The teacher may chose the complete this unit with an in-class test and a 3 page typed paper where each student is expected to provide a written interpretation of matter and to express how their concept of matter has changed over the course of the unit. The class as a whole must develop a final definition for matter.@$:
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Work Sheet 1 – Looking Inside the Atom
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Student #1
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__________________
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Student #2
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__________________
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Dalton’s atomic theory explained in general terms that matter was composed of tiny particles and that each element (e.g. Gold, silver, carbon) consists of uniquely identical particles. In a sense, Dalton’s theory showed only what atoms look like if viewed from the outside. Scientist wanted to understand what “stuff” gives atoms their substance and how this “stuff” was organized.
The Plum Pudding Model
Once scientists discovered electrons (the “stuff”) and associated them with atoms a model was developed to explain what atoms looked like on the inside. This model, called the plum pudding model, said that the “stuff” was evenly distributed throughout the atom to create a solid mass.
(FIGURE HERE)
Ernest Rutherford performed an experiment to test this model. In his experiment he took a thin sheet of gold foil and aimed tiny particles called alpha particles at it. Alpha particles are much like light in that they will easily pass through open space. Rutherford wanted to see if the gold sheet would stop the alpha particles from going through by reflecting the alpha particles like a mirror would reflect light.
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Question 1:
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Why did Rutherford use a thin sheet of foil rather than a square block?
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(HINT: it was not to save cost)
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What did Rutherford observe in his Experiment?
Rutherford found that instead of reflecting the alpha particles like a mirror would reflect light, almost all of the alpha particles went directly through the foil and only a very small amount was reflected. See the figure below.
(FIGURE HERE)
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Question 2:
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How does the reflective property of atoms differ from that of mirror?
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Question 3:
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The stuff is the only part of the atom that can reflect alpha particles. If this is the case, what does the observation from Rutherford’s experiment tell you about the arrangement of the “stuff” in the atom?
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Draw a model of an atom that fits the observation Rutherford made.
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Question 4:
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If you were Rutherford what conclusion would you make about the atom?
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Work Sheet 2 – Determining the number Particles in an Atom
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Name ____________
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Using the Periodic Table and the equations provided complete the following table
Equations
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1.
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The number of protons (p) is equal to the atomic number (z) of the element
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p = z
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2.
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The number of electrons (e) equals the atomic number minus the charge (c)
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e = z - c
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3.
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The number of neutrons (n) is equal to the mass number (m) minus the atomic number.
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n = m – z
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Symbol
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Atomic Number
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Mass Number
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p
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e
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n
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Co
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59
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48
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112
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14
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7
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7
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84
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48
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A1
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27
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60
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144
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Figure 1
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The Plum Pudding Model of the Atom
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Atoms were believed to have there mass evenly distributed with negatively charged ele-ctrons floating in a sea of positive charge. With such a model, Rutherford reasoned that most of the high speed alpha particles will be deflected so that very few actually pass th-rough the atom.
(figure available in print form)
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Figure 2
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Rutherford’s Nuclear Model
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When aim through a thin piece of gold foil, a majority of the particles passed freely through and there was very little deflection. Rutherford concluded that most of the atom is open space and that the deflection was due to the massive nucleus.
(figure available in print form)
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Notes
1 You can find the thoughts on matter by philosophers of antiquity at www.web-lines.com/philosophy/xeno.htm (5/16/99) or by simply searching the Internet for sites that offer this information.
2 The practical significance of quantum number-- n represents the average distance of the electrons from the nucleus, or the size of the shell. l represent the shape of the orbital within the subshell. ml represent the orientation of the orbital in space and ms represents the spin of the electron.
