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DemocritusDemocritus Around 410 BC, Around 410 BC,

originated the idea of originated the idea of the atom.the atom.

400 B.C. - 400 B.C. - Everything in Everything in the world is made the world is made from tiny particles that from tiny particles that cannot be cut.cannot be cut.

atoms are indestructible.atoms are indestructible.

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John DaltonJohn Dalton

1803, Proposed 1803, Proposed New Atomic New Atomic TheoryTheory

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Dalton’s Atomic TheoryDalton’s Atomic Theory 1.1. All matter consists of tiny particles.All matter consists of tiny particles.

2.2. Atoms are indestructible and Atoms are indestructible and unchangeable.unchangeable.

Atoms of an element cannot be created, destroyed, broken Atoms of an element cannot be created, destroyed, broken into smaller parts or transformed into atoms of another into smaller parts or transformed into atoms of another element.element.

3.3. Elements are characterized by the mass Elements are characterized by the mass of their atoms.of their atoms.

All atoms of the same element have identical weights, Dalton All atoms of the same element have identical weights, Dalton asserted. Atoms of different elements have different weights. asserted. Atoms of different elements have different weights.

With the discovery of isotopes, however, the postulate was With the discovery of isotopes, however, the postulate was amended to read, "Elements are characterized by their atomic amended to read, "Elements are characterized by their atomic number".number".

4.4. When elements react, their atoms When elements react, their atoms combine in simple, combine in simple, whole-number ratios.whole-number ratios.

effectively explained both the law of definite proportions and effectively explained both the law of definite proportions and the law of multiple proportions. the law of multiple proportions.

John Dalton (1800)John Dalton (1800)

Link

Law of definite Proportions: chemical compound contains fixed and constant proportions (by weight) of its constituent elements.

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John Joseph ThomsonJohn Joseph Thomson

In 1897, discovered the In 1897, discovered the electronelectron

Experiment: high-Experiment: high-vacuum cathode-ray vacuum cathode-ray tubetube

1904, Calculated Charge 1904, Calculated Charge to Mass Ratioto Mass Ratio

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Cathode Ray-TubeCathode Ray-Tube

C: Cathode (Rays introduced)C: Cathode (Rays introduced) A:A: AnodeAnode

Deflected: When passed through electrical Deflected: When passed through electrical potentialpotential

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Plum Pudding Model

Ernest Rutherford Ernest Rutherford

1. Most particles passed through • atoms mostly empty!

2. Some positive -particles deflected or bounced back!• Thus, a nucleus holds most of an atom’s

mass.

Rutherford shot alpha (Rutherford shot alpha () particles at gold foil.) particles at gold foil. FOUND THE FOUND THE NUCLEUSNUCLEUS!!

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Rutherford’s Results:Rutherford’s Results: Observations:Observations:

99% went through gold foil99% went through gold foil 1% were deflected straight back1% were deflected straight back

Rutherford: “It was . . . As if you had fired Rutherford: “It was . . . As if you had fired a 15-inch shell at a piece of tissue paper a 15-inch shell at a piece of tissue paper and it came back and hit you”and it came back and hit you”

Atoms made of:Atoms made of: Dense, Positive NucleusDense, Positive Nucleus A lot of SpaceA lot of Space

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James ChadwickJames Chadwick Born in England: Born in England:

(1891 – 1974)(1891 – 1974)

1932, discovered 1932, discovered the neutronthe neutron

Many Honors:Many Honors: 1935: Nobel Prize 1935: Nobel Prize

for Physics.for Physics. 1945: Knighted1945: Knighted