Intro to Atoms and Atomic Theory
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Transcript of Intro to Atoms and Atomic Theory
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Chapter 5
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Democritus – thought atoms were indivisible & indestructible Lacked experimental support
4th century B.C.
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John Dalton – (1766-1844)Dalton’s Atomic Theory
1. All elements are composed of tiny, indivisible atoms
2. Atoms of the same element are identical. Atoms of 1 element are different from another element
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Dalton’s Atomic Theory (cont)3. Atoms of different
elements can either physically mix or chemically combine in simple, whole number ratios to form compounds
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Dalton’s Atomic Theory (cont)4. Chemical Reactions occur
when atoms are separated, joined or rearranged. Atoms of 1 element cannot change into another element by a chemical reaction.
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Dalton’s Theory is mostly accepted todayExcept that we now know atoms CAN be divided - into subatomic particles
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JJ Thompson (1856-1940)discovered electrons using a device called a cathode ray tube
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•He sealed gases in a tube fitted at both ends with electrodes (metal disks)
•Connected to a source of high voltage electricity
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A glowing beam formed between the 2 electrodes
He called it the cathode ray
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The cathode ray is attracted to metal plates that have a positive charge
It is repelled by negatively charged plates
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The ray must be composed of negatively charged particles moving at a high speed
He called these particles electrons
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JJ Thomson’s model of the atom is called the plum-pudding model
He thought the electrons were randomly placed throughout the atom, like the currants in a plum pudding
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Ernest Rutherford discovered the nucleusHe used a thin sheet of gold foil and bombarded it with alpha particles (helium nuclei)
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Most particles passed through the foil as he expected
However, some were deflected slightly and came through at an angle
Some particles bounced back at him
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He hypothesized that there must be a dense positive part of the atom, but most of the atom is empty space
Rutherford’s Gold Foil Experiment
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Bohr – proposed that electrons have a fixed energy and move in energy levels around the nucleus – which is why they don’t fall into the nucleus
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The energy levels are like the rungs of a ladder – electrons cannot be in between levels, and need a specific amount of energy to move from one to another
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Dalton – thought atoms were solid and indivisible
JJ Thomson – discovered the electron, & made the plum-pudding model
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Rutherford – discovered the nucleus
Bohr – proposed electrons in orbitals around nucleus