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
 includes: visible light, X rays, ultraviolet and infrared light,
microwaves, and radio waves
 All forms are combined to form electromagnetic
spectrum
Light as a Wave
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 (109nm = 1m)
 frequency: (ν) number of waves that passes a point
in a second, in waves/second
Inversely proportional!
c  
Photoelectric Effect
 when light is shone on a piece of metal, electrons can
be emitted
 no electrons were emitted if the light’s frequency was
below a certain value
 scientists could not explain this with their classical
theories of light
 Ex: coin-operated soft drink machine
Photoelectric Effect
 Max Planck: a German physicist
 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
Planck’s constant (h): 6.626 x 10-34 J*s
E  h
Photoelectric Effect
 Einstein added on to Planck’s theory in 1905
 suggested that light can be viewed as stream of
particles
 photon- particle of EM radiation having no mass and
carrying one quantum of energy
 energy of photon depends on frequency
Photoelectric Effect
 EM radiation can only be absorbed by matter in
whole numbers of photons
 when metal is hit by light, an electron must absorb
a certain minimum amount of energy to knock the
electron loose
 this minimum energy is created by a minimum
frequency
 since electrons in different metal atoms are bound
more or less tightly, then they require more or less
energy
H Line-Emission Spectrum
 ground state- lowest energy state of an atom
 excited state- when an atom has higher potential
energy than it has at ground state
 line-emission spectrum- series of wavelengths of
light created when visible portion of light from
excited atoms is shined through a prism
H Line-Emission Spectrum
 scientists using classical theory expected atoms to be
excited by whatever energy they absorbed
 continuous spectrum- emission of continuous range of
frequencies of EM
radiation
H Line-Emission Spectrum
 Why had hydrogen atoms only given off
specific frequencies of light?
current Quantum Theory attempts to explain this
using a new theory of atom
H Line-Emission Spectrum
 when an excited atom falls back to ground state, it
emits photon of radiation
 the photon is equal to the difference in energy of the
original and final states of atom
 since only certain frequencies are emitted, the
differences between the states must be constant
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
 Can gain energy by moving to a
higher rung on ladder
 Can lose energy by moving to
lower rung on ladder
 Cannot gain or lose while on
same rung of 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:
 did
not work for other atoms
 did not explain chemical behavior of
atoms