1. Introduction to Atoms 8th Grade Science
© Fall 2005, Pflugerville ISD, 8th Grade

2. Who helped Develop the Atomic Theory?
What Do You Think?
Imagine that you have cut a penny in half. Then, you take one piece and half it again. Will this continue forever, or will you come to a point where no more cutting is possible?
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3. *Who helped Develop the Atomic Theory?
List one contribution of each scientist.
Democritus (400 BC)
Dalton ( )
Thompson ( )
Rutherford ( )
Bohr ( )
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4. Who helped Develop the Atomic Theory?
Democritus was a Greek philosopher who theorized that all matter was made of invisible particles called atoms.
Democritus of Abdera,
about BCE
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5. Who helped Develop the Atomic Theory?
Dalton, a British chemist and teacher, theorized that all elements are made up of the same type of atom and that these atoms combine in specific proportions to form compounds.
John Dalton
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6. Who helped Develop the Atomic Theory?
Thomson received the Nobel Prize in physics in 1906 for his discovery of the electron.
Joseph John
“J.J.” Thomson
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7. *Who helped Develop the Atomic Theory?
Rutherford, a former student of Thomson’s from New Zealand, discovered that an atom contains mostly space and that it has a small positively charged nucleus.
Ernest Rutherford
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8. Who helped Develop the Atomic Theory?
He expected his alpha particles to go straight through the foil, and most of them did.
But some of the particles were deflected or bounced straight back!
This showed that a nucleus with a positive charge makes up the center of an atom.
Rutherford’s Gold Foil Experiment
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9. Who helped Develop the Atomic Theory?
Bohr described the motion of electrons around the nucleus.
Niels Bohr
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10. Who helped Develop the Atomic Theory?
Bohr said that electrons orbit the nucleus at specific energy levels, and can move from one level to another.
Bohr’s Atomic Model
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11. Who helped Develop the Atomic Theory?
James Chadwick was awarded the Nobel Prize in Physics in 1932 for his discovery of the neutron. His discovery helped explain the mass of the atom and was instrumental in the development of the atomic bomb.
James Chadwick (1932)
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12. Development of the Atomic Theory
The Modern Atomic Model states that there are regions inside an atom where electrons are likely to be found.
These regions are called electron clouds.
Electron Cloud Model
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13. The Atom
What Do You Think?
What is the smallest thing you have ever seen? How does it compare to the size of an atom?
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14. *What are the parts of the Atom?
The nucleus is the small, dense, positively charged center of the atom.
Protons are positively charged particles in the nucleus
Neutrons are particles in the nucleus that have no charge
Electrons are negatively charged particles which orbit the nucleus.
Nucleus
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15. The Atom
What Do You Think?
What are some differences you use to tell one of your classmates from another?
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16. *What does the information in the box tell me?2 He
4.008
Atomic Number = # of protons
Elemental Symbol
Atomic Mass = # of protons plus neutrons 16

17. Atomic Number- the Number of Protons
The Helium atom has two protons in its nucleus.
This means that it has the atomic number 2.
Helium Atom
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18. Atomic Number- the Number of Protons
The Lithium atom has three protons in its nucleus.
This means that it has the atomic number 3.
Lithium Atom
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19. Atomic Mass Number- Protons plus Neutrons
The Helium atom has two protons and two neutrons in its nucleus.
This means that it has the atomic mass number 4.
Helium Atom
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20. Atomic Mass Number- Protons plus Neutrons
The Lithium atom has three protons and four neutrons in its nucleus.
This means that it has the atomic mass number 7.
Lithium Atom
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21. *What are Isotopes?
Notice that these isotopes of hydrogen have the same number of protons, but different numbers of neutrons.
Isotopes of Hydrogen
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22. Atomic Mass Number- Isotopes
These isotopes are also known as Hydrogen-1, Hydrogen-2, and Hydrogen-3
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23. 3-2-1 Summarize!!!
Write 3 facts you wrote in your notes in complete sentences.
Write 2 facts given to you verbally that you did NOT write down in complete sentences.
Write 1 question that presented itself to you while taking notes.

24. Let’s Review!
- 1 -
In the history of science, what are some ways scientists have described the atom?
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25. What are the parts of an atom, and how is an atom structured?
Let’s Review!
2 -
What are the parts of an atom, and how is an atom structured?
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26. Let’s Review!
3 -
What is the atomic number of an atom? How is this different from the atomic mass number?
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27. Atomic Theory Web Sites to Visit
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