Modern Atomic Theory

Bohr's model of the atom is important because it introduced the concept of the quantum in explaining atomic properties. However, Bohr's model ultimately needed revision becuase it failed to explain the natue of atoms more complicated than hydrogen. It took roughly another decade before a new more complete atomic theory was developed - the modern atomic theory.
 
Louis de Brogllie introduces the wave/particle duality of matter (1921)
 

Traditional (classical) physics had assumed that particles were particles and waves were waves and thats that. However, de Broglie suggested that particles could sometimes behave as waves and waves could sometimes bahave as particles - the wave/particle duality of nature. He suggested a simple equation that would relate the two: Particles have momentum (p), waves have wavelengths (l) and the two are related by the equation

l=h/p

h=Planck's constant = 6.634x10-34 Js

p=(mass)x(velocity)

This wave/particle duality of nature turned out to be a key to the new atomic theory. Here is an examples of using the de Broglie equation:

Question: A ball of mass 0.2 kg is thrown with a velocity of 10 m/s what is its wavelength?

Answer:

This means that the wavelength of the ball is miniscule compared to the size of the ball.

Werner Heisenberg elucidated the Uncertainty Principle (1923)

Classical physics had always assumed that precise location and velocity of objects was always possible. Heisenberg, however discovered that this was not necessarily the case at the atomic level. In particular, he stated that the act of observation interfered with the location and velocity of small particles such as electrons. This is the case because observation requires light and light has momentum. When light bounces off an electron momentum exchange can occur between light and the electron which means the electrons location and velocity have been altered by the act of measurement. This scenerio has important implications to what we can measure at the atomic level.
 
Erwin Schrodinger developed the equation which is used today to understand atoms
and molecules - the Schrodinger Equation (1926)
 

Erwin Schrodinger took the ideas developed by de Broglie, Heisenberg and others and put them together in a single equation that is named after him. Solving this equation can in principle predict the properties and reactivities of all atoms and molecules.

Unfortunately, it is extremely difficult to solve for any but the most simple atoms and molecules.

 
Orbitals and Energies are the central objects that determine the properties of atoms
and molecules in the new Quantum Theory
 

Although the Schrodinger equation is too difficult to solve for any but the simplest atoms/molecules, we can nevertheless extract some essential conclusion from it:

I) 
Energies are quantized:
Atoms and molecules cannot have any energy but only certain energies. This means that energies are "quantized".
 

II) 
The orbitals, associated with each energy, determine where the electrons are located.
Each orbital is determined by a quantum number call the angular momentum quantum number "l". This quantum number can take on the values l=0 (s-orbital), l=1 (p-orbital), l=2 (d-orbital), l=3 (f-orbital) etc.
 
 
These orbitals can be be seen as the "rooms" in which the electrons in an atom "live".

The quantum energies together with the orbitals can be used to explain chemical properties and reactivities.

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C101 Class Notes
Prof. N. De Leon