Electrons in Atoms

1
Chapter 5

• Electrons in Atoms

2
5.1 Light and Quantized Energy

• Nuclear Atoms
• An atoms mass is concentrated in the nucleus,
  surrounded by fast moving electrons
• How do electrons occupy space?
• Why do elements have different properties and
  behaviors?
• In the early 1900s, scientists observed that
  certain elements emit visible light when heated
  in a flame due to electron arrangement

3
Wave Nature

• Electromagnetic Radiation
• Form of energy that exhibits wavelike behavior as
  it travels
• Ex visible light X-rays, microwaves

The EM Spectrum includes all forms of EM
Radiation Energy increases with increasing
frequency
4
Wavelength (?)

• Represented by lambda
• Shortest distance between points on a continuous
  wav
• from crest to crest or trough to trough

5
Frequency (?)

• Represented by nu
• The of waves that pass a given point per
  second.
• SI unit is one hertz (Hz)
• 652 hertz 652 waves/second or 652 s -1

A higher frequency is indicated by a shorter
wavelength A lower frequency has a longer
wavelength
6
Amplitude

• The waves height from the origin to a crest, or
  from the origin to a trough.
• All EM waves travel at the speed of light (3.00 x
  108 m/s) in a vacuum
• Denoted by c
• Formula c ??

7
Problems

• What is the wavelength of a microwave that has a
  frequency of 3.44 x 109 Hz?
• Identify Knowns
• ? 3.44 x 109 Hz (or s-1) ? ?
• C 3.00 x 108 m/s
• Solve using c ??, thus ? c/ ?
• ? 3.00 x 108 m/s
• 3.44 x 109 s-1
• ? 8.72 x 10-2 m

8
Particle Nature of Light

• Quantum concept
• A quantum is the min. amount of energy that can
  be gained or lost by an atom
• German physicist, Max Planck, studied light
  emissions from heated objects
• He proposed that quantum energy is related to
  frequency
• Equantum h? (h is plancks constant)
• Plancks constant is 6.626 x 10-34 J
• J stands for joules, the SI unit of energy

9
Photoelectric Effect

• Photoelectrons are emitted from a metals surface
  when a certain frequency shines on its surface.
• Ex Calculator
• A metal will not eject photoelectrons below a
  specific frequency of incident light
• Einstein proposed that EM radiation has both
  wavelike and particle-like nature
• A photon is a particle of EM radiation with no
  mass with a quantum of energy

10
Photons

• Einstein calculation that a photons energy will
  depend on its frequency
• E photon h?
• The energy of a photon must have a threshold
  value to cause the ejection of a photoelectron
• A dual wave-particle model of light is still
  required for the whole effect.

11
Photon Calculations

• Water drops in the air disperse the white light
  of the sun into a rainbow. What is the energy of
  a photon from the violet portion if it has a
  frequency of 7.23 x 1014 s-1
• You know the frequency and plancks constant.
  Solve for the energy of the photon.
• Formula E h?
• Ephoton(6.626 x 10-34 J s)(7.23 x 1014 s-1)
• Ephoton 4.79 x 10-19 J

12
Atomic Emission Spectra

• The set of frequencies of the electromagnetic
  waves emitted by atoms of the element
• Each elements atomic emission spectrum is unique
  and can be used to determine if that element is
  part of an unknown compound
• Ex strontium atoms emit a red color
• Related to the energy formula of a photon

13
5.2 Quantum Theory and the Atom Bohr Model

• Neils Bohr proposed a quantum model for the
  Hydrogen atom.
• The H atom has a certain allowable energy state
  called ground state.
• Atom gains energy excited state
• The electron moves around the nucleus in circular
  orbits
• The smaller the orbit, the lower the energy

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Hydrogens line spectrum

• Hydrogen is the ground state when the electron is
  in the n1 orbit
• If an energy enters from an outside source, the
  electron moves into a higher level (n2)
• Change in energy results from the difference of
  the higher energy orbit and the lower energy
  orbit.

15
Line Spectrum

• Figure 5-10
• The four electron transitions that account for
  visible lines are shown
• This is called the Balmer series
• The Lyman series are not visible because they are
  UV
• The Paschen series is infrared
• Although good for hydrogen, this model is not
  good for other elements

16
Quantum Mechanical Model of the Atom

• In the mid-1920s scientists had proven the Bohr
  atomic model to be incorrect
• In 1924, Louis De Broglie proposed that electrons
  acts as waves
• This idea accounted for the fixed energy levels
  of Bohrs model

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Electrons as Waves

• Broglie saw that only whole numbers of
  wavelengths are allowed in a circular orbit of
  fixed radius
• The electrons may only have certain possible
  wavelengths, frequencies, and energies
• de Broglie equation ? h/mv
• It predicts that all moving particles have wave
  characteristics

18
Heisenberg Uncertainty Principle

• It is fundamentally impossible to know precisely
  both the velocity and position of a particle at
  the same time.
• To make a measurement of an object, you disturb
  the object
• Can we determine the position of an electron by
  bumping it?
• The act of observing an electron produces an
  uncertainty in the position and motion of it

19
The Schrödinger wave equation

• He derived an equation that treated the Hydrogen
  atoms electron as a wave
• Proposed the quantum mechanical model of the atom
• It makes no attempt to describe the electrons
  path around the nucleus
• It helps to explain the 3-D region around the
  nucleus called the atomic orbital.

20
Hydrogens Atomic Orbitals

• Principle quantum numbers
• Principal energy levels
• Energy sublevels
• Table 5-2