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The Evolution of Early Atomic Models

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The Evolution of Early Atomic Models The Energy Levels of Hydrogen Quantum- the amount of energy required to move an electron from one level to another The Bohr Model ... – PowerPoint PPT presentation

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Title: The Evolution of Early Atomic Models


1
The Evolution of Early Atomic Models
2
Early Models of Atomic Structure
  • The work of Thomson, Rutherford and Bohr

3
Thomsons Model
  • Discovered the electron while studying cathode
    ray tubes in 1897.
  • He received the Nobel Prize in 1906

4
Thomsons cathode ray tube (a vacuum tube with 2
electrodes)
5
His discovery of the electron
  • Thomson found that the cathode ray was a beam of
    negative particles (electrons) and so atoms were
    not indivisible.

6
Thomsons Plum Pudding Model of the Atom
  • He described his atomic model as negative charged
    electrons scattered in a lump of positively
    charged material, like raisins scattered in plum
    pudding ( a popular dessert at the time).

7
Rutherfords Model of the Atom
  • Rutherfords Gold Foil experiment led to the
    development of his atomic model in 1911
  • He received the Nobel prize in 1908.

8
The Gold Foil Experiment
  • Rutherford fired a beam of positively charged
    particles (called alpha particles) at a sheet of
    gold foil a few particles thick.

9
The Gold Foil Experiment
  • Rutherford was expecting results in line with
    Thomsons model, with the stream of positive
    particles passing through the foil.

10
The Gold Foil Experiment
  • Instead, he observed that some of the alpha
    particles were repelled, while most went through
    the foil unchanged.

11
There was only one explanation
  • A dense, very positive charge was condensed into
    one place, called the nucleus
  • The rest of the atom had to be mostly empty space

12
Rutherfords Model of the Atom
13
Understanding the basics of Modern Atomic Theory
  • We need to understand wave properties, and light
    energy

14
Wave Properties
  • Wavelength ?, is the distance between two like
    points on a wave

15
Wave Properties
  • Frequency, v, describes the number of wave cycles
    per second.
  • The unit of frequency is cycles/second (s-1), or
    the Hertz (Hz)

16
Wave Properties
  • Amplitude is the maximum height of a wave,
    measured from the origin of the wave
  • A wave has zero amplitude at certain intervals
    along the wave,called nodes

17
The Electromagnetic Spectrum
  • Short wavelengthlong wavelength
  • High frequencylow frequency
  • High energylow energy

18
  • How are wavelength and frequency related?
  • How are frequency and energy related?
  • What type of electromagnetic radiation has the
    lowest frequency?

19
The Electromagnetic Spectrum
  • All electromagnetic radiation (including visible
    light) travels at the same speed.
  • The speed of light(c) 3.0 x 108m/s
  • 3.0 x 1010cm/s
  • 3.0 x 1017nm/s
  • What travels faster, x- rays or visible light?

20
Questions
  • Which has the highest frequency, red light or
    green light?
  • Which has the longest wavelength, x-rays or
    microwaves?
  • Which has the highest energy, yellow light or
    infrared?

21
Questions
  • What mathematical relationship can you draw about
    wavelength and frequency?

22
The wavelength and frequency of light are
inversely related.
  • C ?v
  • Speed of light wavelength x frequency
  • ( Since c 3.0 x 108 m/s, you will be asked to
    solve for wavelength or frequency.)
  • ? c/v v c/?

23
The Bohr Model
  • In 1912, Niels Bohr adapted Rutherfords model to
    Plancks quantum theory and so developed his
    theory of atomic structure

24
Atoms can give off light
  • Bohrs model explained the atomic emission
    spectrum of hydrogen. For this he received the
    Nobel Prize in 1922. His atomic model is based on
    these ideas.
  • The atomic emission spectrum of an element is
    emission of particular frequencies (colors) of
    light by energized atoms of that element
  • Each atoms emission spectrum is unique

25
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26
The emission spectrum of hydrogen
  • The most prominent spectral lines are violet,
    blue, blue-green, and red.
  • Which of the lines has the lowest frequency?
  • Which of the lines has the shortest wavelength?

27
Atomic Emmision Spectra
28
Atoms can give off lightFirework colorants
  • Red strontium, lithium
  • orange calcium
  • Gold iron
  • Green barium
  • Blue copper
  • Purple strontium copper
  • Silver magnesium

29
BohrsModel
  • In 1913, Bohr proposed his model of the atom. He
    determined that electrons can be located in
    certain discrete energy states, called energy
    levels

30
Bohr related his model to a ladder
  • As a person can stand on one rung of a ladder or
    the next, yet is is impossible for a person to
    stand between the rungs an electron can be found
    in one energy level or the next, but not between
    levels.
  • The only way for the electron to jump to the next
    level is for it to have a quantum leap, which is
    the leap from one energy level to another.

31
  • The energy of the electron has a definite value
    in a stationary orbit. The electron can jump from
    one stationary orbit to another.
  • It it jumps from an orbit of lower energy E1 to
    an orbit of higher energy E2 , it aborbs a
    photon.
  • If it jumps from an orbit of higher energy E2 to
    an orbit of lower energy E1 , it emits a photon.

32
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33
Which jump would produce the highest energy light?
34
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35
The Energy Levels of Hydrogen
  • Quantum- the amount of energy required to move an
    electron from one level to another

36
The Bohr Model of the Atom
  • Quantized energy levels
  • Electron moves in a circular orbit
  • Electron jumps between levels by absorbing or
    emitting photon of a particular wavelength

37
Quantized Energy Levels
  • Since only certain energy changes occur for each
    type of element an atom must contain discrete
    energy levels.

38
Bohrs atomic model was ultimately not successful
  • Bohrs model considered the electron as a
    particle, and classical physics shows that a
    charged particle accelerating around a circular
    path would lose energy, and so the electrons
    would fall into the nucleus.
  • The modern model of the atom considers the
    electron, not as a particle, but as a matter-wave.
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