Aufbau principle

The construction principle is a concept developed by Niels Bohr in 1921 in order to explain the periodic occurrence of the chemical properties in the periodic table of the elements using the properties of the atomic shell can. It is used to determine the arrangement of electrons in the atoms, molecules, or ions in the lowest energy state. The principle is basically a process that describes the gradual filling of the atomic shell with electrons. Electrostatically attracted by the protons in the nucleus examines each newly added electron of the most stable state for themselves. This is located in the atomic orbital, which has the lowest energy and is not yet fully staffed, with the maximum number of electrons in each orbital is given by the Pauli principle. In this orbital, the electron remains, even while filling, so that the atomic shells of all atoms have the same structure in its interior, except that the orbitals concentrate with increasing nuclear charge close to the core.

According to the principle, the electrons fill the orbitals always where the lowest energy state can be achieved before taking orbitals with higher energy (eg 1s before 2s always ). If multiple orbitals are available for an electron with the same energy to choose from, according to Hund's rules, the already partially occupied orbital is preferred.

The construction principle can also be used to describe the arrangement of protons and neutrons in the nucleus.

The Madelung energy scheme

The order in which the electrons occupy the orbitals is described by the rule ( also known as the Madelung rule after Erwin Madelung or Klechkowski rule in some, mostly French -speaking countries ):

• Orbitals with a smaller value are filled before the orbitals with a larger value. : Principal quantum number
• : Quantum number

This behavior of the electrons has been experimentally found by the spectral properties of the elements. The quantum mechanical explanation of the rule is based on the total number of nodes in an orbital, which reflects the energy state.

Specifically, the electrons are in turn incorporated into the following orbitals (see figure on the right ): ⇒ ⇒ 1s 2s 2p 3s ⇒ ⇒ ⇒ 3p 4s 3d ⇒ ⇒ ⇒ 4p 5s 4d ⇒ ⇒ ⇒ 5p 6s 4f ⇒ ⇒ ⇒ 5d 6p 7s ⇒ ⇒ ⇒ 5f 6d 7p ⇒

Note: The Madelung energy scheme can only be applied to neutral atoms in their ground state. (See exceptions) Explanation of the right image: The orbitals of the atomic shell shall be filled in the order of the arrow with electrons. From left to right, the orbitals of the atomic shell are enumerated ( increasing quantum number ) and from top to bottom the shells ( increasing principal quantum number ), each with letters corresponding shortcuts. The superscript numbers indicate, respectively, how many electrons can stay a maximum in the orbital or in the shell. The key for the population value n l takes in this view diagonally to the bottom right. Therefore, all orbitals on lines perpendicular to this direction in each case the same value n l According to the rule, the orbitals with the smaller n values ​​are busy in this case first, ie the individual diagonals are traversed from top right to bottom left. The pale area shown is theoretical because no atoms could be discovered or created with so many electrons and thus required large nuclei.

Exception

Not all atoms follows the occupation of the shells of the above simple construction rule. Reason, relativistic effects, which play an increasingly important role in higher atomic number but have not been considered within this structure rules. Examples of elements that behave differently:

• In lanthanum first occupied one electron orbital of a 5d subshell before 4f is filled; at Actinium occupied according to a 6d electron before 5f is filled. The electrons first occupy empty orbitals within a subshell.

• For copper and chromium an electron of 4s orbital changes in the 3d orbital, so that the 4s orbital is only singly occupied despite its lower energy levels. However, as the 3d orbitals half (Chrome) or completely (copper) are occupied.

The electron configurations beyond the element rutherfordium (atomic number: 104) are not yet clearly confirmed or proven. The following periodic table and the following list gives an overview of the exceptions, while the most striking similarities were summarized.

Excerpt from the Periodic Table

List