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In atomic physics and quantum chemistry, the Aufbau principle (/ ˈ aʊ f b aʊ /, from German: Aufbauprinzip, lit. 'building-up principle'), also called the Aufbau rule, states that in the ground state of an atom or ion, electrons first fill subshells of the lowest available energy, then fill subshells of higher energy. For example, the 1s ...
However there are numerous exceptions; for example the lightest exception is chromium, which would be predicted to have the configuration 1s 2 2s 2 2p 6 3s 2 3p 6 3d 4 4s 2, written as [Ar] 3d 4 4s 2, but whose actual configuration given in the table below is [Ar] 3d 5 4s 1.
[3] As a result of Hund's rule, constraints are placed on the way atomic orbitals are filled in the ground state using the Aufbau principle. Before any two electrons occupy an orbital in a subshell, other orbitals in the same subshell must first each contain one electron. Also, the electrons filling a subshell will have parallel spin before the ...
As an example, the ground state configuration of the sodium atom is 1s 2 2s 2 2p 6 3s 1, as deduced from the Aufbau principle (see below). The first excited state is obtained by promoting a 3s electron to the 3p subshell, to obtain the 1s 2 2s 2 2p 6 3p 1 configuration, abbreviated as the 3p level.
As an example, consider the ground state of silicon. The electron configuration of Si is 1s 2 2s 2 2p 6 3s 2 3p 2 (see spectroscopic notation ). We need to consider only the outer 3p 2 electrons, for which it can be shown (see term symbols ) that the possible terms allowed by the Pauli exclusion principle are 1 D , 3 P , and 1 S .
Two Indian chemical companies have been indicted for allegedly importing ingredients for the highly addictive opioid fentanyl into the United States and Mexico, the U.S. Department of Justice said ...
The most famous example is Vaska's complex (IrCl(CO)(PPh 3) 2), [PtCl 4] 2−, and Zeise's salt [PtCl 3 (η 2-C 2 H 4)] −. In such complexes, the d z 2 orbital is doubly occupied and nonbonding. Many catalytic cycles operate via complexes that alternate between 18-electron and square-planar 16-electron configurations.
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