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Introductory Atomic Theory
Main Idea Summary:
• Elements are composed of atoms, which are the basic building blocks of matter.
• The atoms of a given element are different from the atoms of all other elements.
• Atoms contain positively charged protons, negatively charged electrons, and electrically neutral neutrons.
• The nucleus of an atom is composed of protons and neutrons. The electrons surround the nucleus and
occupy most of the volume of the atom.
• The number of protons in an atom’s nucleus is the atomic number of that element.
• Because atoms are electrically neutral, an atom has the same number of protons and electrons.
• The sum of the number of protons and number of neutrons is the mass number of an atom.
• Atoms with the same number of protons but different numbers of neutrons are called isotopes.
• The atomic mass of an element is expressed in atomic mass units (amu).
• The atomic mass of an element is a weighted average of all the naturally occurring isotopes of that element.
• In the periodic table, the elements are organized into groups (vertical columns) and periods (horizontal
rows) in order of increasing atomic number.
• Elements that have similar chemical properties are in the same group.
• Elements in the periodic table are classified as metals, nonmetals, or metalloids.
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Discussion Sheet 4a – Early Atomic Theory NAME: ______________________
Scientist and Atomic Model
Contributions to Atomic Theory
Dalton Studied ratios in which elements combine in chemical reactions:
1808 • All elements are composed of tiny, indivisible particles
• All atoms of element X are identical. All atoms of element
X are different than atoms of any other element.
• Atoms can chemically combine together in whole number
• Chemical reactions occur when atoms are separated, joined,
or rearranged. Atoms of one element are never changed into
atoms of another element.
Thomson Passed electric current through gases at low pressure using a
cathode ray tube:
• Used a positively charged plate to demonstrate that atoms
contain negatively charged subatomic particles called
• Electrons are extremely small. One electron is about 1/2000
the mass of a hydrogen atom.
• Model called the “plum-pudding” model, named after the
British cuisine, with negatively charged electrons floating in
a positively charged “broth.”
Rutherford Directed a beam of alpha particles at a thin sheet of gold foil:
1911 • Demonstrated that the atom is mostly empty space, with a
very dense core called a nucleus.
• Nucleus of atom is composed of positively charged particles
• Protons are large subatomic particles. One proton is equal to
one atomic mass unit.
• The space between the nucleus and orbiting electrons is
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Worksheet 4.01 – Early Atomic Theory NAME: ______________________
1. With which of these statements would John Dalton have agreed in the early 1800s?
a. Atoms are the smallest particles of matter.
b. The mass of an iron atom is different from the mass of a copper atom.
c. Every atom of silver is identical to every other atom of silver.
d. A compound is composed of atoms of two or more different elements.
2. What experimental evidence did Thomson have for each statement?
a. Electrons have a negative charge. _____________________________________
b. Atoms of all elements contain electrons. ________________________________
3. Would you expect two electrons to attract or repel each other? Why?
4. How did the results of Rutherford’s gold foil experiment differ from his expectations?
5. What is the charge, positive or negative, of the nucleus of every atom?
6. Why is an atom electrically neutral?
7. What technology was available to Thomson in 1897 that was not available to Dalton in 1808?
8. What contribution did Millikan make to the development of atomic theory? Explain.
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Discussion Sheet 4b – Using the Periodic Table NAME: ______________________
In 1932, James Chadwick confirmed the existence of another subatomic particle: the neutron. Neutrons are subatomic particles with
no charge but with a mass nearly equal to that of a proton. Thus, the fundamental building blocks of atoms are the electron, the
proton, and the neutron.
Protons and neutrons are large, and found in the nucleus, at the center of the atom.
The sum of the number of protons and neutrons in an atom is called the mass
number. Protons are positively charged. Neutrons are electrically neutral. On
the other hand, electrons are negative. Electrons are very small, and are found far
away from the nucleus.
The periodic table is an arrangement of all of the elements in the universe. The
first periodic table was devised in the 1800s by a Russian chemist named Dimitri
Mendeleev. He organized all of the known elements based on what atomic mass
each one had. The elements in the modern table are organized according to their
atomic number. There are always two numbers in every entry in the periodic table: the atomic number and the atomic mass.
The atomic number is the number of protons in an atom’s nucleus. In a neutral atom, it is also the number of electrons. The number
of protons an atom has determines what type of atom it is. Every atomic number also corresponds with an atomic symbol. If an
element has an atomic number of 2, each atom of that element has 2 protons, and it can then be identified with the symbol “He,”
which stands for helium. A helium atom may not have more or less than 2 protons. The atomic number is always the smallest
number in any periodic table entry.
The protons and neutrons are the heavier subatomic particles found in the nucleus of an atom. The mass number is the sum of the
number of protons and the number of neutrons in an atom. The atomic mass number is always the largest number in any periodic
If you know the atomic number and mass number of an atom of any element, you can determine the atom’s composition. The atomic
number is equal to the number of protons and the number of electrons. The number of neutrons can be determined from the
number of neutrons = mass number − atomic number
Atomic number = number of protons = number of electrons
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Worksheet 4.02 – Atomic Composition NAME: ______________________
Atomic umber (# of Protons) = (# of Electrons)
Mass umber (# of Protons) + (# of Neutrons)
Isotopes same # of protons, but different # of neutrons
1. Fill in the following table. The first entry has been done for you. (For the purposes of this worksheet, you
may round the number of neutrons to the nearest whole number.)
Number Number Number
Element Symbol of of of
Protons Neutrons Electrons
Lithium Li 3 7 3 4 3
2. What is the difference between the terms “atomic mass” and “mass number”?
3. Why aren’t all of the atomic mass numbers in the periodic table whole numbers? (Hint: It is not possible
for an atom to have a fractional part of a neutron.)
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Laboratory Activity 4A – Thickness of a Thin Aluminum Sheet NAME: ___________________
Your goal is to determine the thickness of a thin sheet of aluminum. Sheets of material are often too thin to
measure with a ruler or calipers, so you will have to do some calculations to find out the thickness of the sheet.
Measure the length and width of a sheet of aluminum, using a centimeter ruler. Using a laboratory balance,
measure the mass of the aluminum sheet. Assuming the density of the aluminum sheet is 2.70 g/cm3, the
accepted density of aluminum, calculate the thickness of the sheet.
Remember: Mass ÷ Volume = Density
Volume = Length × Width × Thickness
Pay special attention to the use of significant figures in all of your measurements and calculations.
Measurement or Trial 1 Trial 2 Trial 3
Type of Sheet “Foil” or “Siding”
Density d 2.70 g/cm3 2.70 g/cm3 2.70 g/cm3
Volume V = m ÷ d
Thickness t = V ÷ (l·w)
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Worksheet 4.03 – Review of Atomic Composition NAME: ___________________
1. Define an atom.
2. Distinguish between protons, electrons, and neutrons in terms of their relative masses and charges. It may
be helpful to construct a table.
3. Discuss the structure of an atom including the location of the proton, electron, and neutron with respect to
4. Complete the following table:
Number Number Number
Element Symbol of of of
Protons Neutrons Electrons
5. Define an atomic mass unit. Which subatomic particles have a mass of one amu?
6. What is an isotope? How do isotopes of an atom differ?
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Discussion Sheet 4c – Finding Average Atomic Mass NAME: ______________________
You may be wondering why some of the atomic mass numbers in the periodic table are expressed in decimal notation, and not whole
numbers. After all, an atom’s mass is the sum of its protons and neutrons, and it is impossible for an atom to have a fraction of a
proton or a neutron. The reason why the periodic table contains decimals is because the atomic mass number in the periodic table
represents the average mass of all of the known isotopes of a given element. Isotopes are atoms that have the same number of
protons but different numbers of neutrons. Some atoms of an element can therefore be heavier than other atoms, due to an extra
neutron in the nucleus.
Some of the carbon found in nature has a mass of 12 (6 protons and 6 neutrons in the nucleus). This isotope of carbon is called
Carbon-12. Some of the carbon found in nature is heavier, due to the presence of an extra neutron. This type of carbon is called
Carbon-13, and has 6 protons and 7 neutrons. The periodic table entry for carbon has an atomic mass of 12.011. Since this number
is so close to 12, we can assume that most of the carbon found in nature is Carbon-12. How does a scientist calculate an average
atomic mass? Well, let’s consider the following problem.
A family contains the following four people. Find the average weight of the family members.
100 pounds 100 pounds 100 pounds 104 pounds
Traditionally, a math student will do this problem by adding up all four of the weights and then dividing by 4, the number of people
in the family. This will get the correct answer, which is 101 pounds:
100 + 100 + 100 + 104
------------------------------- = 101
Here’s another way of looking at this question:
3 out of 4 people in the family are 100 pounds. 3 out of 4 is 75% of the family.
1 out of 4 people in the family are 104 pounds. 1 out of 4 is 25% of the family.
75% × 100 pounds = 7500
+ 25% × 104 pounds = + 2600
100% × Average weight = 10100 = 100% × 101 pounds
By multiplying each percentage by the appropriate weight, and then adding them all up, we can determine what the average of 100%
of the family members will be.
The first method is not useful for calculating average atomic mass because we don’t have the necessary variables. You will find the
second method to be far more convenient and effective when you are trying to find the average atomic mass. Remember to make
sure that all percentages add up to 100%.
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Worksheet 4.04 – Average Atomic Mass NAME: ___________________
Solve the following problems. Use the solution to the example problem as a guide.
EXAMPLE: Use the following information to determine the average atomic mass of chlorine. Two isotopes
are known: chlorine-35 and chlorine-37. The relative abundance of chlorine-35 is 75.4%.
SOLUTION: First, we need to make sure that we know all of the percentages. As the problem states, there are only two
different isotopes of chlorine. Since we know that 75.4% of the sample is chlorine-35, and that all percentages
must add up to 100%, we can find the other percentage by subtraction:
100.0% − 75.4% = 24.6% of the sample must be chlorine-37
Next, we multiply each percentage by each mass, and add up the results, just as in the average weight problem
from Discussion Sheet 4c:
75.4% × 35 = 2639.0
+ 24.6% × 37 = + 910.2
100% × Average mass = 3549.2
Now, all that is left is to divide the outcome by 100% to get the average atomic mass, 35.492. This is the average
atomic mass, in atomic mass units. We can double check our answer with a little bit of logic. Since most of the
sample (75.4%) is chlorine-35, the average atomic mass should be closer to 35 amu than it is to 37 amu. 35.492 is
indeed closer to 35, so our answer makes sense.
1. A research team has just discovered a new element called Wikapedium . Now, they need to determine the
average atomic mass in order to complete an entry for the periodic table. Given the following relative
abundances, calculate the average atomic mass of Wikapedium .
Wikapedium -138: 44.7% Wikapedium -140: 00.5%
Wikapedium -139: 52.3% Wikapedium -141: 02.5%
2. Another new element, Humanium, has two isotopes. 67.52% of the Humanium isotopes have a mass of
256 amu. The rest of the sample is Humanium-257. Find the average atomic mass of Humanium.
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Worksheet 4.05 –Average Atomic Mass NAME: ___________________
Solve the following problems.
1. There are four isotopes of lead. Data on their atomic structure can be found in the table. Find the average
atomic mass of lead in the space below.
Isotope A B C D
Protons 82 82 82 82
Neutrons 122 124 125 126
Percent Abundance 1.37% 26.26% 20.82% 51.55%
2. A new element, Cheesium, has been discovered. 43.2% of all naturally occurring Cheesium has a mass of
292 amu. 46.8% of all Cheesium has a mass of 293 amu. The rest of the Cheesium has a mass of 295 amu.
Find the average atomic mass of Cheesium.
3. There are two naturally occurring isotopes of Cranium, of respective masses 670 and 682. What
percentage of a natural sample is of mass 670 if the periodic table entry reads 671.82? (Remember – all
percentages have to add up to 100%.)
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Laboratory Activity 4B – Average Atomic Mass of Beanium NAME: ___________________
You should create a complete lab report for this Laboratory Activity.
In this lab you will calculate the average mass of the element “Beanium” by taking a sample of this element
and taking a weighted average of its three isotopes – black, red, and pinto. Read the directions before
beginning the laboratory activity. Modify the directions before proceeding if you see fit to do so.
1. Take a cupful of beans from the mixed pile. This is the random sample.
2. Separate the beans by type.
3. In the meantime, have one person in the group measure and record the mass of each type of bean on the
4. Count and record the number of each type of bean.
5. Calculate the percent abundance of each bean isotope – remember that all percentages must add up to
6. Finally, calculate the average mass of Beanium.
SUGGESTED DATA TABLE:
Type of Bean Black Red Pinto
Mass of One Bean
Number of Beans
of One Type
Type of Bean
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Discussion Sheet 4d – The Periodic Table NAME: ______________________
Dimitri Mendeleev created the first periodic table in the 1800s. He listed the elements in columns in order of increasing atomic mass.
He then arranged the columns so that the elements with the most similar properties were side by side. Mendeleev left blank spaces in
the table because there were no known elements with the appropriate properties and masses.
Above, you will find Mendeleev’s periodic table. Notice that there are several question marks next to some of the atomic mass
numbers. Mendeleev was not aware of all of the elements that scientists know about today. Using his table, he was able to predict
the existence of some then-still-unknown elements.
In 1913, Henry Moseley determined the atomic number of the atoms of the elements. Moseley arranged the elements in a table by
order of atomic number instead of atomic mass. That is the way the periodic table is arranged today.
The horizontal rows of the modern periodic table are called
periods. The properties of the elements within a period
change as you move across it from element to element.
The pattern of properties within a period repeats, however,
when you move from one period to the next. This
repetition is known as periodic law.
Each vertical column of the periodic table is called a
group or family. The elements in any group of the
periodic table have similar physical and chemical
Without the help of the periodic table, it would be difficult
to learn and remember the chemical and physical properties of the more than 100 elements. Instead of memorizing
their properties separately, you need only learn the general behavior and trends within the major groups. This gives
you a useful working knowledge of the properties of most elements.
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Worksheet 4.06 – The Periodic Table NAME: ___________________
Use your textbook and a periodic table to answer the following questions:
1. What criteria did Mendeleev use to construct his periodic table of the elements?
2. Relate group, period, and transition metals to the periodic table.
3. Identify each element as a metal or nonmetal.
Element Gold Fluorine Sodium Barium Oxygen Iron
4. Which of the elements in the preceding question are representative elements?
5. Name two elements that have properties similar to those of the element calcium.
6. How did Moseley’s arrangement of the elements differ from that of Mendeleev?
7. Give the symbol of each element:
Any nonmetal in Group 4A Two elements that are liquid Any metal in Group 5A
at room temperature
8. What is the difference between the mass number and the atomic number of an atom?
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Worksheet 4.07 – Concept Review NAME: ___________________
Complete the following passage by filling in the blanks with a term, short phrase, or number. Use your
textbook, notes, and periodic table to help you.
Atoms of each element are ________________ from the atoms of all other elements. Dalton theorized that atoms
are indivisible, but the discovery of ________________ particles changed this theory. We now know that atoms are made
up of electrons, which have a ________________ charge; ________________, which have a positive charge, and
________________, which are neutral. The latter two particles are found in the ________________ of the atom.
It was ________________ who discovered the nucleus of the atom. The nucleus has a ________________
charge and it occupies a very small volume of the atom. In contrast, the negatively charged ________________ occupy
most of the volume of the atom.
The number of ________________ in the nucleus of the atom is the atomic ________________ of that element.
Because atoms are electrically neutral, the number of protons and ________________ in an atom are equal. The sum of
the ________________ and neutrons is the mass number. Atoms of the same element are identical in most respects, but
they can differ in the number of ________________ in the nucleus. Atoms that have the same number of protons but
different mass numbers are called ________________.
The ________________ of an element is the weighted average of the masses of the isotopes of that element.
Two isotopes of sulfur are 32S and 34S. An atom of the sulfur-32 isotope contains _____ protons and _____ neutrons. The
sulfur-34 isotope has _____ protons and _____ neutrons.
Each of the three known isotopes of hydrogen has _____ protons in the nucleus. The most common hydrogen
isotope has _____ neutrons. It has a mass of _____ amu and is called hydrogen-1.
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Worksheet 4.08 – Concept Review NAME: ___________________
Answer the following questions or solve the following problems in the space provided.
1. Complete the following table.
umber umber umber
Element Symbol of of of
Protons Electrons eutrons
carbon 12 6
K 19 21
12 12 12
helium 2 4 2
5 5 6
2. Given the relative abundance of the following naturally occurring isotopes of oxygen, calculate the average
atomic mass of oxygen:
3. In a few sentences, describe the basic structure of an atom, as Rutherford would envision it.
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Worksheet 4.09 – Concept Review NAME: ___________________
Answer the following questions or solve the following problems in the space provided.
1. Dalton, Thomson, and Rutherford had very different models of the atom. In the boxes below, explain what
each scientist would think of an atom of oxygen. Also, include a drawing of what each scientist’s model of
oxygen would look like.
Scientist Dalton Thomson Rutherford
piece of matter.
Identical to every
other atom of oxygen.
Different from any
other type of atom.
2. Fill in the blanks to make the following statements true.
______________ proposed the existence of the electron.
The number of protons and _____________ in a neutral atom are equal.
_____________ are subatomic particles with no charge.
The first modern theory of the atom was proposed by _____________.
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Worksheet 4.10 – Concept Review NAME: ___________________
1. Fill in the blanks to make the following statements true.
___________’s atomic theory states that all atoms are indivisible.
Atoms combine with one another in _____________________ ratios to form compounds.
_____________ used a cathode ray tube to discover the electron.
A neutral subatomic particle is called a ______________.
______________ used gold foil to discover the nuclear atom.
The ___________ number is the number of protons in an atom.
An atom of ____________ has 74 protons.
The atomic mass number is the number of __________ + ___________.
An atom of Beryllium-10 has ______ neutrons.
______________ are atoms that have the same number of protons but different numbers of neutrons.
Forensic chemists use a device called a ______________________ to identify small amounts of unknown
The atomic mass of an element is expressed in __________________________.
2. Given the relative abundance of the following naturally occurring isotopes of Galinskium, calculate the
atomic mass of Galinskium*. Show work.
*Somebody please go thank Mr. Galinski for ALL his wonderful contributions to the wide world of chemistry.