Modern Theory of Atomic Structure

   The Modern Theory of Atomic Structure was developed by Erwin Schrodinger. In this theory, the electron is treated as both a wave and a particle. The result is the Schrodinger Wave Equation, a mathematical statement that describes the behavior of all electrons up to the limits allowed by the Heisenberg Uncertainty Principle. Following is a series of terms and concepts that relate to The Modern Theory of Atomic Structure.




Aufbau Principle

 The Aufbau Principle is a series of procedures that are used when placing electrons on an atom. The term "aufbau" means "to build-up". As applied to elements it refers to the idea that the electronic structure of an atom is built upon the structures of all the atoms with lesser atomic numbers that occurred before it on the Periodic Chart. The Principle consists of three parts:

  • Electrons are placed as close to the bottom of the energy level diagram as possible; at this point they will be located at positions of lowest energy.
  • The Pauli Exclusion Principle must be obeyed, meaning that an orbital will be limited to a maximum of two electrons.
  • Hund's Rule must be obeyed; when placing electrons into a degenerate set of orbitals, there must be one electron in each orbital of the set before any pairing of electrons can take place.

Azimuthal Quantum Number
 This is generally recognized as the second quantum number. It is symbolized with the letter "l". It may have any non-negative integer value from zero up to one less than the value of the Principal Quantum Number. The azimuthal quantum number is used to designate the types of orbitals. For instance, when l = 0 then it designates an s orbital. When l = 1 then it designates a p orbital. When l = 2 then it designates a d orbital. Finally, when l = 3 then it is referring to an f orbital.

de Broglie
 de Broglie is the individual who is credited with the idea of the Duality of the Electron. Because of de Broglie's attempt at describing the electrons as waves, Schrodinger was able to develop the Modern Theory of Atomic Structure.  

Degenerate Orbitals
   Degenerate Orbitals are any orbitals that are located at the same height on the energy level diagram. They have the same energy content. Since an electron is located primarily based on energy content, when an electron is placed in degenerate orbitals there will be equal probability of locating the electron in any orbital of the set. In essence, the electron will not have any preference when choosing amongst degenerate orbitals.

Duality of the Electron
   The Duality of the Electron is the idea that an electron can be described as if it were a wave and particle. Light can be treated as a wave or particle. Light arises from the vibration of electrons. Therefore, it seems logical to be able to describe the electron as either a wave or particle. This concept was first developed by de Broglie. The Duality concept has many varied interpretations. It is useful to be able to visualize an electron as both a wave and a particle.

Effective Nuclear Charge
 Effective Nuclear Charge is the amount of nuclear charge that an electron will experience after taking into consideration all of the screening options contributed by the other electrons in the system. It is the average amount of charge from the nucleus that an electron will experience as a result of the eclipsing, or screening, created by all of the other electrons in the system. Effective Nuclear Charge can be approximately calculated by the application of Slater's Rules.

Electronic Configuration

 Electronic Configuration is the method of listing the locations of all the electrons on a specific atom. The system locates each electron by energy level and orbital type. The number of electrons in each orbital type is indicated with a superscript. For instance, the electronic configuration of Sodium is


This indicates that there are two electrons located in the 1s orbital, two electrons in the 2s orbital, six electrons in the 2p orbital and a single electron in the 3s orbital.

 Isoelectronic Structures are different chemical systems with identical electronic structures. For instance, F-1 will be isoelectronic with Ne. Both systems contain ten electrons. The electrons in each system are distributed in the same way. Such systems will have the same amount of screening.

   Lobes are the arms of an orbital that extent out from the nucleus. They occur on all orbitals except for the s orbital. The lobes are regions that will locate an electron 90% of the time. However, the majority of the time the electron will be located in the outside tips of the lobes.

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Updated June 30, 2008