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TRANSCRIPT
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Early Ideas About
Matter
Chapter 4
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Greek Philosophers • Matter composed of four
elements:
- earth
- air
- water
- fire
• Matter endlessly divided into smaller pieces
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Democritus • Proposed idea that matter was
not infinitely divisible
• Tiny individual particles called atomos
• Cannot be created, destroyed, or divided
• Different kinds of atoms have different sizes and shapes
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Democritus
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Aristotle • Rejected Democritus’
ideas
• Did not believe empty space existed
• Denied the existence of atoms
• 4 elements (earth, fire, air, water)
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Democritus v. Aristotle
• Democritus - unable to support his ideas with experiments
–Could not answer what held atoms together
• Aristotle was more influential
–He rejected Democritus’ ideas
–His ideas went unchallenged for nearly 2,000 years
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John Dalton
• Revived concept of atom during late 18th century
• Work marks beginning of modern atomic theory
• Conducted experiments allowing him to refine and support his hypotheses
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Dalton’s Atomic Theory
• Proposed in 1803
• Matter is composed of extremely small particles called atoms
• Atoms are indivisible & indestructible
• Different types of atoms
–Each type has its own size, mass, and chemical properties
• Atoms are separated, combined, or rearranged during chemical reactions
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Dalton’s Elements
From Dalton’s book A New
System of Chemical
Philosophy
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Problems with Dalton’s Theory
• Atoms are indivisible & indestructible BUT (we know) atoms are divisible into subatomic particles
• Atoms of the same element can have slightly different masses
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Sir William Crookes • Cathode ray tubes led to an accidental discovery:
–Negative particles called electrons are found in all forms of matter
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J. J. Thompson
• Studied negatively charged particles of matter
• Determined the mass of the charged particle (electron) was less than the mass of a hydrogen atom
– Shocking discovery & hard to
accept that there were particles
smaller than atoms
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Robert Millikan
• Determined charge of an electron using an oil-drop apparatus
• Electron has a single unit of negative charge (-1)
• Calculated mass of electron (9.1 x 10-28 g)
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J. J. Thompson & Plum Pudding Model
• Atoms are spherical
• Composed of a uniformly distributed positive charge with electrons residing inside
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Ernest Rutherford - 1911
• Concluded that plum pudding model was incorrect
• Atoms consist mostly of empty space through which electrons move
• Positive charge & all mass contained in dense, center region known as nucleus
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Ernest Rutherford
• Electrons are held in atom by positive charge of nucleus
• Called Nuclear Atomic Model
• Problem – model did not account for ALL mass in an atom
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Thompson v. Rutherford
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Ernest Rutherford
• Redefined nucleus in 1920
• Contained protons with a positive charge
–Proton has equal BUT opposite charge of electron
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James Chadwick
• Showed nucleus contained another subatomic particle called neutron
• Neutron has
no charge
and mass
nearly equal
to proton
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Atoms through time…
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Modern Atomic Theory • All atoms composed of 3 parts:
–Neutrons
–Protons
–Electrons
• Small, dense nucleus with positive charge that contains 99.97% of atom’s mass
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Modern Atomic Theory
• Held together by positive/negative attraction
• Electrons are fast moving & travel through empty space
• Current model: Electron Cloud Model