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1 ATOMIC STRUCTURE Atom Smallest particle of an element that retains its identity in a chemical reaction • ELEMENTS are composed of only one type of atom • COMPOUNDS formed when two or more atoms are CHEMICALLY bonded to form a new substance

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Page 1: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

1

ATOMIC STRUCTURE

AtomSmallest particle of an element that

retains its identity in a chemical reaction

• ELEMENTS are composed of only one type of atom

• COMPOUNDS formed when two or more atoms are CHEMICALLYbonded to form a new substance

Page 2: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

2

AtomSmallest particle of an element that

retains its identity in a chemical reaction

• Atoms first suggested and named by DEMOCRITUS… believed that atoms were indivisible and indestructible

• His ideas were limited because they did not explain chemical behavior and lacked experimental support

• JOHN DALTON used experiments to explain Democritus’ ideas over 2,000 years later…

Dalton’s Atomic Theory1) ALL elements are made of tiny, indivisible atoms

2) Atoms of the SAME element are IDENTICAL… atoms of DIFFERENTelements are DIFFERENT

Page 3: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Dalton’s Atomic Theory3) Atoms can mix together or combine chemically in simple whole number ratios to form COMPOUNDS

4) Chemical reactions occur when atoms are separated, joined, or rearranged… but atoms are NEVERchanged into atoms of another element as a result

Law of Definite ProportionsA compound always has the same elements in exactly the same proportion by mass

• Proposed by Joseph Proust

• Known as the Law of Constant Composition

• Ex: H2O or H2O2

Page 4: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Law of Multiple ProportionsWhen two elements form a compound, the ratio of the masses can be reduced to

whole numbers (subscripts)

• Proposed by John Dalton

• Ex: NO (14:16 = 1:1), NO2 (14:32 = 1:2)

Subatomic Particles

• Dalton was proved mostly right except… ATOMS ARE DIVISIBLE!

• Three Subatomic Particles

-PROTONS: positive charge

-NEUTRONS: no charge

-ELECTRONS: negative charge

Page 5: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Electrons• Discovered by J.J. THOMSON in 1897

• Passed a current through gases at low pressure in a vacuum tube, producing a glowing beam or CATHODE RAY

Cathode Rays

• Originate at the NEGATIVE end of the electrode

• Act the same regardless of the material used

• Deflected by negative magnetic fields

Page 6: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Cathode Ray Experiment

Current

+-

Vacuum tube

Electrodes

Current

+-

Cathode Ray Experiment

Page 7: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Cathode Ray Experiment

Current

+-

The ray is a beam of ELECTRONStraveling from the cathode to the anode!

Cathode Ray Experiment

Current

+

-

When the NEGATIVE end of a magnet is applied, the beam is repelled… ELECTRONS

MUST BE NEGATIVELY CHARGED!!

Page 8: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Cathode Ray Experiment

Current

+

-

No matter what gas or metals were used, the charge-to-mass ratio remained the same…

ELECTRONS ARE PART OF ALL ATOMS!!

Cathode Ray Experiment

Page 9: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Oil Drop Experiment

• ROBERT MILIKAN calculated the charge and mass of the electron

• Charge: one unit of negative charge (-1)

• Mass: 1/1840 the mass of a hydrogen atom (LIGHTEST SUBATOMIC PARTICLE)

Protons and Neutrons

• EUGENE GOLDSTEIN discovered protons (1,840 times larger than an electron)

• JAMES CHADWICK confirmed the existence of neutrons (about the same size as a proton)

Page 10: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Gold Foil Experiment• ERNEST RUTHERFORD shot alpha particles (positively charged) at a thin sheet of gold foil

Lead block

Uranium

Gold Foil

Fluorescent Screen

Hypothesis: alpha particles will pass through the foil WITHOUT changing direction because negatively charged particles spread out in the atoms of the foil will not stop or deflect the alpha particles

Gold Foil Experiment

Page 11: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Gold Foil Experiment

THIS IS WHAT RUTHERFORD EXPECTED!!

Gold Foil Experiment

THIS IS WHAT RUTHERFORD EXPECTED!!

Page 12: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Gold Foil Experiment

Results: most went straight through, some curved, and some came straight back

Gold Foil Experiment

Conclusions: most of the atom is EMPTY SPACE with a dense, positively charged NUCLEUS (protons and neutrons)!!!

+

Page 13: ATOMIC STRUCTURE · did not explain chemical behavior and lacked experimental support •JOHN DALTON used experiments to explain Democritus’ideas over 2,000 years later… Dalton’s

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Gold Foil Experiment


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