The Quantum Mechanical Model of the Atom

Niels Bohr In 1913 Bohr used what had recently been discovered about energy to propose his planetary model of the atom. In Bohr’s model neutrons and protons are contained in the nucleus and electrons move around the nucleus in defined orbits These orbits, also known as “shells” or “energy levels” are designated by an integer: 1,2,3 etc. According to Bohr electrons can ONLY be found within these orbits – not “in between” them (similar to books on a bookshelf), because like light, the energy of the electrons is quantized. Each orbit has a distinct amount of energy associated with it (and different for each element). Orbits closer to the nucleus have lower energy than those further away. Energy must be absorbed for an electron to move from its ground state (the orbit where it would normally reside) to an excited state (any orbit higher in energy)

Electrons are not stable after jumping to higher orbits and readily fall back to their ground states. When this happens energy is released in the form of coloured light. Electrons moving up to a higher level or back down are known as “transitions” Bohr’s planetary model is based on the work he did with hydrogen’s emission spectrum. However, Bohr’s model has some problems: - it only works for hydrogen – a one electron system. For atoms with more electrons, the model fails - the model claims you can tell where an electron is and what it is doing, thereby violating the Heisenberg Uncertainty Principle. - Bohr’s limited instrumentation didn’t allow him to see that the red line in hydrogen's emission spectrum is actually two lines extremely close together. His model could not explain this.

Heisenberg Uncertainty Principle In the 1920’s Werner Heisenberg put forth his Uncertainty Principle which states: at one time it is impossible to calculate both the momentum and the location of an electron in an atom; it is only possible to calculate the probability of finding an electron within a given space.

Louis de Broglie Building upon the work of Einstein, Bohr and others before him, de Broglie developed the idea that any object –including electrons has wavelike properties. Essentially he proposed that all matter consists of waves (on the macroscopic level the wavelength of a item such as a baseball would be so incredibly small that we just don’t see it) While de Broglie didn’t do any experimentation, in 1927 Davisson and Germer performed experiments which did indeed confirm de Broglie’s hypothesis

In 1926 used mathematics and statistics to combine ideas before him (in particular those of de Broglie and Heisenberg) and created Quantum Mechanics. Quantum Mechanics is a branch of physics that uses mathematical equations to describe the wave properties of subatomic particles. From quantum mechanics Schrödinger proposed the quantum mechanical model of the atom According to Schrodinger, you could describe only the probability of where an electron could be. The distributions of these probabilities formed regions of space about the nucleus called orbitals. Orbitals could be described as electron density clouds. The densest area of the cloud is where you have the greatest probability of finding the electron and the least dense area is where you have the lowest probability of finding the electron Electrons can now be described using the four quantum numbers, as we will see. Erwin Schrödinger

Schrodinger’s classical thought experiment to explain quantum theory. The “thought” experiment (it was never actually carried out, and please don’t try this at home) is supposed to explain how a quantum particle (for example an electron) exists as both a particle and a wave and can perform the functions of both of those states. But, when we observe the particle we are seeing it at only a moment in time where the simple act of us observing it has forced it into one of those states.

Modern Quantum Mechanics Since 1932, through continued experimentation, many additional particles have been discovered in the atom. New elements have been created by bombarding existing nuclei with various subatomic particles. Atomic theory has been further enhanced by the concept that protons and neutrons are made of even smaller units called quarks. The quarks themselves are in turn made of vibrating strings of energy. (String theory). There is so much more to talk about in terms of Quantum Mechanics – but I’ll leave some for university!