electrons in an isotope from
its name or symbol.
11. Determine the atomic mass of an element, given the masses and percent
abundance of the elements isotopes.
12. Explain why elements above atomic number 83 are radioactive.
13. Describe how a mass spectrometer is used to determine the relative masses
and abundances of isotopes.
Video: World of Chemistry - The Atom
Please Answer the following question while you watch the video
1. What did Democritus propose?
2. What did Dalton propose?
3. What are the three basic particles of the atom?
4. What are the two regions of the atom?
5. What determines the identity of an atom?
6. How many electrons does an atom have?
7. What is a STM? What can you see with it?
8. What do opposite charges do? What do like charges do?
9. What was Rutherford’s surprise?
10. What Did Rutherford conclude from this surprise?
page 2
Bill Nye - The Atom
Please Answer the following question while you watch the video
1. What is the smallest piece of matter?
2. What are the parts of the atom?
3. What parts of the atom are in the nucleus?
4. What is the charge on a Proton? Neutron? Electron?
5. What is an atom mostly?
6. All matter is made of what?
7. What is produced when you pass electricity through water (electrolysis)? How many parts of hydrogen
gas and oxygen gas do you get from the electrolysis of water?
8. Why is the volume of 2 cups of water greater than the volume of 1 cup of water plus one cup of alcohol?
9. What makes one element (atom) different from another?
10. What is the atomic number?
Discovering Atomic Structure
1. The negatively charged electrode of a cathode ray tube is called the _______________.
a. anode
b. cathode
c. plate
d. magnet
2. The physicist Henri Becquerel discovered radioactivity while studying the phosphorescence of
____________.
a. radium
b. silicon
c. uranium
d. curium
3. Which of the following is not a type of radiation?
a. alpha
b. beta
c. gamma
d. delta
4. Rutherford alpha scattering experiment showed that the nuclear charge must be ______________?
a. positive
b. neutral
c. negative
d. too small to be detected
5. Thomson concluded that cathode rays were composed of particles that were ______________?
a. positive
b. heavy
c. negative
d. visible
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The Millikan Oil-Drop Experiment
In 1909, the American physicist Robert Millikan measured the size of the charge of an electron. Using
the apparatus illustrated below, he introduced a fine mist of oil into a closed chamber. The droplets of oil
passed between two electrically charged plates through which he was able to influence their rates of decent.
Observing the individual droplets through a microscope, Millikan was able to adjust the electrical force so
that the drops moved away from whichever plate had the same charge. He timed the drops’ ate of
movement. From this information, he was able to determine both the charge of an electron and to suggest a
value for its mass. Study the illustration below and answer the following questions.
_____ 1. To cause a negatively charged oil droplet to move upwards; plate Y should have __________.
a. a positive charge b. a negative charge c. an excess of protons d. an excess of neutrons
_____ 2. The purpose of device Z is to _______________.
a. observe the electrical potential difference (voltage) between the plates
b. separate spectral emissions
c. magnify droplets of liquid
d. locate positively charged particles
_____ 3. If a droplet was momentarily suspended between the electric plates, ________________.
a. the number of protons in the droplet equaled the number of electrons
b. there was no charge on the plates
c. the droplet weight was exactly balanced by forces of electrical repulsion/attraction
d. the mass of the electrons in the droplet equaled the mass of the protons
_____ 4. The function of device W is to ________________________.
a. remove excess mist
b. supply protons
c. monitor relative humidity
d. produce a mist from liquid oil
_____ 5. If electrical leads to plates X and Y were reversed, negatively charged oil droplets that had been
moving upwards would ________________________.
a. remain suspended
b. move downwards towards plate Y
c. move upwards towards plate X
d. acquire a positive charge
page 4
The Rutherford Experiment
Our modern view of atomic structure is based to a large extent on the wok that British scientists
Rutherford and Geiger did in 1911. In the classic experiment, positive alpha particles bombarded a sheet of
gold foil. The paths that followed by those particles are illustrated in the diagram below.
Path
1
Path
2
Path
3
Path
4
Atom
Nucleus
_____ 1. Which of the four path was most common?
a. one
b. two
c. three
d. four
_____ 2. Which of the four path was least common?
a. one
b. two
c. three
d. four
_____ 3. Path two was a straight line because of the alpha particles’ _____________________.
a. magnetic repulsion
b. high velocity
c. distance from the gold nuclei
d. interaction with electrons
_____ 4. Path four will most likely _____________________.
a. never be observable
b. be characteristic of only the fastest moving alpha particles
c. be characteristic of alpha particles that move directly towards a nucleus
d. result in an atomic reaction
_____ 5. When Rutherford analyzed his results, he suggested that _____________________.
a. the atom was mostly empty space
b. atoms contained a small dense center
c. the atomic center was positive in charge
d. all of the above were true
page 5
Matching
On the left, write the term or the name from the list that matches the description
alpha particle
Curie
Joliot-Curie
atomic nucleus
Dalton
Millikan
Becquerel
Rutherford
radioactivity
cathode ray
gamma radiation
static electricity
Chadwick
Geiger
Thomson
_____________________________ 1. electrical charges that are not in motion.
_____________________________ 2. stream of particles originating from a cathode
_____________________________ 3. proposed the first atomic model that accounted for the electrical
nature of the atom
_____________________________ 4. measured the size of the charge on an electron
_____________________________ 5. suggested that alpha particles might be rebounding at an angle
approaching 180° after coming close to the nucleus
_____________________________ 6. spontaneous emission of radiation from an element
_____________________________ 7. radiation that is similar to X-rays and not composed of particles
_____________________________ 8. proposed that the atom was a sphere that was of equal density all
the way through
_____________________________ 9. discovered a neutral beam that had high penetrating power.
_____________________________ 10. was surprised when alpha particles bounced back
_____________________________ 11. small core at the center of the atom containing a positive charge
_____________________________ 12. radiation that has a +2 charge
_____________________________ 13. is credited with the discovery of radioactivity
_____________________________ 14. discovered the radioactive element polonium
_____________________________ 15. was first to identify the neutron
Draw and label a diagram of Dalton’s, Thomson’s and Rutherford’s model of the atom.
page 6
Nuclear Symbols
Element symbols are often accompanied by notation that gives information on atomic composition.
The subscript, written to the lower left of an element symbol, represents the atomic number. The
superscript, written to the upper left of the symbol, represents the mass number or the total number of
protons and neutrons. Using this information, complete the following table.
Symbol
12
Atomic #
Mass #
# of protons
# of neutrons
# of electrons
C
6
40
Ar
18
23
Na
11
20
Ne
10
48
Ti
22
40
Ca
20
127
I
53
238
U
92
1. How many protons and electron in a neutral
vanadium atom?
2. How many protons and electron in a neutral
potassium atom?
3. How many protons and electron in a neutral
platinum atom?
4. What is the name of the element that contains 17
protons?
5. What is the name of the element that contains 82 protons?
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6. Write the complete chemical symbol for the atom that
contains 84 protons, 125 neutrons and 80 electrons?
7. Write the complete chemical symbol for the atom that
contains 27 protons, 32 neutrons and 25 electrons?
8. Write the complete chemical symbol for the atom that
contains 73 protons, 108 neutrons and 68 electrons?
9. Write the complete chemical symbol for the atom that
contains 31 protons, 39 neutrons and 28 electrons?
Complete the following table:
Symbol
59
2+
Ni
28
3+
Ce
58
21
Neutrons
Electrons
Symbol
91
Zr
4+
40
140
45
Protons
79
2–
Se
34
3+
Sc
14 4–
C
6
page 8
Protons
Neutrons
Electrons
Nuclear Stability
The stability of a nucleus is dependent upon the ratio of its component particles. In the stable isotopes
of lighter nuclei, the ratio of neutrons to potons is approximately one. The ratio of neutrons to protons in
heavier nuclei approaches a value of 1.5. Ratios falling outside this belt of stability are unstable or
radioactive nuclei.
Complete the following table of stable nuclei. Determine the neutron-to-proton ratio of each and then
plot the points on the grid provided at the bottom of the page.
Isotope Protons Neutrons Neutron-to- Isotope Protons Neutrons Neutron-toproton ratio
proton ratio
4
79
He
2
Br
35
12
90
C
6
20
107
Ne
10
27
127
Al
138
Cl
Ba
56
142
Ca
20
56
I
53
17
40
Ag
47
13
37
Zr
40
Nd
60
158
Fe
26
Tb
65
Neutron-toProton ratio
1.5
1.0
0.5
Atomic
Number
5
10
15
20
25
30
35
page 9
40
45
50
55
60
65
Hydrogen Isotopes
Atoms are made up of subatomic particles, such as protons, neutrons and electrons. The nuclei of
atoms that make up isotopes of an element differ. There are three known isotopes of hydrogen. Make a
drawing that represents each of these isotopes.
e
2p
2n
e
Helium-4
Protium
Hydrogen-1
Deuterium
Hydrogen-2
Tritium
Hydrogen-3
1. Do the numbers of electrons for neutral isotopes of the same elements differ? ________
2. Do the number of protons for such isotopes differ? __________
3. Do the number of neutrons for such isotopes differ? __________
4. Do the atomic numbers for such isotopes differ? Explain.
5. Do the mass numbers for such isotopes differ? Explain.
6. If (1 + 2 + 3)/3 = 2 then why is the average atomic mass for hydrogen 1.0079 and not a whole number?
Review Activity - Development of Atomic Theory
atom
Dalton
isotope
Planck
neutron
atomic number
Democritus
Lavoisier
proton
Thomson
Chadwick
electron
mass number
Proust
subatomic particle
conservation of matter
definite proportions
multiple proportions
Rutherford
More than 2000 years ago, a Greek philosopher named _____________ proposed the existence of very
small, indivisible particles, each of which was called a(n) _____________. The theory that such particles
existed was supported much later, by ____________ who proposed, in his law of ___________ ______
____________, that matter could not be created or destroyed. Then ___________ proposed, in his law of
_________ _________, that the ratio of the masses of elements in any given compound is always the same.
The law of ____________ ___________ , proposed soon after, states that the masses of one element that
combine with a fixed mass of another element in different compounds are in simple, whole number ratios.
An atomic theory based on these laws was developed by _____________, who is credited with the first
modern atomic theory.
page 10
It was later proposed that the atom is in fact divisible into smaller parts each of which is called a(n)
_____________ ___________. These particles include the negatively charged ____________, discovered
by ____________; the positively charged ____________; and the uncharged ____________, discovered by
____________. The latter two particles are present in the ____________, or center of the atom, which was
discovered by ____________ in his gold foil experiments.
The number of positively charged particles in an atom is called its _________ __________. The total
number of positively charged particles and the uncharged particles is called the atom’s _________
___________. An atom that has the same number of positively charged particles as another atom, but a
different number of uncharged particles, is called a(n) ____________.
Average Atomic Mass Problems
1. What is the average atomic mass of the element copper if it is composed of 69.5% of an isotope atomic
mass of 63.00 and 30.5% of an isotope with an atomic mass of 65.00?
2. Calculate the average atomic mass of lithium, which occurs as two isotopes that have the following
isotope atomic masses and abundances in nature: 7.3%, 6.017AMU and 92.7%, 7.018AMU
3. Calculate the average atomic mass of chromium, given the following percent abundances and isotope
atomic masses: 4.350% 49.946AMU; 83.790% 51.941AMU; 9.500% 52.941AMU; 2.360%
53.939AMU.
page 11
4. The four isotopes of lead are listed below, each with its percent abundance. Calculate the average atomic
mass of Pb.
Isotope #1 - atomic mass = 204.01 amu - 1.37%
Isotope #2 - atomic mass = 205.97 amu - 26.26%
Isotope #3 - atomic mass = 207.11 amu - 20.82%
Isotope #4 - atomic mass = 207.89 amu - 51.55%
Analyzing a Spectrograph (Mass Spectrometer)
A mass spectrograph (spectrometer) is an instrument used to separate an element’s isotopes and to
measure their relative abundances. Within this device, beams of an element’s ions are passed through a
strong magnetic field. As they are passed through, they respond to the magnetic force. Ions of greater
mass possess more inertia, or more of a tendency to continue to move in a straight line, and so deviate only
slightly from their projected path. Ions of lesser mass are more greatly influenced by the field and
demonstrate greater deviation. Examine the three mass spectrographs illustrated below and answer the
questions that follow. Note that the upper scale of each spectrograph shows the atomic mass in AMU.
Below each spectrograph the percent abundance of each isotope is indicated.
Atomic Mass
30.0
29.0
28.0
27.0
26.0
25.0
24.0
23.0
(amu)
A
Relative %
100.0
1. a. What is the atomic mass of the isotope of the element represented by spectrograph A? _________
b. What are the name and the element symbol of element A?
page 12
_________
Atomic Mass
30.0
29.0
28.0
27.0
26.0
25.0
24.0
23.0
(amu)
B
Relative %
11.17 10.13 78.70
2. a. Based on the experimentally obtained data in spectrum B calculate the average atomic ________
mass of this element. Show your work.
b. Which isotope deviated most from its straight-line path?
_________
c. What are the name and the element symbol of element B?
_________
Atomic Mass
56.0
55.0
54.0
53.0
52.0
2.38
9.55
83.76
51.0
50.0
49.0
(amu)
C
Relative %
4.31
3. a. Based on the experimentally obtained data in spectrum C calculate the average atomic
mass of this element. Show your work.
________
b. Which isotope deviated most from its straight line path?
________
c. What are the name and the element symbol of element C?
________
page 13
Composition of the Atom - Word Scramble
Use the clues provided to help you unscramble the letter to form words related the development of atomic
structure. The letters in the circle spell out, in order, the name of a famous chemist.
CLUES
1. Pieces of inert metal connected to a power source
2. Element discovered by Marie Curie
3. Atoms with the same atomic number but different mass numbers
4. Radiation consisting of a high speed Helium nuclei
5. Center of the atom
6. Subatomic particle with no charge
7. Element with a mass number of 19.0
8. A mass _________________ , a device used to determine atomic masses. (If you can’t get this one
look at the page before for a hint).
9. A positively charged particle
10. Element with an atomic number of one.
1. T E R D L C E E O
__ __ __ __ __ __ __ __ __
2. L O M U O P N I
__ __ __ __ __ __ __ __
3. S O O S T I P E
__ __ __ __ __ __ __ __
4. H A A L P
__ __ __ __ __
5. S U C U N E L
__ __ __ __ __ __ __
6. N E T N O U R
__ __ __ __ __ __ __
7. N E O L I R F U
__ __ __ __ __ __ __ __
8. E P R T G S C H P O R A
__ __ __ __ __ __ __ __ __ __ __ __
9. T O R N O P
__ __ __ __ __ __
10. O G Y D N E H R
__ __ __ __ __ __ __ __
Name ____________________________
page 14