AtomsAtomsAre They Real?Are They Real?

June 2006June 2006

A Scorpacuda PresentationA Scorpacuda Presentation

What are atoms?What are atoms?

Atoms are submicroscopic particles.Atoms are submicroscopic particles.

Atoms are the basic building blocks of a Atoms are the basic building blocks of a the objects around us.the objects around us.

Are the real?Are the real?

Seven reasons to believe atomic theory:Seven reasons to believe atomic theory:1.1. We can count the number of atoms in an object – We can count the number of atoms in an object –

no fractional atoms.no fractional atoms.

2.2. Compounds of elements have simple ratios.Compounds of elements have simple ratios.

3.3. Velocity and mass of gas particles can be inferred.Velocity and mass of gas particles can be inferred.

4.4. Brownian MotionBrownian Motion

5.5. ScintillationScintillation

6.6. X-Ray Diffraction shows crystal structures.X-Ray Diffraction shows crystal structures.

7.7. Atomic Force Microscope ImagesAtomic Force Microscope Images

Avogadro's NumberAvogadro's Number

Avogadro showed that the number of Avogadro showed that the number of molecules or atoms in one gram is equal molecules or atoms in one gram is equal to the product of a large number and the to the product of a large number and the molecular mass of the molecule (or atom).molecular mass of the molecule (or atom).

A

23A

1 gram = N AMU

N = 6.0221415 10

Counting AtomsCounting Atoms

First we need to know:First we need to know: The weight of the object.The weight of the object. The chemical composition of the object.The chemical composition of the object.

ThenThen1.1. Determine how many moles in the mass Determine how many moles in the mass

(divide weight by the amu)(divide weight by the amu)2.2. Multiply this result by the Avogadro number Multiply this result by the Avogadro number 3.3. Multiply this result by the number of atoms in Multiply this result by the number of atoms in

the molecule.the molecule.

ExampleExample

How many atoms are in 2 grams of How many atoms are in 2 grams of water?A water molecule consists of water?A water molecule consists of one atom of oxygen (atomic weight 16) one atom of oxygen (atomic weight 16) and two atoms of hydrogen (each with and two atoms of hydrogen (each with an atomic weight of 1). an atomic weight of 1).

The atomic weight of a molecule of The atomic weight of a molecule of water is 18. water is 18.

Thus, one mole of water is that quantity Thus, one mole of water is that quantity of water which weighs 18 grams. of water which weighs 18 grams.

Roughly the same…Roughly the same…

Without determining the actual weight of a Without determining the actual weight of a substance, a good rule of thumb to use is substance, a good rule of thumb to use is that a cubic centimeter of solid matter that a cubic centimeter of solid matter contains about 10contains about 102424 atoms. atoms.

1 cubic centimeter

More or less - 10More or less - 102424 Atoms Atoms

Simple RatiosSimple Ratios

Dalton’s atomic theory: matter is composed Dalton’s atomic theory: matter is composed of atoms of several varieties – the elements of atoms of several varieties – the elements of the Periodic Table.of the Periodic Table.

Observed simple ratios of elements in Observed simple ratios of elements in chemical compounds:chemical compounds:

HH22O – two Hydrogen atoms to one Oxygen atom (water)O – two Hydrogen atoms to one Oxygen atom (water) NaCl – one Sodium atom to one Chlorine atom (table salt)NaCl – one Sodium atom to one Chlorine atom (table salt) HH22SOSO44

etc.etc.

Ideal Gas LawIdeal Gas Law

If atoms exist, then they must have mass If atoms exist, then they must have mass and velocity and kinetic energy.and velocity and kinetic energy.

Bernoulli applied Newton’s laws of motion Bernoulli applied Newton’s laws of motion to explain the behavior of gases.to explain the behavior of gases.

Results of several scientists:Results of several scientists:P V = n R TP V = n R T

Gas ParticlesGas Particles

Double the number of gas particles, and Double the number of gas particles, and the number of collisions between the the number of collisions between the particles and walls doubles – pressure particles and walls doubles – pressure doubles.doubles.

TRY IT!TRY IT! Take some balloons and make a balloon Take some balloons and make a balloon

rocket with string, straws.rocket with string, straws. Fill balloons with different amounts of air and Fill balloons with different amounts of air and

release – record how far the balloons travel.release – record how far the balloons travel.

Gas ParticlesGas Particles

Increase the temperature of gas, the Increase the temperature of gas, the average velocity of gas particles average velocity of gas particles increases.increases.

TRY IT!TRY IT! Stretch a balloon over a glass jar or Stretch a balloon over a glass jar or

graduated cylinder.graduated cylinder. Place the jar in heated water – the balloon Place the jar in heated water – the balloon

expands.expands.

Brownian MotionBrownian MotionIn 1905, Einstein showed mathematically that Brownian In 1905, Einstein showed mathematically that Brownian Motion could be modeled as resulting from a force of Motion could be modeled as resulting from a force of random collisions of atoms (or molecules).random collisions of atoms (or molecules).

Simulation:Simulation: http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=41http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=41

Measurements:Measurements: http://www.deas.harvard.edu/weitzlab/research/brownian.htmlhttp://www.deas.harvard.edu/weitzlab/research/brownian.html

TRY IT!TRY IT! DigitalBlue Microscope has a movie mode. Gather pollen or DigitalBlue Microscope has a movie mode. Gather pollen or

carbon dust from flame on spoon and put particles in rubbing carbon dust from flame on spoon and put particles in rubbing alcohol or distilled water. Observe the motion of the particles at alcohol or distilled water. Observe the motion of the particles at highest resolution. (Don’t bump the microscope or table.) Make highest resolution. (Don’t bump the microscope or table.) Make some movies of their motion.some movies of their motion.

ScintillationScintillationScintillation is a flash of light produced in a transparent material by Scintillation is a flash of light produced in a transparent material by an ionization event.an ionization event.

TRY IT!TRY IT! Crunching a Wintergreen LifeSaver® produces scintillation through the Crunching a Wintergreen LifeSaver® produces scintillation through the

process of process of triboluminescencetriboluminescence, making light with friction., making light with friction.

The Exploratorium has a tasty science snack:The Exploratorium has a tasty science snack:http://www.exploratorium.edu/cooking/candy/activity-lightning.html http://www.exploratorium.edu/cooking/candy/activity-lightning.html

The Exploratorium’s explanation:The Exploratorium’s explanation:““When you crush sugar crystals, the stress in the crystal creates When you crush sugar crystals, the stress in the crystal creates

electric fields. Like the electric fields in a lightning storm, these electric fields. Like the electric fields in a lightning storm, these electric fields can rip the outer electrons from molecules. When the electric fields can rip the outer electrons from molecules. When the molecules recombine with their electrons, they emit light. In addition molecules recombine with their electrons, they emit light. In addition to blue and violet light, the crunched crystals produce ultraviolet to blue and violet light, the crunched crystals produce ultraviolet light, which you can’t see. Wintergreen oil converts ultraviolet light light, which you can’t see. Wintergreen oil converts ultraviolet light into visible blue light, making the light you can see brighter.”into visible blue light, making the light you can see brighter.”

X-Ray CrystallographyX-Ray CrystallographySince the size of atoms is similar to the Since the size of atoms is similar to the size of X-Ray wavelengths, the sizes and size of X-Ray wavelengths, the sizes and regular arrangements of atoms in crystals regular arrangements of atoms in crystals can be determined by X-Ray diffraction.can be determined by X-Ray diffraction.

http://www.uni-wuerzburg.de/mineralogie/crystal/teaching/ibasic_b.html

Atomic Force MicroscopyAtomic Force Microscopy

AFM Measurements from the Internet Are AFM Measurements from the Internet Are Consistent With Atomic TheoryConsistent With Atomic Theory

Atomic Theory SummaryAtomic Theory Summary

Seven reasons to believe atomic theory:Seven reasons to believe atomic theory:1.1. We can count the number of atoms in an object – We can count the number of atoms in an object –

no fractional atoms.no fractional atoms.

2.2. Compounds of elements have simple ratios.Compounds of elements have simple ratios.

3.3. Velocity and mass of gas particles can be inferred.Velocity and mass of gas particles can be inferred.

4.4. Brownian MotionBrownian Motion

5.5. ScintillationScintillation

6.6. X-Ray Diffraction shows crystal structures.X-Ray Diffraction shows crystal structures.

7.7. Atomic Force Microscope ImagesAtomic Force Microscope Images