Development of the Modern Atomic Theory
By the end of the 19th century (1800’s), the idea that
Matter & Energy were separate and distinct prevailed.
 Matter – consisted of particles.
o Had mass
o Occupied a definite position in
space
● Energy – consisted of waves
o Massless
o Delocalized-position in space
cannot be specified.
 No intermingling between the two
This all changed with the start of the 20th century and the
work of Max Planck.
Planck – Observed radiations emitted from solid bodies
heated to incandescence (glowing).
This could not be explained by classical physics, which
said that matter could absorb or emit any quantity of
energy. It was continuous.
Planck offered that his observations could be explained by
proposing that energy could be gained or lost in whole
number multiples of the quantity hυ. In other words,
energy behaved like small, independent “particles” of
light energy.
From this work, Planck’s equation was derived.
E = hυ
This was a BIG DEAL, because it indicates that energy
could be quantized. In other words it can behave as a
particle, much like matter, which was contrary to popular
thought; that energy behaves as a wave.
These particles of very specific quantities of light energy
were called quantum (quanta) or photons.
This work led to the Bohr Model.
In 1923, Louis DeBroglie (French) goes in the other
direction and shows that matter can behave as a wave.
Proves mathematically and creates the equation for a
DeBroglie wavelength.
This work showed that any form of matter possesses a
wavelength which is a wave property, so he proved that
matter could behave like waves.
However, the wave properties of matter only become
significant as the form of matter becomes smaller.
This work resulted in what is known as the Wave-Particle
Duality of Nature which states that matter and energy can
both behave as a particle and as a wave, but not at the
same time.
If one wants to understand the behavior of each they must
study both types of properties.
DeBroglie’s work helped establish the area of quantum
physics. The laws of motion that govern/explain the
behavior of extremely small particles moving at or near
the speed of light.
Heisenberg Uncertainty Principle: (1927)
It is impossible to know both the exact location and
the exact momentum of an electron at the same time.
This means the more exactly one knows the location of an
object the less sure they are about how fast it is moving
and vice-versa.
The Result of this work was that it showed that the Bohr
Model was too exact.
The Bohr Model:
 Electrons were in circular orbits a set distance
from the nucleus.
This means he knew exactly where the
electrons were.
 Electrons were traveling in circular orbits with
specific energy and at a set velocity.
This means he knew exactly where the
electrons were going and how fast.
Therefore, according to Heisenberg, Bohr’s Model of the
atom could not be correct.
Schrödinger’s Wave Mechanical Model
(1926-1930)
1. Treated electrons as if they had wave-like
properties instead of particle properties.
2. Electrons do not follow a set circular orbit a
specific distance from the nucleus, but the
electrons are free to travel anywhere within
their respective energy level/region.
3. Energy level was described as a region of
highest probability of finding an electron in a
given place with a specific amount of energy.
4. Regions of highest probability (90%) are
calculated mathematically* and they represent
geometric shapes that describe the volume
occupied by electrons. These regions are called
electron clouds.
*Using the Schrödinger Wave Equation.