Name: Teacher Answer Key
Atomic Theory
Task: Look up the following words in the textbook and complete each portion of the vocab map.
Definition:
Negatively charged subatomic particle
Picture: found outside the nucleus
Electrons
Varies
Sentence: Varies
Definition: Picture:
Positively charged subatomic particle found inside the nucleus
Sentence: Varies
Definition:
The central region of the atom, containing the protons and neutrons, and making up most of the atoms mass
Proton
Nucleus
Varies
Picture:
Sentence: Varies
Definition:
A subatomic particle with neutral charge found inside the nucleus
Sentence: Varies
Neutron
Varies
Picture:
Varies

Name: Teacher Answer Key
Atomic Theory
(Textbook Resource: Holt Science Spectrum)
Task: Complete the following experiment and answer the questions that follow.
Materials: Balance, pennies, marbles, 2 cups (labeled “1” and “2”).
Each cup contains pennies & marbles in a predetermined amount. The pennies & marbles from each cup should be weighed separately, do not combine the pennies & marbles from Cup 1 with the pennies & marbles from Cup 2.
Step 1: Use a balance to find the total mass of the pennies in Cup 1. Mass of pennies (Cup 1) = _varies_
Use a balance to find the total mass of the marbles in Cup 1. Mass of marbles (Cup 1) =__varies_
Step 2: Use a balance to find the total mass of the pennies in Cup 2. Mass of pennies (Cup 2) = _varies__
Use a balance to find the total mass of the marbles in Cup 2. Mass of marbles (Cup 2) =__varies_
1.
Compare the composition of the “compounds” in the 2 cups in terms of the proportions of marbles and pennies by mass. Do the cups contain the same “compound”? Explain. _Because_ the ratio of pennies & marbles is different in each cup, they are not the same compound.
2.
Based on your evidence, do you think that different substances could be made up of the same elements, just different amounts? Explain. Yes. We have a limited number of elements, but those elements are arranged differently (different proportions bond with different elements) to give us all mater that we can observe. Two different items could be made up of hydrogen and oxygen simply by having different hydrogen-oxygen ratios.
(Textbook Resource: Holt Science Spectrum - pgs 113-129)
Task: Research to find the following information.
When did Democritus develop his atomic theory? __400 BC____
What did Democritus suggest? He said that all matter is made up of indivisible units called atoms
What does the word indivisible mean? _It cannot be divided.___________________
Did everyone support Democritus’s theory? __No_ Why or why not? He did not have evidence
When did Dalton develop his atomic theory? __1808________
What did Dalton’s Atomic Theory say? All matter consists of indivisible particles called atoms. Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements. Atoms of same/different elements can combine with each other to form compound atoms. Atoms are the smallest unit of matter that can take part in a chemical reaction.
What law is used as evidence to support Dalton’s theory? _Law of definite proportions_____
When did Thomson develop his atomic theory? __1897_______
What instrument did Thomson use to develop his theory? ___cathode ray tube___
How was Thomson’s model different from Dalton’s model of the atom? _It contained electrons and said that the atom could be divided_
What was Thomson’s model called? Plum pudding model
What did Rutherford say about the structure of the atom? It contained a massive central region called the nucleus.
What was Rutherford’s experiment called? __Gold foil experiment___________________

Name: Teacher Answer Key
Atomic Theory
As a result of his experiment, what did Rutherford discover? __The nucleus of the atom_____
When did Bohr develop his theory? _1913______
What does Bohr’s theory say? _Electrons can be found in specific energy levels orbiting the nucleus __
Task: Using the information from your research, complete the timeline to show the development of the atomic theory.
The timeline should include

The following scientist: Democritus, John Dalton, JJ Thomson, Ernest Rutherford, and Niels Bohr

The date,

A description of their major contributions,

An illustration of the model of the atom at that specific date.
400 BC
Democritus said that all matter is made up of tiny indivisible units called “atomos”
1897 J.J. Thomson Using the cathode ray tube discovered that atoms contained negatively charged particles called electrons – developed his plum pudding model
1913 Niels
Bohr said that electrons can be found in specific energy levels orbiting the nucleus
1808 John Dalton studied the composition of gases to determine that: All matter consists of indivisible particles called atoms. Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements. Atoms of same/different elements can combine with each other to form compound atoms. Atoms are the smallest unit of matter that can take part in a chemical reaction.
1911 Ernest Rutherford conducted his gold foil experiment to determine that the atoms mass is contained a dense center called the nucleus.
Summary: Describe how scientific knowledge is open to change as new evidence is observed. As we see with the atomic theory; as technology advances, new evidence is observed and if the evidence does not match a previous theory, that theory is modified in order to take into account the new evidence. We don’t continue to think of the atom in the same way that Democritus and Dalton thought of the atom because we now know that subatomic particles exist.

Name: Teacher Answer Key
Atomic Theory
(Textbook Resource: Holt Science Spectrum)
Task: Read the following paragraphs, and complete the exercises below.
Seeing Atoms: The STM
The idea that everything is made up of small particles called atoms was first proposed by Greeks in the fifth century BCE. At that time, there was no experimental evidence to support this theory, and it did not gain much acceptance.
Much later, in the early 1800s, British scientist John Dalton made a convincing argument, based on experimental evidence that supported the existence of atoms. Atoms are much too small to see, but
Dalton found evidence for them in things he could observe, such as reactions between compounds.
Because of the work of Dalton and others, the atomic theory soon became widely accepted by scientists.
How the STM Sees Atoms
Until recently, all evidence for the atomic theory was indirect. However, an exciting development in
1981 made it possible to see atoms for the first time. This new technology, the scanning tunneling microscope (STM), uses an electric current to probe the surface of a material. The STM measures how electrons are distributed on the material’s surface. The results are used to create a computer generated image of the atoms on the surface.
The first image created by an STM showed a layer of gold atoms. Since then, the atoms of many different substances have been observed with STMs, including silver, nickel, platinum, and silicon. At this time, STMs are used primarily for imaging atoms. STMs have also been used to move single atoms from one location to another, and other potential applications are being researched.
EXERCISES – Discuss the answers to the following questions then, on your own, write your response.
1. What is the most important difference between the Greek theory of atoms and Dalton’s atomic theory?
Dalton was able to provide evidence.
2.What is the significance of the invention of the STM?
It will allow us to make even more observations of the atom and potentially provide new evidence for the continued evolution of our knowledge of the atom
Task: Use the items provided to build Dalton’s model of the atom, J.J. Thomson’s model of the atom,
Rutherford’s model of the atom, and Bohr’s model of the atom then answer the following questions.
Materials: Modeling clay (several colors)
Questions
1.
Compare Thomson’s atomic model with Rutherford’s atomic model. Explain how they are the same and how they are different. Thomson’s model of the atom only included electrons and they were scattered all throughout the atom. Rutherford’s model included the nucleus and showed that the electrons were found surrounding that nucleus.
2.
Does the term indivisible still describe the atom? Explain. No. Because we know that it can be divided into subatomic particles (nucleus, electron clouds, protons, neutrons, electrons, etc.)

Name: Teacher Answer Key
Atomic Theory
Task: Examine the diagram and think about the discoveries that each scientist made to develop the atomic theory to answer the following questions.
1. What is the center of the atom called? _The nucleus_
2. The center of the atom is made up of positively charged particles and particles that have no electrical charge. Name the two particles.
Particles with a positive charge = protons____
Particles with no charge = _neutrons_______________
3. Negatively charged electrons are found in clouds outside the center of an atom. Why are electrons attracted to the center of the atom?
The negative charge of the electrons is attracted to the positive charge of the protons found in the nucleus.
Use the items provided to build a model of a neutral Carbon atom. Draw your model (be sure to label what each color represents) & explain it when you’re done.
Materials: 1 bowl, 30 balls (10 red, 10 black, 10 blue), colored pencils (red, blue, black), periodic table
Draw
Red represents ___protons or neutrons__________.
Yellow represents __protons or neutrons.
Black represents ____electrons
How many protons did you use? _6__
How many neutrons did you use? ___6___
How many electrons did you use? __6____
What does the bowl represent? The nucleus_____
Explain where you placed your subatomic particles and why you placed them in that location. Protons and neutrons were placed inside the bowl since the bowl represented the nucleus. Electrons were placed around the outside of the bowl since they are found in electron clouds outside the nucleus. _
How did you select the amount of protons, neutrons, and electrons that you chose to use?
Using the periodic table provided, the number of protons was based on the atomic number, the number of neutrons was found by taking the atomic mass minus the atomic number and the number of electrons equals the number of protons since the atom is neutral.

Name: Teacher Answer Key
Atomic Theory
(Online Resource: phet.colorado.edu)
Task: Complete the lab simulator to answer the following questions.
1. Google Phet Build an Atom
2. Click on the first link.
3. Click on the RUN NOW button
4. Open the boxes called Symbol, Mass Number, and Net Charge.
5. Experiment by putting some protons into the nucleus of the atom (on the X).
Fill in the table to the right to keep track of what you are learning about protons.
When you finish, put the protons back into the bowl.
Mass number?
Changes
Charge? positive
Stays in the center
(on the
X)?
Yes or No
6. Experiment by putting some neutrons into the nucleus of the atom (on the X).
Fill in the table to the right to keep track of what you are learning about neutrons.
When you finish, put the neutrons
Mass number?
Changes
Charge?
No charge
Stays on the X?
Yes or No back into the bowl.
Symbol changes on the periodic table?
Yes or No
Symbol changes on the periodic table?
Yes or No
7. Experiment by putting some electrons into the nucleus of the atom (on the X).
Fill in the table to the right to keep
Mass Charge? Stays on Symbol changes on track of what you are learning about electrons. number? the X? the periodic table? negative
When you finish, put the electrons
Stays Yes or No Yes or No back into the bowl. the same
8. Look over your data tables for protons,
neutrons and electrons.
Two things we noticed are: 1. ____varies________________________
2. __varies_________________________________
9. Put 3 protons into nucleus of the atom. Fill in the following:
Name of atom:__Lithium____ atom or ion? ___ion___ net charge? ___+3______
10. Decide how you will build a neutral atom that is stable
.
Practice making atoms using your ideas.
List the steps that youwould take to build a neutral atom starting with protons:
1.
First I choose _varies_ protons and put them in the center (nucleus) of the atom.
2.
Varies
3.
Varies

Name: Teacher Answer Key
Atomic Theory
Extension: Play the game!