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History and Structure of the AtomFrom Democritus to...

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History of Atomic Theory

• Democritus (from about 440 BC) • coined the term atom which means uncuttable • He felt that if you kept cutting matter smaller and

smaller eventually you will no longer be able to cut any further.

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3

John Dalton (1766-1844)3

• Felt that an atom was indivisible • spherical in shape • Model: Sphere • Analogy: Billiard ball

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JJ Thomson (1856-1940)

• Discovered electrons • Felt that an atom was negatively charged

particles floating in a positive soup • Model: Charges floating around • Analogy: Raisin bun or Plum Pudding

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The Atom• Thompson proved in 1897 that cathode rays were

particles from inside the “a-tomos” or “un-cuttable” atom

• Nagaoka pictured a planetary model for the atom, with electrons around a central mass

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Analogy

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Hantaro Nagaoka (1865-1950)

• Developed the planetary model of atom • Model: Positive sphere with electrons orbiting in a

ring • Analogy: Saturn

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Ernest Rutherford (1871-1937)

• Discovered the nucleus, the proton and first split an atom

• Model: An atom was a small positively charged nucleus surrounded by electrons orbiting around it.

• Analogy: A beehive

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Rutherfords Famous Experiment

• Called the Gold Foil Experiment • Alpha particles were fired at thin gold foil • A detector encircled the foil and lit up when hit with

alpha particles. • If the plum pudding model were true it was expected

most particles would go straight through the foil with only slight deflection.

• However, in the experiment, some particles were deflected back at a sharp angle proving the existence of a small, dense, and positively charged nucleus.

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Text

Plum pudding expected results. Particles all go through.

Actual results. Some are deflected back due to a nucleus

Rutherford's results

Data table for 50 “shots”

# timesStraight Through

Slight Deflection

Bounce Back

IIII II

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Another view of the experiment9

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Niels Bohr

• Worked out details of atomic structure. Notably orbital layers. Solved problems related to Rutherfords model.

• Model: Electrons orbit in rings at different distances from the nucleus.

• Analogy: Planets orbiting the sun

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Erwin Schroedinger (1887-1961)

• Major discoveries in quantum mechanics. Nature of electrons in atoms.

• Model: Electrons exist in a probability distribution around the atom. Kind of like a cloud.

• Analogy: A spinning fan blade.

Radioactivity• Marie and Pierre Curie discovered that

some isotopes are unstable: they spontaneously decay and give off radiation

What are the effects of radiation?

Up and Atom!

You wouldn’t like me when I’m angry...

Check your understanding

P. 279 even Steven

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Quick Quiz: Historical Atom

1. What did Dalton say an atom was like? 2. What did JJ Thomson say an atom was like? 3. What did Rutherford say an atom was like? 4. What are the 3 parts of an atom?

1. A billiard ball, or a pool table ball 2. Raisin bun 3. A beehive 4. Proton, Neutron, Electron

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Parts of the Atom13

The Nucleus

Electrons

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Parts of the Atom14

The Nucleus

Electrons Mass=0 Charge=-1

Protons Mass=1 Charge=+1

Neutrons Mass=1 Charge=0

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Atoms on the Periodic Table

3 Be 7.0

Atomic # = Number of protons

neutrons + protons Atomic mass

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Try This16

Atomic # Element Symbol Atomic Mass

Be

Na

Fe

I

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To Determine Other Numbers 17

To find Do this

Proton # same as atomic number

Electron # same as atomic number for neutral atoms

Neutron # =atomic mass-atomic number

Mass =Prot # + Neut#

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Element Atomic # e- # Mass # Neut #

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P 15

16 8

18

Fill in the blanks

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Summary19

• The model of the atom has progressed over time from a simple ball to the mysterious quantum structure.

• An atom contains protons, neutrons and electrons.

• You can calculate the number of each using the atomic number and atomic mass

• Electrons are negative, protons are positive, and neutrons are neutral

Check your understanding

P. 283 Odd Todd

Isotopes• Elements have a characteristic number of

protons, – e.g. Hydrogen always has one proton

• They may have different numbers of neutrons

• Neutrons help to glue together the nucleus of mutually repulsive protons

Radiation

• Radiation from the nucleus comes in three flavors: – α (Alpha): massive, “slow”, low penetration. He2+ – β (Beta): fast, low mass, medium penetration. e- – γ (Gamma): high energy E-M radiation. Gamma ray

photon.

Check your understanding

P. 289 even Stephen

f(x)=100(0.5)^x

-0.5 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

20

40

60

80

100

x

y

% Remaining vs. Half-lives elapsed

• Activity level: • 1 bequerel = Decays per second • Ex: what is the activity level for 12000 decays per

minute?

• 12000decays/60s • =200 Bq

f(x)=100(0.5)^x

-0.5 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

20

40

60

80

100

x

y

% activity vs. Half-lives elapsed

Half Life λ

• When a radioactive isotope decays, the amount remaining can be described by a half life relation:

λt

AA ｜⎠⎞｜⎝

⎛=

2

1

0

• Ex 1: find the amount remaining of 55g of Ne-18 in 26s, if it has a half life of 13 s

λt

AA ｜⎠⎞｜⎝

⎛=

2

1

0

• Ex 1: find the amount remaining of 55g of Ne-18 in 26s, if it has a half life of 13 s

• How many half lives? • 26/13=2

• Amount remaining=55x1/2x1/2=12g

Check your understanding

P. 295 q's 4-9

Chapter Review

P. 295 q's 4-9 Choose 10 vocab p. 302 P. 304 q's 1-10, choose 5 of 11-18