1. Section 2: Defining the Atom
An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus. K
What I Know W
What I Want to Find Out L
What I Learned

2. 6(A) Understand the experimental design and conclusions used in the development of modern atomic theory, including Dalton’s Postulates, Thomson’s discovery of electron properties, Rutherford’s nuclear atom, and Bohr’s nuclear atom.
2(H) Organize, analyze, evaluate, make inferences, and predict trends from data.
3(F) Research and describe the history of chemistry and contributions of scientists.
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Defining the Atom

3. Essential Questions What is an atom?
How can the subatomic particles be distinguished in terms of relative charge and mass?
Where are the locations of the subatomic particles within the structure of the atom?
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Defining the Atom

4. Vocabulary Review New model atom cathode ray electron nucleus proton
neutron
Defining the Atom
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5. The Atom
The smallest particle of an element that retains the properties of the element is called an atom. An instrument called the scanning tunneling microscope (STM) allows individual atoms to be seen.
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Defining the Atom

6. The Electron
When an electric charge is applied, a ray of radiation travels from the cathode to the anode, called a cathode ray. Cathode rays are a stream of particles carrying a negative charge. The particles carrying a negative charge are known as electrons. This figure below shows a typical cathode ray tube.
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Defining the Atom

7. The Electron
J.J. Thomson measured the effects of both magnetic and electric fields on the cathode ray to determine the charge-to-mass ratio of a charged particle, then compared it to known values. The mass of the charged particle was much less than a hydrogen atom, then the lightest known atom. Thomson received the Nobel Prize in 1906 for identifying the first subatomic particle—the electron.
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Defining the Atom

8. The Electron
In the early 1910s, Robert Millikan used the oil-drop apparatus shown below to determine the charge of an electron.
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Defining the Atom

9. the mass of a hydrogen atom
The Electron
Charges change in discrete amounts—1.602 × 10–19 coulombs, the charge of one electron (now equated to a single unit, 1–). With the electron’s charge and charge-to-mass ratio known, Millikan calculated the mass of a single electron.
the mass of a hydrogen atom
Defining the Atom
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10. The Electron
Matter is neutral. You know that matter is neutral from everyday experiences. You do not receive an electric shock (except under certain conditions) when you touch an object. If electrons are negative, then how is matter, which is made up of electrons, neutral? J.J. Thomson proposed a model of the atom to answer this question. J.J. Thomson's plum pudding model of the atom states that the atom is a uniform, positively charged sphere containing electrons.
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Defining the Atom

11. The Nucleus
In 1911, Ernest Rutherford studied how positively charged alpha particles interacted with solid matter. By aiming the particles at a thin sheet of gold foil, Rutherford expected the paths of the alpha particles to be only slightly altered by a collision with an electron.
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Defining the Atom

12. The Nucleus
Although most of the alpha particles went through the gold foil, a few of them bounced back, some at large angles.
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Defining the Atom

13. The Nucleus Rutherford concluded that atoms are mostly empty space.
Almost all of the atom's positive charge and almost all of its mass is contained in a dense region in the center of the atom called the nucleus.
Electrons are held within the atom by their attraction to the positively charged nucleus.
The repulsive force between the positively charged nucleus and positive alpha particles caused the deflections.
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Defining the Atom

14. Completing the Model of the Atom
All atoms are made of three fundamental subatomic particles: the electron, the proton, and the neutron. Atoms are spherically shaped. Atoms are mostly empty space, and electrons travel around the nucleus held by an attraction to the positively charged nucleus.
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Defining the Atom

15. Structure of the Atom Anatomy of an Atom Concepts in Motion
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Defining the Atom

16. Completing the Model of the Atom
Scientists have determined that protons and neutrons are composed of subatomic particles called quarks. Scientists do not yet understand if or how quarks affect chemical behavior. Chemical behavior can be explained by considering only an atom's electrons.
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Defining the Atom

17. Review Essential Questions Vocabulary What is an atom?
How can the subatomic particles be distinguished in terms of relative charge and mass?
Where are the locations of the subatomic particles within the structure of the atom?
Vocabulary
atom
cathode ray
electron
nucleus
proton
neutron
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Defining the Atom