L2 ELECTRON CONFIG

• Electronic Configuration

  •  Also called electronic structure or electron configuration.
  • The probable distribution of electrons in their atomic orbitals.
  • The arrangement of electrons in orbitals around an atomic nucleus. 
  • It describes how electrons are distributed in its atomic orbitals. 
• • The order in which electrons are placed into the orbitals is based on the order of their energy. This is referred to as the Aufbau principle. 
• • (1) Aufbau Principle
  • Named after the German word ‘Aufbeen’ which means ‘build up’.
  • States that electrons will occupy the orbitals having lower energies before occupying higher energy orbitals.
  • Electrons are filled in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p…
  • The s, p, d, and f subshells can accommodate a maximum of 2, 6, 10, and 14 electrons, respectively.
• • (2) Pauli Exclusion Principle
  • States that a maximum of two electrons, each having opposite spins, can fit in an orbital.
  • This principle can also be stated as “no two electrons in the same atom have the same values for all four quantum numbers”.
  • Therefore, if the principal, azimuthal, and magnetic numbers are the same for two electrons, they must have opposite spins.

  
• (3) Hund’s Rule

  • This rule describes the order in which electrons are filled in all the orbitals belonging to a subshell.
  • It states that every orbital in a given subshell is singly occupied by electrons before a second electron is filled in an orbital.
  • In order to maximize the total spin, the electrons in the orbitals that only contain one electron all have the same spin (or the same values of the spin quantum number).

  
• • The standard notation often yields a long electron configuration (especially for elements having a relatively large atomic number). 
  • abbreviated or condensed notation may be used instead of the standard notation. 
  • An abbreviated notation consists of the symbol of the last noble gas before that atom, followed by the configuration of the remaining electrons.
• • How to Write an Electron Configuration
  • The symbols used for writing the electron configuration start with the shell number (n) followed by the type of orbital and finally the superscript indicates how many electrons are in the orbital.

  
• REVIEW AND PRACTICE ONLINE

  
• • Exception
  • It is important to note that there exist many exceptions to the Aufbau principle such as chromium and copper. These exceptions can sometimes be explained by the stability provided by half-filled or completely filled subshells. 
  • The electron configuration of Chromium (Z=24) is [Ar] 4s13d5 and not [Ar] 4s23d4, as we might expect. 
• • A similar case is observed for Copper, whose electron configuration is [Ar] 4s13d10 rather than [Ar] 4s23d9. 
  • The reason for these irregularities is that a slightly greater stability is associated with the half-filled (3d5) and completely filled (3d10) subshells.