Electronic structure

1
Electronic structure

• Electronic structure of atoms
• Where are the e-s located and how are they
  arranged

2
How can we probe an atom to find out?

• light

3
Light

• Electromagnetic radiation
• Is the energy in the form of light which travels
  through space as waves at a velocity of 3.0 x
  108 m/s (the speed of light)
• A small part of the Electromagnet Spectrum

4
light

• Considered an Electromagnetic Wave
• Waves can build each other up or cancel each
  other Interference
• Waves can bend around edges Diffraction

5
Waves

• All forms of electromagnetic radiation exhibits
  wave properties
• Velocity, wavelength, amplitude, and frequency
• Velocity (c) 2.998 x 108 m/s in vacuum (speed
  of light)
• Wavelength (?) lambda - distance from start of
  one wave to start of next wave (nm)
• Shorter the wavelength, the stronger the energy
• Period (T) Time for one wave to go by (seconds)
• Frequency (f or v) nu - Number of waves passing
  by in one second (hz) 1/T
• Also 1 / ?

6
Wavelengths and Visible light

• Red low energy, longest wavelength
• Orange
• Yellow
• Green
• Blue
• Indigo
• Violet high energy, shortest wavelength

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waves

• Speed of wave (c) ?
• c ? f
• C speed of light
• (c) 2.998 x 108 m/s in vacuum
• If ? 550 nm then f ?
• If f 7.52 x 1014 Hz then ? ?

8
Light

• Considered as particles
• Photons or Quantas
• Intensity or Power of light depends on the height
  of the wave or number of photons
• Energy of light though depends on the
  frequency/wavelength
• Red light (750 nm) does not expose film
• Violet light (450 nm) does expose film and causes
  sunburns

9
Energy of light

• Energy hf or (hv)
• h Planks constant 6.62 x 10-37 J/Hz
• V frequency
• E energy of a quantum of light (photon)
• Since c ? v
• Then we can say that v c/ ?
• Therefore, E hc/?
• H Planks constant
• C speed of light

10
Energy and electromagnetic radiation

• r E hv hc / ?
• r E energy change
• h Plancks constant, 6.63 x 10-34 joule sec
• v frequency of radiation
• ? wavelength of radiation
• c the speed of light, 3.00 x 10 8
  meters/sec (c ? f)

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Practice

• What is the frequency of red light that has a
  wavelength of 700.0 nm? What is the energy of a
  photon of this light?
• What is the frequency of violet light that has a
  wavelength of 410.0 nm? What is the enrgy of a
  photon of this light?

12
Electromagnetic radiation

• When atoms absorb energy in the form of
  electromagnetic radiation, electrons jump to
  higher energy levels
• When electrons drop from higher to lower energy
  levels, atoms give off energy in the form of
  electromagnetic radiation (light)
• The energy level changes for the electrons of a
  particular atom are always the same, so atoms can
  be identified by their emission and absorption
  spectra

13
Atomic Structure

• Photon given off when electron moves from higher
  level to lower level.
• Energy emitted Ehi - Elo hf hc/?

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Atomic Structure

• In sodium the yellow line is ? 589.0 nm
• calculate f c/ ?
• calculate ?E hf
• calculate ?E for one mole of electrons
• The difference in two sodium energy levels is
  then 203.1 kJ/mol

15
Review

• Positively charged nucleus is always pulling at
  the negatively charged electrons around it
• Electrons have potential energy that increases
  with their distance from the nucleus
• like gravitational potential energy of a brick on
  the third floor of a building
• This is greater than if the brick was on the
  first floor

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Energy of an electron is quantized

• Remember from electron configuration energy
  levels are fixed
• Electrons can only exist at specific energy
  levels, separated by specific intervals the rows

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Hydrogen Atom
n5
n4
n3
n2
n1

• Ryberg discovered that the energy of each
  emission is related to a whole number multiple
  n (row number)
• ?E -R (1/n hi2 - 1/n lo2) , R 2.180 x 10-18 J

18
Quantized energy of an electron in a hydrogen atom

• Can be found if you know its principal quantum
  number or shell (row number)
• Energy of an electron
• En -2.178 x 10-18 / n2 joules
• En the energy of the electron
• N the principal quantum number of the electron