Warm-up: To be turned in
• Use your periodic table to determine which
of the following would have a larger atomic
radius:
•
•
•
•
Phosphorous or Silicon
Sodium or Lithium
Chlorine or Fluorine
Neon or Krypton
Ch. 4 Arrangement of
Electrons in Atoms
4.1 The Development of a New
Atomic Model
Light
• Before 1900, scientists thought that light
behaved only as wave
• discovered that also has particle-like
characteristics
Light as a Wave
• electromagnetic radiation:
– form of energy that acts as a wave as it travels
• All forms are combined to form
electromagnetic spectrum
Electromagnetic spectrum
High Frequency
Gamma Rays
Short
Wavelength
X-Rays
Ultraviolet Rays
Visible Light
Infrared Rays
Microwaves
Low Frequency
Radio Waves
Long
Wavelength
Light as a Wave
• all form of EM radiation travel at a speed of
3.0 x 108 m/s in a vacuum
• it has a repetitive motion
• wavelength: (λ) distance between points on
adjacent waves; in nm (1nm= 10-9m)

• frequency: (ν) number of waves that
passes a point in a second, in
waves/second
c
Inversely proportional!
What is the wavelength of X-rays having a
frequency of 4.80 x 1017 Hz?
Photoelectric Effect
• when light is shone on a piece of metal,
electrons can be emitted
– Didn’t happen if the light’s frequency was
below a certain value
• scientists could not explain this with their
classical theories of light
Photoelectric Effect
• Max Planck- 1877
• suggested that an object emits energy in
the form of small packets of energy called
quanta
• quantum- the minimum amount of energy
that can be gained or lost by an atom
E

h

Planck’s constant (h): 6.626 x 10-34 J*s
Calculate the energy of a photon of radiation
with a frequency of 4.3 x 1015 Hz.
Photoelectric Effect
• Einstein added on to Planck’s theory in
1905
• suggested that light can be viewed as
stream of particles called photons
– no mass and carry one quantum of energy
– Energy depends on frequency
Bohr Model
• created by Niels Bohr
(Danish physicist)
in 1913
• linked atom’s electron with emission
spectrum
• electron can circle nucleus in certain
paths, in which it has a certain amount of
energy
Bohr Model
• Electron gains energy by
moving to a higher rung
on ladder
• Loses energy by moving
to lower rung on ladder
Bohr Model
a photon is
released that has
an energy equal to
the difference
between the initial
and final energy
orbits
Bohr Model
• problems:
– True only for hydrogen atom
– did not explain chemical behavior of
atoms