Atoms!

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Atoms!

Lynn A. Melton

University of Texas at Dallas

Mini-CAST February 21, 2009

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Website

• http://www.chemchapterzero.com

• Lots of ideas there.

• It you use this material in the classroom, it may take you a month or more to work through the material..

• The “hands on” stuff will count as labs.

• No algebra!

http://www.chemchapterzero.com/

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Fundamental Concepts ofChemistry

• Atoms

• Bonding/Molecules/Reactions

• Structure/Properties– Activity of molecule derives from its structure

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Some Fundamental Skills

• Ability to work with models

• “Seeing without seeing”

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#1 Models

A model is a step onthe staircase of understanding

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#1 Models

Working scientists use manyof the steps. They use the simplestmodel that works, since the highersteps generally require morecomplex mathematics.

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#1 Models

Models are generally not completely“true”. They generally explains somethings well and other things poorly.

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#1 Models

Which step is best for us?

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#2 Seeing Without Seeing

• Most chemists see atoms moving when they talk about reactions. Maybe they have a “tv screen” in the front of their brain.

• It takes students a long time (sophomore year of college?) to acquire this skill.

• How can we intentionally start to build this skill in younger students?

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#3 Atoms

• A conceptual chemistry problem:• “A piece of normal (dirty) copper wire is held in a

gas flame. It becomes bright copper “pink”. When it is removed from the flame and allowed to cool, it becomes black.

• Question: Does the blackened copper wire weigh more, same, or less than when it was in the flame?”

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#3 Atoms

• A secondary school teacher was in the class, and came to me for help with this homework problem.

• She could tell me that the flame cleaned the surface of the dirty copper wire and that oxygen from the air reacted with the clean surface to produce copper oxide, which is black.

• She went back and forth as to whether the answer was “more”, “same”, or “less”. She was guessing.

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#3 Atoms

• I tried to help. Knowing that she once had taught Home Economics, I said, “Go to the grocery store and fill a basket with oranges. Now put a layer of avocados on top of the oranges. Does the basket weigh more, same or less when I add the avocados?

• “Oh, Dr. Melton, of course it weighs more”.• She could reasons well enough, but when she was

asked about atoms, she turned off her reasoning.• The atomic world was ARCANE.

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#3 Atoms

• The atomic world was ARCANE.– Known or understood by only a few: arcane

economic theories. – adj : requiring secret or mysterious knowledge;

"the arcane science of dowsing“– Definitions from dictionary.com

• In the arcane world, the normal rules do not work, and you might as well guess.

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#3 Atoms

• If a sassy ninth grader asked you “So why – other than you and the book say so – should I accept that the world is made of atoms? After all, I cannot see atoms.”

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#3 Atoms

• Your answer has three parts:– Define an atom carefully– Data #1:Atomic Force Microscopy (in #5)

(the world is granular)– Data #2: Mass Spectrometry (in #5)

(the particles have different weights)

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#3 Atoms

• Definition of an atom– Rip any piece of the world apart, but you may

use only the energies available to the ancients – horses, flames, and lightning. When you cannot rip the smaller pieces apart any longer (to produce only neutral particles) then those last (neutral) particles are ATOMS.

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#3 Atoms

• The weight of anything in the world is the same, regardless of how finely you divide it.

• Or, when you add up the weight of all the pieces, you get the weight of the original thing.

• The world is granular; it is– Sand rather than shampoo– Grapes rather than jello

• The world is tinkertoys – molecules are built from atoms

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#3 Atoms

• Words that may come up. (If they don’t ask, don’t bring them up; Keep to the simple model)– Electron, proton, neutron: subatomic particles, they will

be discussed as more complex MODELS– Element: a group of atoms all of which have the same

number of protons– Ion: a atom in which the number of electrons is not the

same as the number of protons– Isotopes: atoms that have the same number of protons

but different numbers of neutrons

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#3 Atoms

• What do we need to know about atoms?– What is your weight?– What can I build with you?

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#4 AtomsSeeing Without Seeing

• The garbage bag contains models of atoms, but you may not use your eyes to see them.

• Same routine as before, but B starts out as the “doer”. Put both hands in the bag. A starts out as the “recorder”.

• Halfway through switch with your other team (1 – 2, 3 – 4, etc.)

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AtomsWhat data do we have?

• Atomic Force Microscopy– A very sensitive probe is scanned across the

surface, and the force on the probe is measured– By using electronics to keep the force constant,

we can – line by line – generate a profile of the surface

– The best instruments can “feel” individual atoms.

– Conclusion: the world is granular.

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AtomsWhat data do we have?

• Atomic Force Microscopy (neat websites)• http://www.mee-inc.com/afm.html • http://www.rhk-tech.com/hall/NaCl-mica.html • http://stm2.nrl.navy.mil/how-afm/how-afm.html • http://www.omicron.de/index2.html?/results/

atomic_resolution_on_si_111_7x7_in_non_contact_mode_afm/~Omicron

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AtomsWhat AFM data do we have?

Silicon surface

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NaCl (salt) surface

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• Conclusion:

– The world “feels” granular.

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AtomsWhat MS data do we have?

• Mass Spectrometry separates atoms (actually ions) according to their differing masses.

• Different masses have different trajectories!

• Real mass spectrometers require a very good vacuum, and they are expensive.

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AtomsWhat MS data do we have?

• Mass Spectrometry separates atoms (actually ions) according to their differing masses.

• Neat websites!• http://www.chem.arizona.edu/massspec/

example_html/examples.html • http://www.cea.com/cai/simstheo/mspectra.htm • http://www.chemguide.co.uk/analysis/masspec/

elements.html

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#5 AtomsWhat MS data do we have?

The different elements have different masses.

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Mass Spectrum

What well known compound gives rise to this result?

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AtomsAFM and MS

• AFM – The AFM box allows students to mimic the measurements made with a real AFM. Maybe you can feel individual atoms?

• MS – The mass spectrometer allows students to mimic the measurements made with a real mass spectrometer. Do you want to see the trajectories of your atoms?

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Should I use this approach in my class?

• It (probably) will help students with the fundamental concepts of chemistry.

• Perhaps you are constrained by the sequencing of chemistry instruction?

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• Perhaps you are constrained by the sequencing of chemistry instruction?

• 8th grade ??? [pre-AP chemistry AP chemistry Freshman Chemistry degree in chemistry]

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• It (probably) will help students with the fundamental concepts of chemistry.

• Perhaps you are constrained by TEKS and TAKS?

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• Perhaps you are constrained by TEKS and TAKS?

• “Which letter in this model of a boron atom represents a neutron?” (TAKS grade 8 science April 2006)

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Atoms!