Download Atomic Spectroscopy and the Bohr Model

7.1-7.3 The Nature of Light – Atomic
Spectroscopy and the Bohr Model
The Nature of Light
• The quantum-mechanical model (a.k.a. the electron
cloud model) – An atomic model that explains the
strange behavior of electrons.
– Electrons determine the chemical and physical properties of
elements.
• Electromagnetic radiation – a type of energy embodied
in oscillating electric and magnetic fields.
• The speed of light (c): 2.998X108 m/s  on your
reference table.
The Nature of Light (Continued)
• Amplitude: the vertical height of the crest (or the depth of the
trough. In light a higher amplitude means more intense light.
• Wavelength (λ): the distance between two adjacent crests. Units
are meters (or nano- or…)
•
More energetic waves have higher amplitudes and shorter wavelengths.
• Frequency (ν): the number of waves (cycles/s), or s-1, or Hz.
c=λν
For Visible light: light that can be
seen by the human eye,
wavelength determines color. Red
has the longest wavelength and
violet has the shortest.
Let’s Try a Sample Problem
A laser dazzles the audience in a rock concert by
emitting green light with a wavelength of 515 nm.
Calculate the frequency of the green light.
c=λν
c 2.998X108 m/s
ν = --- = --------------------- = 5.82X1014 Hz
λ 5.15X10-7 m
The Electromagnetic Spectrum
• Other than visible light, what other types of
electromagnetic radiation are there?
– Low energy waves: radio waves, microwaves,
Infrared (IR) waves.
– High energy waves: Ultraviolet, x-rays, and gamma
rays
The Particle Nature of Light
• The photoelectric effect: the observation that many
metals emit electrons when light shines upon them.
– Only high frequency light, transfers energy into the electron,
excites and dislodges the electrons within the metal. The
energy of a photon has to at least be equal to the binding
energy of the electron to eject the electron.
– Albert Einstein explained that light energy comes in packets
(a photon or quantum) of light.
– E = hν
• E = amount of energy in a light packet
• h = Planks constant = 6.626X10-34 J s  in your reference table
hc
– E = ----λ
Let’s Try a Practice Problem
The energy required to dislodge electrons from sodium metal
via the photoelectric effect is 275 kJ/mol. What wavelength in
nm of light has sufficient energy per photon to dislodge an
electron from the surface of sodium?
hc
E = ----λ
hc
(6.626X10-34 J s)(2.998X108 m/s)
λ = ------ = ------------------------------------------ ----------------------- =
E
(2.75X105 J/mol) (1 mol/6.02X1023 electrons)
λ = 4.35X10-7 m = 435 nm
Atomic Spectroscopy and the Bohr
Model
• Atomic spectroscopy: the study of electromagnetic
radiation absorbed and emitted by atoms.
• When atoms absorb a specific amount of energy, electrons
become excited to a higher energy level, and then relax and
emit the energy in the form of light energy (photons).
• If we slow down this light using a prism or spectrometer,
we can see the constituent colors that make up the color
light that we are seeing. This series of lines is called the
emission spectrum. This bright line spectrum is used to
identify elements. Example on the next page.
Bohr’s Model of The Atom
• In Bohr’s model (this is not the modern model of
the atom), electrons traveled around the nucleus
in circular orbits, and each orbit existed at a fixed
distance from the nucleus.
– Each orbit represented a different energy level, and
the electron only released the amount of energy
analogous to the difference between the two
stationary orbits.
7.1-7.3 pg. 330 #’s 38, 40 and 42.
Read 7.4-7.5 pgs. 309-320